Electrodynamically Confined Microscale Lasers · Rachit Sharma aus Bhilai, Indien Max-Planck-Institut fu¨r die Physik des Lichts Erlangen, 2009. Als Dissertation genehmigt von den

Electrodynamically Confined

Microscale Lasers

Den Naturwissenschaftlichen Fakultaten derFriedrich-Alexander-Universitat Erlangen-Nurnberg

zurErlangung des Doktorgrades

vorgelegt von

Rachit Sharma

aus Bhilai, Indien

Max-Planck-Institut fur die Physik des LichtsErlangen, 2009

Als Dissertation genehmigt von den NaturwissenschaftlichenFakultaten der Universitat Erlangen-Nurnberg

Tag der mundlichen Prufung: 28.07.2009

Vorsitzenderder Promotionskommission: Prof. Dr. Eberhard Bansch

Erstberichterstatter: Prof. Dr. Lijun Wang

Zweitberichterstatter: Prof. Dr. Min Xiao

i

Acknowledgments

This thesis would have been impossible without the significant helps and con-tributions from my various co-workers in the research group of Professor Wang atthe Max Planck Institute for the Science of Light, Erlangen. Apart from being agreat help in the research work, they have also influenced my life and education ina very positive way during my stay in Germany. Therefore, I would like to take thisopportunity to express my deep sense of gratitude towards them.

First and foremost, I would like to thank Prof. Lijun Wang for giving me theopportunity to pursue full-time graduate studies in his group, thus providing mewith a chance to realize my long cherished dream of making an original contributionto science. Prof. Wang has been an immense source of motivation, encouragementand knowledge to me. He granted me with enormous freedom to pursue research inmy field of interest and, at the same time, provided the right kind of guidance tomake sure that my efforts were always in accordance with scientific rigor. Despitehis busy schedule, he was always accessible for any sort of academic or non-academichelp that I needed during the course of this project.

I also extend sincere thanks to my labmates Jan Schaefer and Dr. Jessica Mon-dia. The three of us spent countless but enjoyable hours in the lab together whilesetting up experiments and acquiring data. I specially thank Dr. Mondia for teach-ing me various important laboratory skills during the initial phases of my doctoralwork and also for proof reading this thesis. I also thank Dr. Harald Schwefel forproof reading the theoretical background chapter.

Furthermore, I thank Dr. Zehuang Lu who, on numerous occasions, helped insolving the technical problems in the lab. In addition, thanks to Dr. Stefan Malzerfor helping me with the SEM images. Thanks also to Dr. Quanzhong Zhao forhelping me at various instances with the femtosecond micromachining setup. I alsothank Prof. Gottfried Doehler for his useful and far-reaching suggestions. Moreover,I thank Ben Sprenger for helping us obtain the ZnO tetrapod samples. I also thankall the members of the Wang group for their efficient help, creative suggestions, en-lightening discussions, and pleasant social interactions. Finally, I specially thank myparents and sisters whose support and encouragement have served as an invaluablemotivation during my doctoral research.

Rachit Sharma

ii

iii

Zusammenfassung

Die konstante Weiterentwicklung von modernen optoelektronischen und ver-wandten Technologien in Richtung kleinerer Bauteile bringt einen steigenden Bedarfan miniaturisierten Lichtquellen mit sich. Mikrolaser sind in dieser Hinsicht beson-ders viel versprechend wegen ihrer mikroskopischen Große, geringen Laserschwelleund schmalen Ausgangsbandbreite. Die vorliegende Arbeit konzentriert sich auf dieEntwicklung und Untersuchung von drei solcher Laserquellen, und zwar einem ZnOTetrapoden Laser, einem Glyzerin Mikrotropfchen Laser und CdSe/ZnS Quanten-punkt Laser. Eine elektrodynamische Falle vom Endkappen-Typ wird verwendet,um die Laser-aktiven Mikroteilchen raumlich zu beschranken. Ein gutegeschalteterNd:YAG Laser (10 Hz, 10 ns) wird zur optischen Anregung verwendet. Wir zeigendie experimentelle Realisierbarkeit der elektrodynamischen Isolation und Mikropo-sitionierung von ZnO-basierten Nanostrukturen, um ihre intrinsischen optischenEigenschaften unter atmospharischen Bedingungen zu untersuchen. Mit Hilfe einerElektrospray-Technik wird eine verdunnte Losung von ZnO Tetrapoden (in Methanol)in die elektrodynamische Falle gespruht. Anschließend werden die Fallenparameterder verdampfenden Methanol-Losung angepasst, bis eine einzelne ZnO Tetrapoderaumlich isoliert in der Falle zuruckbleibt. Laseraktivitat im UV (ca. 390 nm)bei einem Schwellstrahlungsfluss von 10 mJ/cm2 wird von einzelnen und mehrerengleichzeitig gefangenen ZnO Tetrapoden mit typischen Beinlangen von 15-25 µmbeobachtet. Daruber hinaus wird die prazise Mikromanipulation von gefangenenTetrapoden uber eine Lange von 100 µm gezeigt. Wir demonstrieren außerdem Ra-man Laseraktivitat in Mikrotropfchen aus reinem Glyzerin und prasentieren Langzeit-messungen des typischen Laser-Blinkverhaltens (an/aus). Single- und MultimodenRaman-Laseraktivitat (bei ca. 630 nm) werden bei Tropfchendurchmessern von10.3 µm und 44.7 µm erreicht und gezeigt. Typische Laserschwellen zwischen 200-390 mJ/cm2 werden gemessen. Das Lasersignal tritt in zeitlich getrennten undfast symmetrischen Haufungen auf, die mit zunehmender Verdampfungsrate desTropfchens an Frequenz zunehmen und an Dauer abnehmen. Durch eine Varia-tion von Glyzerin-Konzentration und Pumpleistung gelingt es uns zu demonstri-eren, dass das Blinkverhalten durch Doppelresonanz im verdampfenden Tropfchenverursacht wird und dass es durch Kontrolle der Verdampfungsrate manipuliert wer-den kann. Schließlich demonstrieren wir Single- und Multimoden Laseraktivitat(bei ca. 640 nm) von CdSe/ZnS-Quantenpunkt-dotierten Mikrotropfchen bei 9 µmund 34 µm Tropfchendurchmessern und bei Laserschwellen von ca. 50 mJ/cm2.Spektrale Blauverschiebungen der Lasermoden von bis zu 2 nm und des spek-tralen Verstarkungsbereichs der Quantenpunkte von 3.2 nm werden bei zunehmenderPumpleistung beobachtet. Außerdem deuten unsere Ergebnisse darauf hin, dassdie zur Laseraktivitat minimal benotigte Quantenpunktkonzentration mehr als zweiGroßenordnungen unter der bisher angenommenen theoretischen Grenze liegen kann.

iv

v

Abstract

As modern-day optoelectronics and related technologies are constantly movingtowards smaller dimensions, there is an increasing need to develop efficient minia-ture light sources. Microcavity lasers are very promising in this respect due to theirmicroscale sizes, low lasing thresholds, and narrow output linewidths. This workfocuses on the development and study of three such lasers, namely, the ZnO tetra-pod laser, the glycerol microdrop Raman laser, and the CdSe/ZnS quantum dotmicrodrop laser. An “end-cap” type electrodynamic trap is used to spatially confinethe lasing microparticles. A Q-switched Nd:YAG laser (10 Hz, τ ∼10 ns) is usedfor optical excitation. We experimentally show the viability of electrodynamicallyisolating and micropositioning ZnO-based nanostructures to investigate their intrin-sic optical nature under atmospheric conditions. An electrospray technique is usedto spray a dilute solution of ZnO tetrapods (in methanol) into the electrodynamictrap. Subsequent tuning of trapping parameters, as the methanol evaporates, leadsto the stable confinement of a single ZnO tetrapod in free space. UV lasing (around390 nm), with threshold fluence around 10 mJ/cm2, is observed from single andmultiple trapped ZnO tetrapods with typical leg lengths of 15-25 µm. Moreover,precise translational micromanipulation of a trapped tetrapod is shown up to arange of 100 µm. We further demonstrate Raman lasing from a trapped pure glyc-erol microdrop and present long-term measurements of the lasing blinking (on/off)behavior. Single and multimode Raman lasing (around 630 nm) are achieved andshown for glycerol drops of 10.3 µm and 44.7 µm in diameter, respectively. Typicalthreshold fluences are measured to be between 200-390 mJ/cm2. Lasing is found tooccur in temporally separated and nearly symmetric bursts which increase in fre-quency and decrease in duration as the evaporation rate of the drop is increased. Byusing drops of different glycerol concentrations and by varying the pump fluence,we conclusively demonstrate that the Raman lasing blinking is caused by doubleresonances in the evaporating drop and that it can be manipulated by controllingthe drop’s evaporation rate. Finally, we demonstrate single and multimode lasing(around 640 nm) from CdSe/ZnS doped microdrops, of diameters 9 µm and 34 µm,respectively, at threshold pump fluences of around 50 mJ/cm2. Blue-shifts of up to2 nm for the lasing modes and 3.2 nm for the quantum dot gain profile are observedwith increasing pump fluences. Moreover, our results indicate that the minimumquantum dot concentration required for lasing can be more than two orders of mag-nitude lower than the previously reported theoretical limit.

vi

Contents

Acknowledgments i

Zusammensfassung iii

Abstract v

List of Figures xi

List of Tables xix

1 Introduction 1

1.1 Towards Microscale Lasers . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1.1 Semiconductor Nanowires and Tetrapods . . . . . . . . . . . . 3

1.1.2 Microcavity Raman Lasers . . . . . . . . . . . . . . . . . . . . 5

1.1.3 Quantum Dot Microcavity Lasers . . . . . . . . . . . . . . . . 7

1.2 Motivation and Goal . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.3 Outline of the Thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2 Theoretical Background 13

2.1 Basics of Quadrupole Ion Trapping . . . . . . . . . . . . . . . . . . . 13

2.2 Lasing Mechanism in ZnO Tetrapods . . . . . . . . . . . . . . . . . . 19

2.3 Theory of Whispering Gallery Modes in Spherical Microcavities . . . 20

2.3.1 The Mie Theory . . . . . . . . . . . . . . . . . . . . . . . . . . 21

2.3.2 The Ray Model . . . . . . . . . . . . . . . . . . . . . . . . . . 27

2.4 Fundamentals of Raman Scattering . . . . . . . . . . . . . . . . . . . 28

viii CONTENTS

2.5 Brief Theory of Colloidal Quantum Dots . . . . . . . . . . . . . . . . 30

3 Experimental Details 33

3.1 The Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

3.2 Pump Lasers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

3.3 The Electrospray . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

3.3.1 Brief Theory of Electrospray Ionization . . . . . . . . . . . . . 35

3.3.2 Electrospray for the Experiment . . . . . . . . . . . . . . . . . 39

3.4 The Endcap Trap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

3.5 The Imaging System . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

3.6 Spectral Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . 44

3.6.1 Signal Collection Optics . . . . . . . . . . . . . . . . . . . . . 44

3.6.2 The Spectrometer . . . . . . . . . . . . . . . . . . . . . . . . . 45

4 The Electrodynamically Confined Single ZnO Tetrapod Laser 49

4.1 ZnO Tetrapods: Preparation and Structural Properties . . . . . . . . 49

4.2 Electrodynamic Trapping of a Single ZnO Tetrapod . . . . . . . . . . 51

4.3 Optical Investigations of Trapped ZnO Tetrapods . . . . . . . . . . . 54

4.3.1 Photoluminescence and Raman Spectra . . . . . . . . . . . . . 54

4.3.2 UV Lasing in a Single ZnO Tetrapod . . . . . . . . . . . . . . 56

4.3.3 UV Lasing in Multiple ZnO Tetrapods . . . . . . . . . . . . . 60

4.4 Micromanipulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

4.4.1 Translational Control . . . . . . . . . . . . . . . . . . . . . . . 63

4.4.2 Charge Determination . . . . . . . . . . . . . . . . . . . . . . 65

4.4.3 Towards Rotational Control . . . . . . . . . . . . . . . . . . . 68

4.5 Study of ZnO Tetrapods on a Glass Substrate . . . . . . . . . . . . . 70

4.5.1 Experimental Setup . . . . . . . . . . . . . . . . . . . . . . . . 70

4.5.2 Lasing on Substrate vs Lasing in Trap . . . . . . . . . . . . . 72

4.5.3 Q Factor Estimation of Lasing Modes . . . . . . . . . . . . . . 75

4.5.4 Transverse Whispering Gallery Modes on the Tapered Legs . . 76

4.6 Summary of the Chapter . . . . . . . . . . . . . . . . . . . . . . . . . 81

CONTENTS ix

5 Raman Lasing in Electrodynamically Trapped Glycerol Microdrops 83

5.1 Trapping of a Single Pure Glycerol Microdrop . . . . . . . . . . . . . 83

5.2 CW Raman Spectroscopy of a Trapped Glycerol Microdrop . . . . . . 86

5.3 Observation of Raman Lasing Near 630 nm . . . . . . . . . . . . . . . 88

5.4 The On/Off Behavior of Raman Lasing . . . . . . . . . . . . . . . . . 94

5.4.1 Interpretation: The Double Resonance Effect . . . . . . . . . . 96

5.4.2 Effect of Microdrop Evaporation on the On/Off Behavior . . . 99

5.5 Doping the Glycerol Microdrop with Ag Nano-aggregates . . . . . . . 103

5.5.1 Background: Surface Enhanced Raman Scattering . . . . . . . 103

5.5.2 Ag Nanoaggregate Properties . . . . . . . . . . . . . . . . . . 106

5.5.3 Effects of Nanoaggregate Inclusion . . . . . . . . . . . . . . . 109


6 The Quantum Dot Microdrop Laser in an Electrodynamic Trap 115

6.1 The CdSe/ZnS Core-Shell Quantum Dots . . . . . . . . . . . . . . . . 115

6.2 Whispering Gallery Modes in the Quantum Dot Doped Microdrop . . 118

6.3 Lasing from Quantum Dots in the Trapped Microdrop . . . . . . . . 121

6.3.1 Single and Multimode Lasing . . . . . . . . . . . . . . . . . . 121

6.3.2 Threshold Measurements . . . . . . . . . . . . . . . . . . . . . 123

6.3.3 Microdrop Evaporation Effects: Blue Shift of Lasing Modes . 125

6.4 Low Quantum Dot Density in the Lasing Microdrop . . . . . . . . . . 127


7 Conclusion 131

7.1 Summary of the Thesis . . . . . . . . . . . . . . . . . . . . . . . . . . 131

7.2 Future Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133

Bibliography 137

x CONTENTS

List of Figures

1.1 Schematics of optical microcavities taken from literature (see text for

references). (a) A Fabry-Perot type microcavity based on distributed

bragg reflectors (DBRs). (b) Common Whispering Gallery Mode type

microcavities, i.e. microsphere, microdisk, and microtorroid. (c) A

photonic crystal defect microcavity. . . . . . . . . . . . . . . . . . . 2

1.2 SEM images, taken from literature, of various ZnO nanostructures

which have shown stimulated emission (see text for references). (a)

nanowire, (b) tetrapod, (c) nanocomb, and (d) nanoribbon. . . . . . 4

1.3 Images of nanostructure junctions and assemblies taken from liter-

ature (see text for references). (a) SEM image of a fused junction

between a GaN nanowire and a SnO2 nanoribbon (Vertical element is

the nanowire). (b) Dark field image of a three-dimensional assembly

of GaN nanowires and SnO2 nanoribbons (Horizontal elements are

nanoribbons). (c) SEM image of a diode structure based on a single

ZnO tetrapod. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.4 Previous observations of microcavity Raman lasing (see text for refer-

ences). (a) Raman lasing from a spherical silica microcavity. The in-

set shows a microsphere coupled to a fiber taper. (Pump ∼ 1555 nm,

Lasing ∼ 1670 nm.) (b) Raman lasing from a glycerol microdrop

pumped at 532 nm on a superhydrophobic surface. The lasing drop

is shown in the inset. . . . . . . . . . . . . . . . . . . . . . . . . . . 6

xii LIST OF FIGURES

1.5 (a) Schematic of a single photon source, taken from literature, based

on a self-assembled quantum dot and a DBR microcavity. (b) A

UV illuminated 20 µm diameter CdS/ZnS nanocrystal-microsphere

composite used to achieve blue lasing around 470 nm. (c) An opti-

cal micrograph of a tapered fiber-coupled toroidal microcavity laser

showing CdSe/ZnS nanocrystal emission from the whispering-gallery

modes. (see text for figure references) . . . . . . . . . . . . . . . . . 7

2.1 Simulated images of the oscillating trapping potential at times (a)

t = 0, and (b) t = 12f

. The angular frequency of the oscillation is

2πf . The blue circle represents the trapped particle. . . . . . . . . . 14

2.2 (a) A schematic of the conventional quadrupole Paul trap. (b) An

image of the three electrodes of a real Paul trap taken from literature

(see text for reference). . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.3 Microphotograph of a trapped aluminum particle taken from litera-

ture (see text for reference). The secular and the micro motions of

the particle can be seen in the image. (b) A similar pattern observed

for a chalk particle in our endcap trap. The image is acquired by

scattering a red laser off the particle. . . . . . . . . . . . . . . . . . . 18

2.4 A schematic depicting the lasing mechanism in a leg of a ZnO tetra-

pod. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2.5 (a) A schematic of a linearly polarized z-travelling wave incident on

a scattering microparticle. (b) The geometrical optics based expla-

nation of whispering gallery modes. . . . . . . . . . . . . . . . . . . 22

2.6 (a) Effect of quantum confinement on the excitonic energy levels in

quantum dots. (b) A laboratory picture showing the red-shift of the

photoluminence wit increasing quantum dot diameters (Photo cour-

tesy: Andrey Rogach, LMU, Munich). . . . . . . . . . . . . . . . . . . 31

3.1 A schematic of the salient features of our experimental setup. . . . . . 34

LIST OF FIGURES xiii

3.2 (a) A schematic depicting the mechanism of electrospray ionization.

(b) Demonstration of the electrospray characteristics in our lab using

a micropipette with water as the fluid. A blue laser is scattered off

the spray for imaging purposes. . . . . . . . . . . . . . . . . . . . . . 36

3.3 (a) A picture of our electrospray setup used for sample introduction

into the trap. (b) Images of the spray of a 50 % glycerol solution

for different voltages applied to the needle. The values of the corre-

sponding voltage is written next to each image. . . . . . . . . . . . . 38

3.4 (a) A simulated drawing of our endcap trap. (b) An image of the real

trap. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

3.5 Our 40X high magnification optics. (a) A schematic showing the

arrangement of the optical components (b) A picture of the actual

optics showing the microscope objective and the achromat attached

in a 1 inch diameter tube. (c) The magnified image of a 25±.29 µm

trapped polystyrene bead used for calibration purposes. . . . . . . . 43

3.6 (a) Schematic of our inbuilt signal collection optics. (b) A picture of

the actual signal collection optics showing the two achromats mounted

on a 1 inch diameter tube inside a PVC cage. (c) A picture showing

the two ends of the multimode fiber used for guiding the collected

signal to the spectrograph. . . . . . . . . . . . . . . . . . . . . . . . 45

3.7 Calibration test of our spectrometer system for the (a) 300 lines/mm,

and (b) 1200 lines/mm, gratings. . . . . . . . . . . . . . . . . . . . . 47

4.1 SEM images of (a) a cluster of tetrapods, (b) a single tetrapod, (c)

an end facet of a tetrapod leg, and (d) the center of the tetrapod. . . 50

4.2 High magnification (40X) image of (a) a trapped cluster of tetrapods

(b) a single trapped tetrapod. The trapped particles are illuminated

with a green laser and the scattered light is used for imaging. . . . . 52

4.3 Optical properties of a single trapped ZnO tetrapod, (a) The PL

spectra and (b) The CW Raman spectra. . . . . . . . . . . . . . . . 55

xiv LIST OF FIGURES

4.4 Single pump pulse excitation PL spectra of the tetrapod in Fig. 4.5a at

three different pump fluences of 2 mJ/cm2, 20 mJ/cm2, and 90 mJ/cm2.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

4.5 High magnification (40 X) images of (a) a single trapped lasing ZnO

tetrapod and (b) multiple trapped lasing tetrapod. The images are

taken for a CCD exposure time of 30 ms and a pump fluence of

20 mJ/cm2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

4.6 (a) Spectral evolution of a single tetrapod, shown in Fig. 4.5a, as

a function of the fluence averaged for 200 pump pulses. (b) A plot

of the lasing threshold behavior for a single tetrapod. Each point

represents the average of 20 scans and their corresponding error bars

and each scan covers 10 pulses. To minimize the background PL we

integrate around the lasing peak centered at 388.5 nm. . . . . . . . . 59

4.7 (a) Spectral evolution of multiple tetrapods, shown in Fig. 4.5b, as a

function of pump fluence averaged for 200 pulses. (b) A plot of the

lasing threshold behavior for multiple tetrapods. Each point repre-

sents the average of 20 scans and their corresponding error bars, each

scan covers 10 pulses. To minimize the background PL we integrate

from 386 to 393 nm. . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

4.8 Demonstration of translational micromanipulation of a single lasing

tetrapod. Three positions of the tetrapod are shown for voltages of

-7 V, -9 V, and -11 V applied across the DC bar electrodes. . . . . . 64

4.9 Use of micromanipulation to determine the charge on a single trapped

tetrapod. (a) Images showing the position of the trapped tetrapod

for different voltages on the DC electrodes. (b) Plot of the tetrapod

distance from the trap center as a function of the voltage on the DC

electrodes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

4.10 The schematic of the inverted microscope setup. The inset shows the

image (under white light illumination) of a 12±.25 µm bead used for

calibration of the system. . . . . . . . . . . . . . . . . . . . . . . . . 71

LIST OF FIGURES xv

4.11 The magnified images (50 X objective) of a tetrapod on a glass sub-

strate acquired with the inverted microscope setup under a) white

light illumination, and b) lasing conditions. . . . . . . . . . . . . . . 73

4.12 The lasing threshold behavior for the tetrapod of Fig. 4.11. The spec-

tra are averaged for 200 excitation pulses and integrated in the range

between 385-393 nm. These measurements are performed without a

pinhole. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

4.13 (a) The PL spectra of a 25 µm leg length tetrapod acquired with

the inverted microscope. (b) The zoomed in spectra for the mode at

388.7 nm used for the Q factor estimation. . . . . . . . . . . . . . . . 76

4.14 (a) A magnified image (100 X objective) of an optically pumped tetra-

pod leg showing the WGMs along the taper. The inset is a similarly

magnified image of the same leg under white light illumination (b)

Comparison of the WGM behavior observed in (a) with theoretical

simulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

4.15 (a) The schematic of a hexagonal tetrapod waveguide taken from lit-

erature (see text for reference). The white arrows inside the hexagon

represent the propagating WGMs. (b) The SEM image of the end

facet of one our tetrapods showing its hexagonal shape. . . . . . . . . 78

4.16 (a) Image of the center (core) of an optically excited tetrapod. (b)

Image of the WGMs on one of the legs of the tetrapod shown in (a).

(c) Normalized spectra (acquired by selective imaging) of different

regions of the leg in (b). The insets in each spectra show the image

of the corresponding region. . . . . . . . . . . . . . . . . . . . . . . . 80

5.1 A trapped 45 µm diameter glycerol microdrop.(a) CCD image of the

green light scattered from a trapped and coarsely centered microdrop

(b) High magnification (40X) image of the same microdrop after pre-

cise centering. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

5.2 Normalized CW Raman Spectra of a 45 µm pure glycerol microdrop. 87

xvi LIST OF FIGURES

5.3 Raman lasing spectra of electrodynamically trapped glycerol micro-

drops. Black curve -Raman spectrum of glycerol drops between 612-

662 nm under CW (532 nm) excitation. Blue curve - multimode lasing

at 628.4 nm and 630.9 nm, respectively, from a 44.7 µm drop. Red

curve (scaled up 5 times) - Single mode lasing at 629.6 nm from a

10.3 µm drop. The blue and red curves are measured for single pulse

Q-switched (532 nm) excitation of the drop. . . . . . . . . . . . . . . 89

5.4 (a) High magnification (40X) image of a trapped 37 µm pure glycerol

drop under green CW illumination showing the 3 typical glare spots.

(b) The same drop, as in (a), under lasing conditions (pump light

filtered) exhibiting the characteristic red lasing spots. (c) Schematic

to explain the occurrence of the pair of red lasing spots. . . . . . . . . 91

5.5 Raman lasing threshold behavior of a 35 µm pure glycerol drop. The

corresponding spectra above and below threshold are shown by the

insets at the top and bottom, respectively. . . . . . . . . . . . . . . . 93

5.6 Temporal evolution of the Raman lasing intensity for a 44.7 µm pure

glycerol drop at a pump fluence of 490 mJ/cm2. . . . . . . . . . . . 95

5.7 Schematic to explain the concept of “input resonance.” The vertical

green arrows represent the plane wave monochromatic pump light.

The circles, A, B, and C depict three microcavities with similar di-

ameters and the same refractive index. . . . . . . . . . . . . . . . . . 97

5.8 Temporal evolution of the Raman lasing intensity for (a) a 45.3 µm

70 % glycerol drop at a pump fluence of 490 mJ/cm2 (b) a 46.5 µm

40 % glycerol drop at a pump fluence of 490 mJ/cm2. . . . . . . . . 100

5.9 Temporal evolution of the Raman lasing intensity for (a) a 43.8 µm

pure glycerol drop at a pump fluence of 490 mJ/cm2 (b) a 45.6 µm

pure glycerol drop at a pump fluence of 785 mJ/cm2. . . . . . . . . . 102

LIST OF FIGURES xvii

5.10 (a) A near field scanning optical microscopy image of a typical Ag

nanoaggregate taken from literature (see text for reference) (b) SEM

image of a single Ag nanoparticle (c) SEM image of a Ag nanoaggre-

gate (d) SEM image of several Ag nanoaggregates. . . . . . . . . . . 106

5.11 Measured absorption spectra of the Ag nanoparticles (black dotted

curve) and aggregates (red solid line). The green mark at 532 nm

shows the pump wavelength and the red mark around 630 nm shows

the Raman lasing wavelength. . . . . . . . . . . . . . . . . . . . . . . 107

5.12 (a) A high magnification (40 X) image of a 35 µm pure glycerol drop

doped with Ag nanoaggregates under green CW illumination. (b)

Comparison of the CW Raman spectra of the drop in (a) with a

similar drop of pure glycerol. Both the spectra are recorded at the

same pump fluence for an EMCCD exposure time of 30 s. (c) Zoomed

in spectra of Fig. 5.12b around 650 nm. The dotted red circles are

used to highlight the measured WGMs. . . . . . . . . . . . . . . . . . 111

6.1 (a) A pictorial representation of the structure of a CdSe/ZnS quan-

tum dot taken from literature (Photo Courtesy: Evident Technolo-

gies Inc.). (b) Photoluminescence and absorption properties of our

CdSe/ZnS quantum dots. . . . . . . . . . . . . . . . . . . . . . . . . 117

6.2 Observation of Whispering Gallery Modes from the quantum dot

doped microdrops of sizes, (a) 9 µm, and (b) 34 µm. The zoomed-in

spectra of (b) around 625 nm is shown in the corresponding inset. . . 120

6.3 Lasing from the quantum dot doped microdrops of sizes (a) the 9 µm

drop and (b) the 34 µm drop. The QD concentration in drops shown

in (a) and (b) are 1.13 µM and 0.57 µM, respectively. The corre-

sponding pump fluences are 56.25 and 75 mJ/cm2, respectively. . . . 122

xviii LIST OF FIGURES

6.4 (a) Spectral evolution of the 34 µm drop as a function of the increasing

pump fluence. Note that the spectra at 30 mJ/cm2 is multiplied by

100 for better visibility. (b) The lasing threshold behavior of the

34 µm drop. (c) The CCD image of a lasing microdrop of diameter

40 µm. Except for the size, this drop is similar to the 34 µm drop in

every other respect. . . . . . . . . . . . . . . . . . . . . . . . . . . . 124

6.5 The blue shift of the lasing modes with increasing pump fluences. (a)

Blue shift of the overall spectra, (b) Blue shift of individual modes,

(c) Blue shift of the gain region. Note that the center position, in (c),

is determined by a gaussian fit of the gain region. . . . . . . . . . . . 126

7.1 (a) A picture of our linear octupole trap. (b) A CCD image of chalk

dust under green illumination trapped in our linear trap. . . . . . . . 134

List of Tables

5.1 Bond assignment to the different Raman peaks observed in Fig. 5.2. . 88

xx LIST OF TABLES

Chapter 1

Introduction

Realization of efficient miniature light sources, for applications in fields such as opto-

electronics, nanotechnology, optical communication, and optofluidics, is an ongoing

challenge. Optical microcavities serve as ideal microscale resonators and, in combi-

nation with a variety of gain media, have played a pivotal role in the advancement

of such lasers. This thesis looks at the development and characterization of three

fundamentally different microcavity lasers based, respectively, on a ZnO tetrapod,

a glycerol microdrop, and a CdSe/ZnS quantum dot doped microdrop. To elucidate

their intrinsic properties, these lasers are studied under atmospheric condition in an

electrodynamic trap.

1.1 Towards Microscale Lasers

The ingenious idea of combining a gain medium with a resonant feedback cavity

to achieve amplified stimulated emission of light, the scheme behind what we call

a “Laser” today, was first proposed by Schawlow and Townes in the late 1950s [1].

The concept was soon actualized and the first ever laser, based on a ruby crystal,

was developed in 1960 [2]. Later, the same concept was used to develop a variety of

laser sources such as dye lasers, CO2 lasers, Helium-Neon lasers, and semiconductor

lasers [3]. Due to their unique properties like monochromaticity, directionality, and

spatial coherence, lasers brought about a technological revolution in diverse fields

such as optoelectronics, spectroscopy, communication, precision manufacturing, and

medicine. However, as science advanced, and still is, towards smaller dimensions,

2 Introduction

Figure 1.1: Schematics of optical microcavities taken from literature (see text forreferences). (a) A Fabry-Perot type microcavity based on distributed bragg re-flectors (DBRs). (b) Common Whispering Gallery Mode type microcavities, i.e.microsphere, microdisk, and microtorroid. (c) A photonic crystal defect microcav-ity.

the bulky size of the conventional lasers became disadvantageous.

For the realization of efficient miniature (microscale) laser sources, an important

requirement was the resonant recirculation of light in volumes of the order of µm3,

i.e., a microresonator. In the late 1980s, with the advent of “Optical Microcavities”,

this requirement was realized and lasing was demonstrated in compact microscale

systems [4, 5]. Such “Microcavity Lasers” soon gained popularity due to their their

tiny sizes, low lasing thresholds, and narrow output linewidths. To date, miniature

lasers have been developed based on three fundamentally distinct microcavity con-

figurations. These are the Fabry-Perot (FP) type, the Whispering Gallery Mode

(WGM) type, and the Photonic Crystal (PC) type [6].

The FP type cavity, in principle, requires two reflecting surfaces each with a

step-like refractive index variation. For high efficiency, however, distributed Bragg

reflectors are common where the wavelength selective feedback is provided by epi-

taxially grown Bragg mirrors. Such cavities have been extensively employed in the

development of vertical cavity surface emitting lasers (VCSELs) [4, 7]. These cav-

ities, when grown in the form of a pillar, are known as “Micropillar cavities” (see

Fig. 1.1a, taken from literature [6]). The WGM type cavities, on the other hand, are

based on the whispering gallery effect where light is confined by consecutive total

1.1 Towards Microscale Lasers 3

internal reflections along a closed surface. WGM type lasers have been developed

with a variety of structures such as microspheres (microdrops in liquid media) [8, 9],

microdisks [10], and microtorroids [11]. A schematic of these common WGM type

microcavities, taken from literature [6], is shown in Fig. 1.1b. The PC type micro-

cavities [12], however, are not relevant to this thesis and therefore, are not reviewed

here.

The choice of the appropriate gain medium depends on the desired laser charac-

teristics, such as output wavelength, lasing threshold, tunability and physical state

(solid or liquid). Numerous linear/non-linear material systems, in combination with

the microcavities described above, have been used for the development of microscale

lasers. The broad range of gain media, studied in the past, makes a comprehensive

review of the subject beyond the scope of this thesis. However, the following sub-

sections present an overview of a variety of material systems most relevant to this

work.

1.1.1 Semiconductor Nanowires and Tetrapods

Semiconductor nanostructures (structures with one or more dimensions) have re-

cently gained popularity as they have shown promise as potential building blocks

for nanometer scale optical and electronic devices [13]. Apart from efficient lasers [14,

15], semiconductor nanostructures have also been employed in development of pho-

todetectors/optical switches [16], transistors [17], light emitting diodes (LEDs) [18]

and gas sensors [19]. Specially, nanostructures of wide bandgap semiconductors

such as ZnO and GaN have found a variety of applications in UV optoelectronics.

Nanowires are the most elementary form of nanostructures which typically have

cross-sections of 5-500 nm and lengths ranging from hundreds of nanometers to tens

of microns (see Fig. 1.2a). Single nanowires of ZnO and GaN have been used to

develop efficient UV/blue Fabry-Perot type microcavity lasers, where, the two end

facets act as the cavity mirrors [14, 15]. Another important structure is the tetrapod

(see Fig. 1.2b) which comprises of four nanowires joined at a central core in a tetra-

hedral geometry [20, 21, 22]. UV lasing from individual legs of a single ZnO tetrapod

4 Introduction

Figure 1.2: SEM images, taken from literature, of various ZnO nanostructures whichhave shown stimulated emission (see text for references). (a) nanowire, (b) tetrapod,(c) nanocomb, and (d) nanoribbon.

has been reported [23, 24]. Other ZnO nanostructures which have shown stimulated

emission characteristics include nanocombs [25] and nanobelts [26]. Scanning elec-

tron micrograph (SEM) images of these nanostructures, taken from literature [27],

are shown in Fig. 1.2c and d, respectively.

Recently, considerable efforts have also been devoted to the development of inte-

grated optical and electrical circuits based on semiconductor nanostructures. Trans-

port and assembly of nanowires (ZnO, GaN, SnO2, and Si) in water have been

demonstrated with optical traps [28, 29]. Fig. 1.3a, taken from [28], is the SEM

image of a fused junction between a GaN nanowire and a SnO2 nanoribbon. Simi-

larly, Fig. 1.3b shows the dark field image of a three-dimensional assembly of GaN

nanowires and SnO2 nanoribbons (taken from [28]). In addition, optical trapping

of a single potassium niobate (KNbO3) nanowire was also used to realize a tunable


Figure 1.3: Images of nanostructure junctions and assemblies taken from literature(see text for references). (a) SEM image of a fused junction between a GaN nanowireand a SnO2 nanoribbon (Vertical element is the nanowire). (b) Dark field image ofa three-dimensional assembly of GaN nanowires and SnO2 nanoribbons (Horizontalelements are nanoribbons). (c) SEM image of a diode structure based on a singleZnO tetrapod.

nanowire nonlinear probe based on second harmonic generation [30]. Other than

optical trapping, techniques such as focused ion beam deposition have been used

to realize complex electronic circuit elements, such as Schottky photodiodes [31].

Fig. 1.3c shows such a device, based on a ZnO tetrapod, fabricated on a substrate

with tungsten (W) and platinum (Pt) metal electrodes (image from [32]). In this

work, we will demonstrate lasing from a single ZnO tetrapod structure levitated in

an electrodynamic trap.

1.1.2 Microcavity Raman Lasers

Nonlinear optical gain media have also attracted considerable attention in the past.

Raman lasers, especially, are quite advantageous due to their ability to operate at

virtually any wavelength within the spectral transparency window of the gain mate-

rial [33]. The combination of Raman gain and a high Q microcavity can be used to

reduce the otherwise high threshold for stimulated Raman scattering. Efficient low-

threshold solid state microcavity Raman lasers have been developed using materials

such as silica [34] and calcium fluoride [33]. Fig. 1.4a, taken from literature [34],

shows the far infrared lasing of an ultralow threshold silica microsphere Raman laser.

6 Introduction

Figure 1.4: Previous observations of microcavity Raman lasing (see text for refer-ences). (a) Raman lasing from a spherical silica microcavity. The inset shows amicrosphere coupled to a fiber taper. (Pump ∼ 1555 nm, Lasing ∼ 1670 nm.) (b)Raman lasing from a glycerol microdrop pumped at 532 nm on a superhydrophobicsurface. The lasing drop is shown in the inset.

Microscale lasers based on optically active liquid solution as the gain media

are of significant importance in the field of optofluidics [35]. Such lasers based

on the Raman effect are, however, not much advanced. Several investigations of

Raman effects in liquid microdrops have been reported. Structural resonances [36]

and size dependence [37] in the Raman spectra of liquid microdrops were reported

about two decades ago. Around the same time, stimulated Raman scattering from

microdroplets at WGM resonances was also observed [38]. In most of these studies,

glycerol was recognized as a favorable medium due to its high Raman gain and

low vapor pressure (under standard conditions). However, practical applications

of the Raman effects in glycerol microdrops are still to come. The most important

reason being that stable laser operation, with glycerol microdrops, is hard to achieve

due to the temporal output intensity fluctuations (or blinking). Previous reports

have attributed these temporal bursts of Raman lasing to the double resonance

phenomena in the slowly evaporating glycerol microdroplet [39, 40].

Recently, the Raman lasing characteristics of glycerol microdrops on a superhy-

drophobic surface were reported [41]. Fig. 1.4b, taken from this source, shows the

Raman lasing spectra of a glycerol microdrop when pumped at 532 nm. A picture

of the lasing drop is shown in the inset. Interestingly, this paper proposed that the


blinking is caused by the spatial phase distortions in the circulating mode due to

thermally induced random density fluctuations in the lasing microdrop. In this the-

sis, lasing behavior and blinking properties of electrodynamically trapped glycerol

microdrops will be presented.

1.1.3 Quantum Dot Microcavity Lasers

Colloidal quantum dots, due to their small size, precise bandgap tunability, and

easily manipulable surface properties, have recently gained popularity as gain media

in miniature laser sources. Early studies and experiments on quantum dots began

in the late 1980s with the interest of examining the change in physical and chemical

properties of semiconductors as a function of reducing size and dimensionality. Based

on the quantum confinement effect, these studies resulted in the understanding that

the particle size and the surface chemistry are critically responsible for the properties

of nanometer sized semiconductor particles [42, 43]. Since then, efficient lasers, and

even single photon sources, based on epitaxially grown, self-assembled quantum dots

have been successfully developed [44, 45]. Fig. 1.5a shows the schematic of a single

photon source based on a self-assembled quantum dot and a DBR microcavity (taken

from [6]).

Figure 1.5: (a) Schematic of a single photon source, taken from literature, based ona self-assembled quantum dot and a DBR microcavity. (b) A UV illuminated 20 µmdiameter CdS/ZnS nanocrystal-microsphere composite used to achieve blue lasingaround 470 nm. (c) An optical micrograph of a tapered fiber-coupled toroidal mi-crocavity laser showing CdSe/ZnS nanocrystal emission from the whispering-gallerymodes. (see text for figure references)

8 Introduction

The development of lasers based on chemically synthesized colloidal quantum

dots (or nanocrystals), compared to their self-assembled counter parts, has been

rather slow. The main reason has been that the colloidal nanocrystals developed in

the early 1990s were extremely sensitive to photo-oxidization and readily damaged

by reaction with the surroundings. Around mid 1990s, however, more stable, inor-

ganically capped core/shell type quantum dots came into existence [46]. In spite of

several efforts, lasing was still not achievable even with the core/shell type struc-

tures. In the next few years, it was found that the non radiative losses owing to

Auger recombinations, which were fairly high due to the nanometer sizes of the

colloidal quantum dots, were prohibiting stimulated emission [47]. Closely packed

films (very high density of colloidal quantum dots) were soon realized as a solution

to this problem and stimulated emission was observed with CdSe nanocrystals [48].

Since then, several microcavity lasers based on colloidal quantum dots have

been developed. Early this decade, lasing was observed with the combination of

CdSe quantum dots, in both solid state [49] and solutions [50], and a cylindri-

cal microcavity. A distributed feedback laser based on CdSe/ZnS nanocrystal-

titania composite was also demonstrated around the same time [51]. Recently, mi-

crosphere [52, 53] and microtorroidal [54] type core/shell nanocrystal lasers have also

been reported. Fig. 1.5b (taken from [53]) shows a UV illuminated 20 µm diame-

ter CdS/ZnS nanocrystal-microsphere composite used to achieve blue lasing around

470 nm. Fig. 1.5c (taken from [54]) is an optical micrograph of a tapered fiber-

coupled toroidal microcavity (principal diameter∼60.6 µm) laser showing CdSe/ZnS

nanocrystal emission from the whispering-gallery modes. In this work, lasing from

an electrodynamically trapped microdrop doped with CdSe/ZnS quantum dots will

be demonstrated.

1.2 Motivation and Goal

In spite of the extensive research in the field of microscale lasers, there are still a

number of issues which have not been addressed in the past. First of all, most of the

existing studies on ZnO or other semiconductor nanostructures have been carried

1.2 Motivation and Goal 9

out on substrates. Although optical trapping of nanostructures has been successful,

it has only been achieved in solutions yet. The powerful tool of electrodynamic trap-

ping has never been used to trap, study, or to investigate optical properties of ZnO

nanostructures or other nanomaterials. Electrodynamic trapping is advantageous

because it can enable studies at atmospheric conditions. Also, as a particle trapped

in air is completely isolated from external influences (like a substrate), its intrinsic

properties can be investigated. Moreover, such a particle can possibly be precisely

micromanipulated by probing with external electric/optical fields.

In the field of microscale Raman lasers, liquid state microcavities are far less

evolved than their solid counterparts. The reason behind the on/off lasing behavior

in such lasers remains uncertain. Different investigation techniques, i.e., optical

levitation and superhydrophobic surface, have resulted in contrary explanations for

the on/off behavior of glycerol microdrops. Investigation of the on/off behavior

effects in an electrodynamic trap have not been reported to date. The results of

such a study, when compared with the two existing explanations, can conclusively

resolve this ambiguity.

Liquid-state colloidal quantum dot lasers are still in their adolescent phase with

only one such previous demonstration in a cylindrical microcavity [50]. Other cavity

geometries remain unexplored till date. Spherical microdrops, which are known

for their narrow linewidths and very high quality factors, can be advantageous for

this purpose as the quantum dots can be located inside them, hence increasing

the efficiency compared to the evanescent gain type geometry [53]. Therefore, the

quantum dot microdrop laser can be a significant improvement and possibly be a

step towards the development of practically applicable liquid-state quantum dot

lasers.

The goal of this thesis is to develop and study, with the help of electrodynamic

trapping, three novel miniature laser sources. These are the ZnO tetrapod laser,

the liquid-state glycerol microcavity Raman laser, and the CdSe/ZnS quantum dot

microdrop laser. These lasers, in combination, practically cover the whole wave-

length spectrum from UV to VIS/Near IR. The UV range can be accessed by the

10 Introduction

ZnO tetrapod laser which operates at∼390 nm. The output of the Raman laser,

when pumped at 532 nm, is in the red∼630 nm. This wavelength, in principle,

can be tuned over the whole transparency range of glycerol by varying the pump

wavelength. The quantum dot laser, in addition, can be conveniently chosen to op-

erate at any desired wavelength in the VIS/Near IR by selecting quantum dots of

appropriate diameter.

1.3 Outline of the Thesis

This thesis, over a span of seven chapters, is presented in four stages i.e., theoreti-

cal background, experimental details, results, and conclusion. The basic theoretical

knowledge behind the concepts relevant to this thesis are discussed in Chapter 2.

The chapter starts with the fundamentals of quadrupole ion trapping. A brief math-

ematical analysis, starting from a quadrupole potential, is carried out to derive the

secular and micro motion trajectories of a particle in a Paul trap. The mechanism

of the Fabry-Perot type lasing is ZnO nanowires in then discussed. Subsequently,

the Mie theory of WGMs is addressed where the solution to the Helmholtz equation

(in spherical polar coordinates) is concisely presented to obtain the characteristic

equations for the resonances. A short description of the Ray model of WGMs also

follows. After presenting the fundamental concepts of Raman scattering, the chapter

concludes with a discussion of the Quantum Confinement Effect in semiconductor

quantum dots.

Chapter 3 is devoted to the details regarding the experimental setup. The chap-

ter starts with an overview of the complete setup which is followed by the specifica-

tions of the various pump lasers used in our experiments. Subsequently, the theory

of electrospray ionization is discussed briefly and its is shown. Later, the design

of the endcap trap and the high magnification optics are illustrated. In the end,

the features of our spectral data acquisition setup, along with its calibration, are

presented.

Chapter 4 focuses on the development of the electrodynamically confined ZnO

tetrapod laser. Beginning with the preparation and structural properties of our sam-

1.3 Outline of the Thesis 11

ples, the chapter proceeds by presenting our technique to trap a single ZnO tetrapod.

UV lasing in single and multiple trapped tetrapods is subsequently demonstrated.

In addition, translational micromanipulation of a trapped tetrapod is shown and

its charge value is estimated. Towards the end, the chapter discusses our investiga-

tions with the tetrapods on a glass substrate using an inverted microscope. Single

tetrapod lasing in the trap is compared with that on the substrate. A lower-limit

estimate of the quality factor of the lasing modes is given. Finally, WGM type

modes on the tapered tetrapod leg are observed and their existence is supported by

theoretical calculations.

Chapter 5 presents our investigations of Raman lasing in electrodynamically

trapped glycerol microdrops. It commences with our technique to trap a single glyc-

erol microdrop. Following this, the CW Raman spectroscopy of a glycerol microdrop

is shown. Subsequently, drop size dependent single and multi mode Raman lasing

in observed. The on/off behavior of the Raman lasing is investigated by changing

the concentration of glycerol in the microdrops and the pump fluence. Ultimately,

a brief theory of surface enhanced Raman scattering is provided and the observed

effects of doping the glycerol microdrop with metal nanoaggregates are discussed.

The electrodynamically trapped quantum dot microdrop laser is explained in

Chapter 6. The chapter begins with the properties of our CdSe/ZnS core/shell

quantum dots. Coupling of the quantum dot emission to the WGMs, under CW

excitation, is later shown in microdrops of different sizes. Subsequently, single and

multimode lasing from the doped microdrops along with the threshold behavior

are demonstrated. In addition, the spectral blue shifts in the lasing microdrop are

investigated as a function of the pump fluence. The chapter ends with a comparison

of the quantum dot concentration in our lasing microdrops with that predicted by

theory. Chapter 7 presents the conclusion of this work along with the scope for

future research.

12 Introduction

Chapter 2

Theoretical Background

This chapter is dedicated to the physics behind the theoretical concepts relevant

to this work. A detailed consideration of each theoretical aspect is, however, be-

yond the scope of this thesis. Therefore, only the most important fundamentals,

required to understand the experimental results, are presented here. Appropriate

references are provided for readers seeking extended theoretical background. The

chapter starts with a basic overview of quadrupole ion trapping in section 2.1. Sub-

sequently, the lasing mechanism in ZnO tetrapods is briefly discussed in section 2.2.

Following this, the theory of Whispering Gallery Modes (WGMs) in a spherical

microcavity is addressed in section 2.3 with the help of both Mie and ray models.

The chapter proceeds with a concise overview of Raman scattering in section 2.4.

Finally, the chapter concludes with an introduction to colloidal quantum dots and

their properties in section 2.5.

2.1 Basics of Quadrupole Ion Trapping

Electric field based spatial confinement is possible in a charge-free region in the pres-

ence of a three dimensional potential minimum. In such a region, a charged particle

experiences a restoring force from all the surrounding positions and hence, is stably

trapped. According to Earnshaw’s theorem [55], a static potential (φ) cannot have

a potential minimum and a zero Laplacian (∇2(φ) = 0) simultaneously. Hence, in

the electrostatic regime, the trapping of a charged particle is prohibited. The con-

dition for stable trapping, however, can be satisfied by electrodynamic potentials

14 Theoretical Background

(oscillating in time). If a saddle like potential (in the X-Y plane) oscillates with an

angular frequency of Ω = 2πf , as shown in Fig. 2.1 a and b, the overall effect is a

rotation (at a frequency of Ω) about the Z-axis passing through its center. Hence,

this resulting potential, known as the pseudopotential, has a bowl like shape and

can confine particles in the x-y plane. Similarly, a three dimensional pseudopoten-

tial leads to a total spatial confinement of the particle and hence, is the basis for

electrodynamic trapping. This outstanding idea was proposed and demonstrated by

Wolfgang Paul [56] for which he was awarded the 1989 Nobel prize in physics.

The most common form of complete electrodynamic confinement is the “Three

Dimensional Quadrupole Ion Trapping” where the pseudopotential has a quadratic

variation in X, Y, and Z coordinates. The conventional “Paul Trap” [56] is a widely

known trap geometry for this purpose. It consists of three electrodes (a ring and

two endcaps) with hyperbolic curvatures arranged and electrically connected in a

fashion as shown in Fig. 2.2a. The ring has a radius of r0 and the endcaps are

separated by a distance 2z0, where r20 = 2z2

0 . Fig. 2.2b, taken from literature [57],

shows the real electrodes of a conventional Paul trap. A brief theory for basic

understanding of quadrupole ion trapping will follow. More details can be found in

literature [56, 57, 58].

Figure 2.1: Simulated images of the oscillating trapping potential at times (a) t = 0,and (b) t = 1

2f. The angular frequency of the oscillation is 2πf . The blue circle

represents the trapped particle.

According to the voltage scheme shown in Fig. 2.2a, the electric potential inside

2.1 Basics of Quadrupole Ion Trapping 15

Figure 2.2: (a) A schematic of the conventional quadrupole Paul trap. (b) Animage of the three electrodes of a real Paul trap taken from literature (see text forreference).

a Paul trap is given by

φ(x, y, z) =Vtr

2r20

(Cxx2 + Cyy

2 + Czz2), (2.1)

where Cx, Cy, and Cz are dimensionless constants. From the Laplace’s equation, we

get

∇2[φ(x, y, z)] =Vtr

2r20

(2Cx + 2Cy + 2Cz) = 0,

=⇒ (Cx + Cy + Cz) = 0. (2.2)

For the Paul trap, Cx = Cy = 1 which gives Cz = −2. Hence, Eq. (2.1) can be

rewritten as

φ(x, y, z) =Vtr

2r20

(x2 + y2 − 2z2). (2.3)

To change into cylindrical coordinates, we substitute x = r cos θ and y = r sin θ in

Eq. (2.3) to get

φ(r, θ, z) =Vtr

2r20

(r2cos2θ + r2sin2θ − 2z2),

=⇒ φ(r, z) =Vtr

2r20

(r2 − 2z2). (2.4)

This is the simplified expression for the pseudopotential in cylindrical coordinates.

Now, let us first consider the radial direction. The force acting on a trapped particle

of charge e and mass m along the radial direction can be expressed as

e(−dφdr

) =−erVtr

r20

= md2r

dt2. (2.5)


The external AC voltage Vtr is of the form

Vtr(t) = Vdc + V0cos(2πft), (2.6)

where Vdc is the DC offset, V0 is the amplitude of the trapping voltage, f is the

trapping frequency, and t is the time. Using Eq. (2.5) and (2.6), and by defining a

dimensionless variable ζ = πft, we get

d2r

dζ2=

−er[Vdc + V0cos(2ζ)]

π2f 2mr20

, (2.7)

which can be rearranged to get

d2r

dζ2+

eVdc

π2f 2mr20

r +eV0 cos(2ζ)

π2f 2mr20

r = 0. (2.8)

By substituting

ar =eVdc

π2f 2mr20

=4eVdc

mΩ2r20

, and qr =−eV0

2π2f 2mr20

=−2eV0

mΩ2r20

, (2.9)

in Eq. (2.8) we getd2r

dζ2+ arr − 2qrcos(2ζ)r = 0. (2.10)

A similar analysis for the Z coordinate can be carried out to get

d2z

dζ2+ azz − 2qzcos(2ζ)z = 0, (2.11)

where

az = −2ar =−8eVdc

mΩ2r20

, and qz = −2qr =4eV0

mΩ2r20

. (2.12)

Eq. (2.10) and (2.11) are the equations of motion of the trapped particle in

the radial and axial directions, respectively. Both equations are of the same form

and resemble the well known Mathieu equation. The solutions to these equations

represent the motional trajectory in their respective coordinates. A particle is stably

trapped only when the solution to both these equations are simultaneously stable.

It is obvious that the nature of the solutions depends on the parameters ai and

qi, where i = r, z. Therefore, the values of these parameters, given by Eq. (2.9)

and (2.12), play an important role in determining the properties of the trap.

2.1 Basics of Quadrupole Ion Trapping 17

The motion of a trapped particle, in the r and z directions, are independent of

each other. Let us define a dimensionless constant βi =

√

ai +q2

i

2where i = r, z.

The solutions to the equations of motion are stable only when βi is a purely real

non-integer number [58]. In this case, the trajectory of the particle in a coordinate

i is given by

i(ζ) = An=+∞∑

n=−∞K2ncos[(2n± βi)ζ] +B

n=+∞∑

n=−∞K2nsin[(2n± βi)ζ], (2.13)

where i = r, z, the integer n = −∞ to +∞, A is a purely real constant, B is a

purely imaginary constant, and K2ns are the amplitudes of oscillations. The cosine

and sine terms show that the motion of the particle is periodic. To calculate the

periodicity of the motion, let us define

(2n± βi)ζ = Ωseci,n · t, (2.14)

where t is the time and Ωseci,n , also known as the secular frequency, is the angular

frequency of the particle motion. Since ζ = πft = Ωt2

, the general expression for the

secular frequency is given by

Ωseci,n =

(2n± βi)Ω

2, (2.15)

and, the fundamental frequency of motion of the trapped particle, Ωseci,0 , is given by

Ωseci,0 =

βiΩ

2=

Ω

2

√

ai +q2i

2. (2.16)

The trajectory (r(t) or z(t)) of a trapped particle, given by Eq. (2.10) and (2.11),

comprises of two components. First is the slow “secular motion” (rs(t) or zs(t))

which determines the mean position of the particle at a time t. The frequency

of the secular motion is given by Eq. (2.15). The other component is the rapid

oscillations of tiny amplitudes around a given mean position. This is called the

“micromotion” (rm(t) or zm(t)). Therefore, the coordinates of the particle can be

expressed as

r(t) = rs(t) + rm(t), and z(t) = zs(t) + zm(t). (2.17)


The assumption rs(t) >> rm(t), drs(t)dt

<< drm(t)dt

, and ar << qr are valid in most

practical cases. Based on these assumptions, at a given mean position rs(t), the

micromotion is obtained by double integration of Eq. (2.5) to be,

rm(t) =−qrrs(t)cosΩt

2. (2.18)

Similar analysis for the z coordinate gives,

zm(t) =−qzzs(t)cosΩt

2. (2.19)

This shows that the periodicity of the micromotion oscillations is the same as the

trap driving frequency of Ω. Substituting the micromotion in Eq. (2.17) gives the

overall motion of the particle in r and z directions.

Fig. 2.3a, taken from literature [56], can be used for a better understanding of

the particle motion. It shows a microphotography image of both the secular and the

micro motion of a trapped aluminum particle. Fig. 2.3b shows the similar motion

of a chalk particle in our endcap trap. This CCD image is acquired by scattering a

red laser off the particle. The secular motion is clearly visible, however, the pattern

is a bit distorted compared to Fig. 2.3a as our trap is not an ideal Paul trap. The

micromotion cannot be seen most likely due to the low magnification and the low

frame-rate of the imaging system.

Figure 2.3: Microphotograph of a trapped aluminum particle taken from literature(see text for reference). The secular and the micro motions of the particle can beseen in the image. (b) A similar pattern observed for a chalk particle in our endcaptrap. The image is acquired by scattering a red laser off the particle.

2.2 Lasing Mechanism in ZnO Tetrapods 19

2.2 Lasing Mechanism in ZnO Tetrapods

Zinc Oxide (ZnO) is a II-VI semiconductor with a direct bandgap of 3.4 eV and a

hexagonal wurtzite type crystal structure. The wide bandgap makes it a favorable

material for blue/UV optoelectronics applications, for example, for fabrication of

LEDs and diode lasers. The binding energy for the excitons in ZnO is about 60 meV.

At room temperatures, this value is considerably larger than the thermal energy

∼ KT (k=Boltzmann’s constant, T= Temperature) which is of the order of 25 meV.

Hence, the room temperature photoluminescence (PL) in ZnO is largely excitonic

in nature. A detailed account on the optical and electronic properties of ZnO along

with its technological applications can be found in literature [59].

The exciton Bohr radius of ZnO is about 1.8 nm. This value is too small com-

pared to the size of ZnO based nanoparticles, such as nanowires or tetrapods. And

hence, quantum confinement effects are absent in such nanoparticles. Recall that

tetrapods consist of four nanowire legs joined together in a tetrahedral geometry to

a central core. Typically, the individual legs are 50-500 nm in diameter and 1-30 µm

in length. As explained below, such a geometry enables them to act as efficient

microscale resonators.

An optical resonator must have two essential characteristics, i.e., waveguiding

and resonant feedback. The symmetric and smooth sidewalls allow for the UV PL

to be guided along the length of the tetrapod legs (nanowires). According to the

classical optical waveguide theory, the guiding properties of the hexagonal (cross-

section) legs can be approximated to those of a cylindrical waveguide. For a leg

(in air) of radius r and refractive index n, the approximate fractional mode power

guided inside it is given by [60]

η = 1 −[

(

2.405

e1

V

)2

V −3

]

, (2.20)

where the normalized frequency V = 2πr√

n2−1λ

, n=2.2, and λ=390 nm. The above

equation tells us that for diameters greater than 200 nm, the leg has more than 90%

guiding efficiency for the lowest order mode. Therefore, for such diameter sizes, most

of the PL (excluding surface contribution) is coupled to the bounded axial modes


of the leg while very little is lost as radiation. Hence, the condition of efficient

waveguiding is satisfied.

The other important aspect is the resonant feedback. The refractive index of

ZnO (2.2) is fairly high compared to that of air. At the end facets, this index

contrast corresponds to a reflectivity of about 14% and hence, leads to the reflection

of the guided modes. The facets, therefore, behave like two mirrors of the Fabry-

Perot cavity. Such a cavity has equally spaced resonances with a free spectral range

∆ν = c2nL

, where L is the leg length. Therefore, lasing can occur, at these resonances,

as the guided PL is fed back into the leg at the end facets. If the FWHM of the PL

is represented by ∆νPL, the number modes expected in the laser emission are given

by ∆νPL/∆ν. Moreover, due to such a configuration, the laser emission (at the end

facets) is directed preferentially along the length of the leg. Fig. 2.4 can be used for

better understanding of the lasing mechanism.

Figure 2.4: A schematic depicting the lasing mechanism in a leg of a ZnO tetrapod.

2.3 Theory of Whispering Gallery Modes in Spher-

ical Microcavities

Optical microcavities are structures known for their ability of confining light in small

dimensions by resonant feedback. In the case of spherical microcavities, most of the

light is trapped by recirculation along the circumference of the sphere. Depending

2.3 Theory of Whispering Gallery Modes in Spherical Microcavities 21

on the size and refractive index of the sphere, the confinement occurs for a range of

optical modes. Each mode can be characterized by its frequency and the spatial field

distribution. These are known as the Whispering Gallery Modes (WGMs) and are

named after Lord Rayleigh’s observations at St. Paul’s Cathedral in London. The

term Morphology Dependent Resonances (MDRs) is also used commonly to address

them. The existence of WGMs, for a spherical microcavity, can be explained by

rigorous solution of the Maxwell’s equation according to the Mie Theory. They can

also be understood with the help of a rather simplistic model based on geometrical

optics. The following two sections will address each of these approaches individually.

2.3.1 The Mie Theory

A detailed derivation of WGMs from first principles using Mie theory, available in

literature [61, 62, 63, 64], is mathematically exhaustive and is beyond the scope of

this thesis. Therefore, only the salient features of the approach will be discussed here

for basic understanding of the Mie theory. Note that the intermediate mathematical

calculations are not shown for the sake of conciseness. Consider a non-conducting,

charge free spherical particle with a radius of a and refractive index n. The origin

of the coordinate system is located at the center of the particle. A plane polarized

z-travelling wave is incident on the particle. The electric fields associated with the

incident and the scattered wave (outgoing) are denoted by Ew and Es, respectively.

The electric field inside the particle is given by Epar. The corresponding magnetic

fields will be represented by the same subscripts on the letter H. A schematic of

this situation is depicted in Fig. 2.5a. The incident electric field Ew is x-polarized

and can therefore be expressed as

Ew = E0eikze−iωtx, (2.21)

where E0 is the amplitude, k is the wavenumber, ω is the frequency, i =√−1, t is

the time, and x is the unit vector for the x-coordinate. Note that the bold fonts

represent vector quantities in this analysis. In such cases with harmonic time de-

pendent fields (e−iwt), the vector wave equation can be represented in the Helmholtz

form. The WGMs and their spatial distribution are represented, respectively, by the


eigenvalues and the eigenfunctions of the Helmholtz equation. The mathematical

Figure 2.5: (a) A schematic of a linearly polarized z-travelling wave incident on ascattering microparticle. (b) The geometrical optics based explanation of whisperinggallery modes.

analysis is carried out along the following main steps. First, the scalar solutions of

the Helmholtz equation in spherical polar coordinates are obtained. These scalar

functions are then used to reconstruct the vector wavefunctions which also satisfy the

Helmholtz equation. Subsequently, expressions for Ew, Es, and Epar are obtained

as a superposition of the vector wavefunctions. Finally, the singularities in Epar and

Es, which represent the WGMs, are found by applying the boundary conditions.

To start, the Helmholtz equation for a wavefunction ψ in spherical polar coordi-

nates (r, θ, φ) is written as

1

r

∂

∂r

(

r2∂ψ

∂r

)

+1

r2 sin θ

∂

∂θ

(

sin θ∂ψ

∂θ

)

+1

r2 sin θ

∂2ψ

∂φ2+ k2ψ = 0. (2.22)

The solution to the above equation can be obtained by separation of variables.

If a substitution of the form ψ(r, θ, φ) = R(r)Θ(θ)Φ(φ) is used, three differential

equations exclusively in r, θ, and φ, respectively, are obtained. Note that R, Θ, and

Φ are independent of each other and are functions of their respective coordinates.

Each of these equations is much simpler than the Eq. (2.22) and can be solved by

conventional methods for differential equation.


The Azimuthal variable Φ(φ) has two linearly independent solutions, Φo (odd)

and Φe (even), which are given by

Φo = sinmφ and Φe = cosmφ. (2.23)

The solutions for Θ(θ) can be expressed in terms of the associated Legendre poly-

nomials in the following form

Θ = Pml (cos θ), (2.24)

where l and m are integers which depend on the boundary conditions. Moreover,

solutions for the radial dimension can be expressed in terms of the Bessel functions

(Z) as

R(ρ) =

√

π

2ρZl+ 1

2

(ρ) = zl(ρ), (2.25)

where ρ = rk and zl(ρ) is the spherical Bessel function. Depending on the problem,

zl is replaced by one of the three types, i.e., the spherical Bessel (first kind or jl),

Neumann (nl), or Hankel functions (hl). A good description of the Legendre and

the various types of Bessel functions can be found in [65].

Now, the pair of scalar solutions to the Eq. (2.22) can therefore be written as

ψoml = zl(ρ)Pml (cos θ) sinmφ, and ψeml = zl(ρ)P

ml (cos θ) cosmφ, (2.26)

where the subscripts o and e still refer to the odd and even solutions of Φ, respec-

tively. Now, for such a scalar function ψ (solution of the Helmholtz equation), vector

solutions TE and TM (which also satisfy the Helmholtz equation) can be created

as [64]

TE = ∇× (rψ), and TM =1

ΛM

∇× TE, (2.27)

where r is an arbitrary radial vector and ΛM is a constant. From Eq. (2.26) and

(2.27), the vector solutions corresponding to ψoml and ψeml can be calculated as

TEoml = ∇× (rψoml), TE

eml = ∇× (rψeml) (2.28)

and

TMoml =

1

ΛM

∇× TEoml, TM

eml =1

ΛM

∇× TEeml. (2.29)


The vector functions TE and TM represent (are proportional to) the electric fields in

the transverse electric and transverse magnetic modes, respectively. Therefore, the

incident electric field Ew can be expanded in terms of TEoml, TE

eml, TMoml, and TM

eml.

Using the orthogonal properties of the TE and TM vector functions, the expression

for Ew and Hw can be simplified and are given by

Ew =∞

∑

l=1

El[(TEo1l)j − i(TM

e1l)j], (2.30)

Hw =−kωµ0

∞∑

l=1

El[(TEe1l)j + i(TM

o1l)j], (2.31)

where El = ilE02l+1l(l+1)

, µ0 is the permeability of free space, and the subscript j

represents that jl(ρ) is used as the Bessel function in Eq. (2.26). jl(ρ) is used

because, unlike the other Bessel functions, it does not have a singularity at the

origin.

The field distribution inside the particle should also be finite at the origin. There-

fore, Epar and Hpar are also expanded by using jl(ρ) (here ρ = nkr) to get

Epar =∞

∑

l=1

El[cl(TEo1l)j − idl(T

Me1l)j], (2.32)

Hpar =−nkωµ

∞∑

l=1

El[dl(TEe1l)j + icl(T

Mo1l)j], (2.33)

where µ is the permeability of the particle and cl and dl are expansion coefficients.

Recall that the origin of the coordinate system is located at the center of the particle.

Therefore, the outgoing scattered field (Es) need not be non-divergent at the origin

as it is shielded by the particle. However, Es must have well behaved asymptotic

properties. The spherical Hankel functions hl(ρ), in their asymptotic form, are very

close to the representation of a spherical wave. h1l (ρ) and h2

l (ρ) represent outgoing

and incoming spherical waves, respectively. Therefore, for the expansion of the

outgoing scattered field, h1l (ρ) is used in the Eq. (2.26), rather than jl(ρ), to obtain

Es =∞

∑

l=1

El[ial(TMe1l)h − bl(T

Eo1l)h], (2.34)

Hs =−kωµ0

∞∑

l=1

El[ibl(TMo1l)h + al(T

Ee1l)h], (2.35)


where al and bl are expansion coefficients and the subscript h reflects the use of

h1l (ρ). Now, the boundary condition of such a scattering problem indicates that the

sum of the incident and the scattered fields must equal the field inside the particle

at all the points on the boundary (r = a). The same should also hold true for the

corresponding magnetic fields. For a known l, these boundary conditions can be

used to calculate the values of al, bl, cl, and dl as

al =nγl(Y )γl(X) − γl(X)γl(Y )

nγl(Y )ζl(X) − ζl(X)γl(Y ), bl =

γl(Y )γl(X) − nγl(X)γl(Y )

γl(Y )ζl(X) − nζl(X)γl(Y ), (2.36)

cl =−mi

γl(Y )ζl(X) − nζl(X)γl(Y ), dl =

−ninγl(Y )ζl(X) − ζl(X)γl(Y )

, (2.37)

where γl(X) = Xjl(X), ζl(X) = Xh1l (X) (similar for Y ), the dots represent the

derivatives, X = ka, and Y = nX. Now, if cl is resonant, i.e., considerably large

(in theory, infinity), Eq. (2.32) indicates that the electric field inside the particle

corresponding to the TE mode will be dominant. Similar situation occurs for TM

modes if dl is at resonance. This enhancement of the local fields inside the particle

represent the WGM type resonances. Similarly, the TM and the TE modes of

the scattered field are dominant at the resonances of al and bl, respectively. In the

expression for the expansion coefficients, al and dl have the same denominators. The

condition of resonance is obtained when the denominator goes to zero. Therefore, the

TM polarized field inside the particle and the TM scattered field are simultaneously

resonant. Same is true in the TE case as bl and cl also have the same denominators.

Mathematically, the conditions for a TM and TE resonance are given, respectively,

by

nγl(Y )ζl(X) = ζl(X)γl(Y ), and γl(Y )ζl(X) = nζl(X)γl(Y ). (2.38)

The expressions represented in Eq. (2.38) are known as the characteristic equations

of WGM resonances. The characteristic equations do not involve the incident field

and hence, indicate that the WGM resonances are solely a function of the particle

and its surroundings. The dimensionless variable X, also expressed as 2πa/λ, is

known as the size parameter. In general, X is the most significant parameter along

with n in determination of the WGMs of a given spherical particle.


The spatial field distribution of a TE or TM WGM can be characterized with

the help of three integers, namely l, m, and s. l is known as the mode number and is

defined as half the number of field maxima covered in one equatorial roundtrip along

the circumference of the particle. m is known as the Azimuthal mode number and

describes the field variation with respect to φ. However, in the case of a spherical

particle, m becomes expendable due to the spherical symmetry of the system. Now,

Eq. (2.38) can be satisfied for multiple values of X. For these different X values,

the particle size (or the wavelength) is different and hence, the effective radial field

distribution is different. This effect is characterized by the integer s, known as the

order number, which is defined as the number of radial field maxima present inside

the boundary of the particle. Hence, a WGM in a spherical particle can be uniquely

and completely defined by specifying the l, and s integers. For mathematical nota-

tion of WGMs, the mode and order numbers are used as subscripts to the expansion

coefficient. For example, a TE mode (cl,s) of mode number 10 and order number

5 is denoted as c10,5. As mentioned before, the scattered fields are also enhanced

simultaneously with the field inside the particle. Therefore, the scattered TE (bl)

and TM (al) modes can also be characterized by the mode and order numbers of

their respective cl,s and dl,s. For example, the scattered mode (TE) corresponding

to c10,5 is be expressed as b10,5.

For a given l, the linewidths of the resonances of different orders become broader

with increasing order numbers. The Quality factor (in short referred to as Q factor or

Q) is the measure of the linewidth of a resonance and is defined as Q = Xc

∆X= λc

∆λ=

νc

∆ν. Here, Xc and ∆X refer to the center value and the FWHM linewidth of the

resonance, respectively. The same notation is used for the wavelength (λ) and the

frequency (ν). Q-factors of the order of 105 to 106 and above are usually considered

good. Such Q values produce ultranarrow resonance linewidths and therefore, enable

the particle to serve as an ideal optical resonator.


2.3.2 The Ray Model

The concept of WGMs in a spherical particle can also be qualitatively explained

with the help of geometrical optics. Consider the picture of the circular particle

(radius a, refractive index n) shown in Fig. 2.5b. The critical angle of total internal

reflection (TIR) θc, at the particle boundary, is given by sin−1(1/n). Now consider a

ray, travelling inside the particle, incident at a point A on the boundary at the same

angle θinc (Fig. 2.5b). If θinc > θc, the ray will undergo TIR and will not escape the

particle. The reflected ray, due to the symmetry of the system, will repeatedly hit

the boundary at an angle θinc and will in each case experience TIR. Let us imagine

the situation that the ray travels one roundtrip and returns to the point A after NR

number of reflections. If the final ray at A is in phase with the initial ray, a standing

wave resonance is generated which is nothing but a WGM.

For large values of NR, the ray practically approaches point A at grazing inci-

dence (tangentially) and hence, the overall path in one roundtrip is close to the cir-

cumference of the particle. Therefore, the phase difference generated in a roundtrip

(k · 2πa · n) must be an integral multiple of 2π. Hence, the condition for resonance

is given by,

2π

λ· 2πa · n = l · 2π,

=⇒ n · 2πa = l · λ, (2.39)

where l is defined as the mode number and the other parameters are as defined

before. Hence, a WGM is generated whenever the optical path length of particle

circumference is an integral multiple of the wavelength of the light (λ). Note that

the above analysis assumes that most of the circulating light field is located near

the particle boundary. However, this is only true is the case of 1st order WGMs.

Therefore, the ray picture and the Eq. (2.39) are not valid for understanding of

higher order modes.


2.4 Fundamentals of Raman Scattering

To understand the mechanism of Raman scattering, let us consider that an electro-

magnetic wave of frequency ν0 is incident on a material with polarizability α. If the

amplitude of the local optical field is E0, the oscillating dipole moment (M) induced

in a molecule can be expressed as,

M = αE0cos(2πν0t), (2.40)

where t is the time. In the above equation, however, α is not a constant and

depends on the instantaneous position of the atoms in the molecule. If the atomic

motion is periodic with a frequency of νi, then α = α0 + αicos(2πνit). Here, α0 is a

constant and αi is the maximum change in polarizability due to the atomic motion.

Substituting this in Eq. (2.40) gives,

M = [α0 + αicos(2πνit)]E0cos(2πν0t)

= α0E0cos(2πν0t) + αiE0[cos2π(ν0 + νi)t + cos2π(ν0 − νi)t]. (2.41)

Now, recall that the induced oscillating dipole acts as a source of radiation. The

scattered light is nothing but this emitted radiation. As seen from Eq. (2.41), the

induced dipole moment is a superposition of three periodic functions with frequencies

ν0, ν0 + νi, and ν0 − νi, respectively. Therefore, the scattered radiation also consists

of these three frequencies. The scattering at the frequency ν0, same as the incident

wave, is the elastic process such as Rayleigh or Mie scattering. However, the other

two frequencies are shifted from ν0 and constitute the Raman scattering.

The physical process behind the generation of these three frequencies can be

explained as follows. Consider a molecule in the ground state and for simplicity,

assume the ground state energy as the zero reference. Now, an incident photon

can excite it to a higher virtual state (unstable) of energy hν0. In most cases,

the molecule returns back to the initial ground state giving rise to a photon with

frequency ν0. However, it can also happen that the molecule returns to an excited

vibrational/rotational state which has an energy of hνi hence, releasing a photon

at a lower frequency of νs = ν0 − νi. This is known as the Stokes line. There

2.4 Fundamentals of Raman Scattering 29

is also a possibility that a molecule in the excited state (with an energy of hνi)

absorbs another incident photon of frequency ν0, gets excited to a different virtual

level, and subsequently decays to the initial ground state releasing a photon at

a frequency of νas = ν0 + νi. This is known at the Anti-stokes line. Note that

the efficiency of Raman processes is typically over 1000 times smaller than that of

Rayleigh scattering. Also, at room temperatures, the stokes process is considerably

more efficient than the anti-stokes.

Polyatomic molecules can have multiple stokes and anti-stokes lines due to the

different values of νi corresponding to the various possible vibrational/rotational

states. The normal modes of vibration are typically created by stretching (symmetric

or antisymmetric) and bending (rocking, twisting etc.) of the constituting atoms

relative to each other. The rotational modes, on the other hand, correspond to the

rotation of the molecule along different axes. Note that the explanation of Raman

scattering is presented here in its most simplistic form. A more interested reader is

advised to refer to the literature for further details [66, 67].

The nature of the Raman scattered light is dependent on the intensity of the

incident light. If N0 and Ns are the photon numbers corresponding to the incident

and stoke shifted light, respectively, then for low values of N0, Ns is proportional to

N0. This is known as spontaneous Raman scattering. However, at very high N0, the

Raman photons can stimulate the molecule to decay in the vibrational/rotational

state. Hence, the relation between N0 and Ns becomes non-linear and the process

is called stimulated Raman scattering (SRS). In SRS, the intensity of the scattered

light is considerably higher than the spontaneous case. Mathematically, we have [66],

Ns(t) = Ns(0)eAN0t + AN0t, (2.42)

where A is constant which depends on νs, ν0, the scattering medium, and the po-

larization of the photons. In cases when AN0t << 1, the above equation shows the

linear relation between N0 and Ns, i.e., the spontaneous regime. However, if this is

not the case then Ns grows exponentially with time and hence, SRS occurs. The

optical gain achieved in SRS is characterized by the Raman gain cross-section (g).


The maximum value of g is mathematically expressed as [66],

g =c2NσD

ǫshν2sν0π∆νs

, (2.43)

where c is the speed of light, N is the number of scattering molecules per unit

volume, σD is the spontaneous Raman scattering cross-section per unit solid angle,

ǫs is the dielectric constant of the scattering medium at frequency νs, h is the Planck’s

constant, and ∆νs is the FWHM linewidth of the scattered photons at frequency

νs. When SRS is coupled to an optical feedback cavity with a narrow linewidth, the

Raman gain competes with the cavity losses. Above a certain N0 (threshold), the

gain exceeds the loss and we have what is called a Raman laser.

2.5 Brief Theory of Colloidal Quantum Dots

In semiconductors, the electron-hole pair created, when an electron makes a transi-

tion from the valence to the conduction band, is described by the term “Exciton”

and the natural physical separation between the electron and the hole is called the

“Exciton Bohr Radius.” This natural limit can be calculated by balancing the cen-

tripetal force on the electron with the mutual coulomb attraction force between the

electron and the hole. If rex is the exciton bohr radius, m0 is the electron rest mass,

e is the electron charge, v is the electron velocity, and ǫ0 is the permittivity of free

space, then we have,

m0v2

rex

=e2

4πǫ0r2ex

,

=⇒ rex =e2

4πǫ0m0v2. (2.44)

For most semiconductors, the typical values for rex lie around a few nanometers.

Therefore, in the bulk regime, rex is negligible compared to the actual size of the

semiconductor. Therefore, the exciton is free to extend to its natural limit and to

move freely throughout the semiconductor crystal. However, when the size of the

semiconductor approaches rex, the exciton is confined by the physical boundaries

of the material. This effect is known as the “Quantum Confinement Effect.” As a

2.5 Brief Theory of Colloidal Quantum Dots 31

consequence, the excitonic energy states are now determined by the eigenvalues of

the solution of the time independent Schroedinger equation for the relevant boundary

conditions. For the simplest cases, the exciton can be assumed to behave as a particle

in a box to obtain the following eigenvalues

En = Ebulk +n2h2π2

2R2

[

1

me

+1

mh

]

, (2.45)

where En is the nth excitonic energy eigenvalue, Ebulk is the band gap of the bulk

semiconductor, R is the physical dimension of the confinement, me is the mass of

the electron, and mh is the mass of the hole. Note that in the above equation,

the coulomb and the correlation energies of the exciton are neglected for simplicity.

The above equation tells us that the energy separation between the adjacent levels

(En+1 − En) is inversely proportional to R2. For large values of R, the levels are

closely spaced giving rise to a continuum of exciton energies, i.e., the bulk behavior.

Therefore, in this case, the bandgap shows no size dependence and has a fixed value

of Ebulk.

Figure 2.6: (a) Effect of quantum confinement on the excitonic energy levels inquantum dots. (b) A laboratory picture showing the red-shift of the photoluminencewit increasing quantum dot diameters (Photo courtesy: Andrey Rogach, LMU,Munich).

For small nanoscale materials like colloidal quantum dots, however, the energy

levels are considerably discretized as shown in Fig. 2.6a. Therefore, the effective

bandgap (E1) increases as an inverse square of the decreasing sizes and vice-versa.


Hence, a change in the size of the quantum dot, due to addition or deletion of a

few atoms, can bring about a considerable change in its spectral properties. In

fact, the effective bandgap can be precisely tuned throughout the visible and near

infrared just by controlling the size. This behavior is demonstrated in Fig. 2.6b. The

viles, containing colloidal quantum dots, are arranged in ascending order of dot sizes

(from left to right) and excited with UV radiation. The diameter of the quantum

dots varies from 2-5 nm and the corresponding photoluminescence changes from

green to red. Hence, unlike bulk semiconductors, quantum dots can be designed to

operate at an application specific spectral region.

The name, colloidal quantum dots, refers to nanoscale semiconductor particles (2-

10 nm) dispersed in the form of a colloidal solution. For their synthesis, two or more

desired reacting chemicals are sequentially introduced in a solvent (usually organic).

This leads to chemical replacement reactions followed by the nucleation process

which gives rise to nanometer sized clusters of atoms or molecules in the solution.

The size, composition, and concentration of these clusters can be controlled by

manipulating the reaction dynamics parameters, such as the solvent characteristics,

the reacting chemicals, time of reaction etc. Their spectral properties are monitored

during the nucleation process and the reaction is stopped when desired properties

are achieved. Colloidal quantum dots can be stabilized in a variety of organic and

inorganic solvents by coordinate bonding of appropriate molecules on their surfaces,

hence making them versatile for a variety of applications. Further details about the

theory, preparation, properties, and applications of colloidal quantum dots can be

found in literature [68, 69].

Chapter 3

Experimental Details

In this chapter, the design and functionality of our experimental setup are presented.

After a brief overview of the experimental setup in section 3.1, the latter sections

individually address each of the various aspects and provide further details. Sec-

tion 3.2 discusses the characteristics of the various laser sources used. A brief theory

and design of our sample introduction technique, the electrospray, is presented in

section 3.3. Subsequently, the design of our endcap trap is shown in section 3.4.

Finally, the imaging optics and the spectral measurement scheme are discussed in

sections 3.5 and 3.6, respectively.

3.1 The Setup

The basic schematic of our experimental setup is illustrated in Fig. 3.1. The endcap

trap is located inside a grounded metallic housing (also called trap chamber) which

has six access ports (along each axis of the cubical chamber). The figure shows a

cross-sectional view where the trap axis is perpendicular to the plane of the paper.

The two ports along this axis are used for making electrical contacts to the endcaps

and the bar electrodes. The bottom port, with a glass window, is used for optical

excitation of the trapped particles. The top port allows for the sample introduction

through an aperture via electrospray. The left and right ports are used for spectral

measurements and imaging, respectively.

The setup can be considered to comprise of five feature, namely, the optical

pump, the electrospray, the endcap trap, the imaging optics, and the spectral mea-

34 Experimental Details

surement system. The following sections in this chapter will individually address

each of these aspects in details.

Figure 3.1: A schematic of the salient features of our experimental setup.

3.2 Pump Lasers

We use three different laser sources in our experiments. All the lasers are carefully

aligned along a common beam path which passes through the center of the trap.

Focusing optics can be introduced in this path to achieve a particular spot size at

the center. Flipper mirrors are employed to choose the required laser beam for a

desired application.

For initial alignment of the trapped particles in the trap, the “RLDD532-1”

diode laser (manufactured by Roithner Laser Technik GmbH, Vienna) is used. The

cylindrical laser head has fairly small dimensions (φ16 mm x 60 mm) which makes

it easily mountable for optical alignment purposes. It has a Nd:YVO4/KTP crystal

inside which provides a linearly polarized CW output of 1 mW at a wavelength of

532 nm. The output beam has a gaussian profile and a diameter of about 5 mm.

More details on this laser can be found on the internet [70].

For carrying out measurements which require high pump intensities, such as

Raman spectroscopy, we use the “Millenia Pro 5s” laser from Spectra Physics Inc.

3.3 The Electrospray 35

It is a diode pumped frequency-doubled Nd:YVO4 laser and provides CW output at

a wavelength of 532 nm in the power range between 0.2-5 W. The beam is linearly

polarized, about 8 mm in diameter, and has a Gaussian intensity profile.

For pumping of our microlasers, we use the pulsed “Quanta Ray (LAB-130-10)”

laser from Spectra Physics Inc. It is a flashlamp pumped Q-switched Nd:YAG laser

with a repetition rate of 10 Hz and a pulsewidth of 10 ns. External triggering by

a 2V TTL pulse (10 Hz) is used to ensure a fixed repetition rate. The laser can

be exclusively operated in the first (1064 nm), second (532 nm), third (355 nm), or

fourth (266 nm) harmonic mode. We only use the 355 nm (for the ZnO tetrapod

laser) and the 532 nm (for the Raman and Quantum dot microdrop lasers) beams in

our experiments. The output beam is linearly polarized with a Gaussian intensity

profile and a diameter of about 1 cm. The output energies of the pulses at 355 nm

and 532 nm are 90 mJ and 200 mJ, respectively. However, the pulses are externally

attenuated with a λ/2 plate and a polarizing beam splitter cube to achieve the

desired energy.

The output powers of the Millenia and the Quanta Ray lasers are measured

with a portable thermal power meter (Model-407A) from Spectra Physics Inc. The

sensitive area of the detector head has a diameter of about 1.8 cm and can measure

powers in the range of 1 mW to 20 W. More details about the Millenia laser, the

Quanta Ray laser, and the 407A power meter can be found at the Spectra Physics

website [71].

3.3 The Electrospray

3.3.1 Brief Theory of Electrospray Ionization

For our experiments, we employed electrospray ionization as the microparticle gen-

eration technique as it fulfills the necessary requirements. Firstly, it is capable of

producing charged drops with diameters of the order of tens of microns. Secondly,

it is versatile to work efficiently for a variety of liquids (such as methanol, water,

and glycerol). Moreover, it is a simple structure which can be easily mounted and


quickly removed.

The electrospray is one of the most commonly used techniques for generation of

charged microdrops, macromolecules, or ions at atmospheric conditions. It has wide

applications in the fields of mass spectrometry, biology, and nanotechnology [72, 73].

The basic working mechanism of the process is depicted in Fig. 3.2a. The desired

fluid is filled inside a metallic capillary tube and placed close to a grounded aperture

(or plate). Application of a high positive potential to the capillary causes it to repel

the positive charges in the liquid. As this electrostatic repulsion overcomes the

surface tension forces, a positively charged microdrop is produced. The subsequent

size reduction of this travelling drop (by evaporation) increases the surface charge

density. As the Rayleigh stability limit is reached [74], coulomb explosion causes

the drop to repeatedly split into smaller entities and hence, gives rise to a positively

charged spray. Negatively charged drops can be produced by reversing the polarity

of the high voltage in Fig. 3.2a. However, this is often complicated by generation of

an electric discharge due to the field emission of electrons from the sharp negatively

charged capillary tip.

Figure 3.2: (a) A schematic depicting the mechanism of electrospray ionization. (b)Demonstration of the electrospray characteristics in our lab using a micropipettewith water as the fluid. A blue laser is scattered off the spray for imaging purposes.

3.3 The Electrospray 37

For a positive voltage of Vtip applied to the capillary tube, the electric field (Etip)

at the tip is given by [75],

Etip =Vtip

rtip ln( 4drtip

), (3.1)

where, rtip is the inner radius of the capillary tip and d is the distance between

the tip and the ground electrode. As this field becomes comparable to the surface

tension, the fluid starts getting pulled out of the capillary in a half-ellipsoidal shape.

At a threshold voltage VtipT , the ellipsoid changes into a cone shape and the fluid

starts spraying.

The cone formed right before the spraying action starts is known as the “Taylor

cone.” In the electrostatic case, when the spray is about to begin (Vtip = VtipT ),

the surface of the Taylor cone can be assumed to be an equipotential surface for

conducting liquids [76]. Also, at the threshold voltage, the pressures due to the

electrical forces and the surface tension must balance each other at all points on

the conical surface. Therefore, with the tip of the cone as the origin, the electric

potential of the taylor cone (Vtaylor(r, θ)) in spherical coordinates can be calculated

to be [76],

Vtaylor(r, θ) = Vs + Cr1

2P 1

2

[cos(π − θ)], (3.2)

where Vs and C are constants and P1/2 is the Legendre polynomial of degree 0.5. If

θT is the half angle of the Taylor cone, Vtaylor(r, θT ) corresponds to the equipotential

conical surface and hence, must be constant. Therefore, the term Cr1

2P 1

2

[cos(π−θT )]

in Eq. (3.2) must be zero for all values of r. This implies that θT is such that the

value of P 1

2

[cos(π − θT )] is zero. From this, the value of θT can be calculated to be

49.30. This is also known as the Taylor angle. For Vtip > VtipT , a fluid jet, which

eventually turns into the spray, appears to emerge from the tip of the Taylor cone.

This operation is very stable and is known as the cone-jet mode. In the cone-jet

mode, the angle of the Taylor cone decreases with increasing flow rates [76].

A demonstration of the cone-jet mode operation, achieved in our lab, is illus-

trated in Fig. 3.2b. A micropipette with a metal coated tip as the capillary and


Figure 3.3: (a) A picture of our electrospray setup used for sample introduction intothe trap. (b) Images of the spray of a 50 % glycerol solution for different voltagesapplied to the needle. The values of the corresponding voltage is written next toeach image.

water as the liquid are used for this purpose. A blue laser beam is scattered off

the spray for imaging purposes. The different regions namely, the Taylor cone, the

jet, and the spray are clearly visible in the image. The half angle of the observed

Taylor cone can be measured to be 44.50 from the image. This value is close but

smaller than the Taylor angle (49.30) and hence, the observation agrees with the

expected behavior. Note that the spray of Fig. 3.2b consists of particles ranging

from submicron to a few microns in size. It is only shown here to demonstrate the

theoretically expected behavior. The electrospray employed in our experiments, to

study microdrops in the range of 10-50 µm, is discussed in the next section.

3.4 The Endcap Trap 39

3.3.2 Electrospray for the Experiment

The basic setup of our electrospray is shown in Fig. 3.3a. A gastight glass syringe

(from Hamilton Bonaduz AG [77]) with a metallic needle (0.5 mm inner diameter) is

used to hold the fluid. The needle is connected to a PHYWE high voltage DC power

supply [78]. In the image, the system is mounted above an aperture (grounded).

The falling microdrops can be observed with back illumination imaging.

Fig. 3.3b shows the back illuminated images of the microdrops generated by our

electrospray at different voltages. The liquid used here is a solution of 50 % glycerol

in water. At 2.5 kV, drops are produced but they are not charged enough for coulomb

explosion to happen. The situation improves at 3 kV, and the spray becomes fully

operational around a voltage of 3.5 kV. At higher voltages (∼4.5-5 kV), the spray

is observed to become very unstable due to electric discharge between the tip and

the aperture.

The setup shown in Fig. 3.3a employs a test aperture. In the real experiments,

however, the needle tip is placed about 1 cm above the aperture (diameter∼3 mm)

in the grounded top port of the trap chamber. The charged drops falling through

this aperture are subsequently selectively trapped by our endcap trap. The coming

section shows the mechanical design and the setup of our trap in details.

3.4 The Endcap Trap

Our experiments aim at carrying out optical studies on isolated single microscale

particles. It has been known for more than two decades that such isolation, for

charged particles, can be achieved with the help of three dimensional quadrupole

electric potentials. The conventional Paul trap [56] has been and is still widely used

in this respect. For our purposes, however, it is disadvantageous because the fairly

closed construction leaves very little room for optical probing and investigations.

An alternate arrangement, which provides the necessary trapping potential with

considerable optical access to the trapped particles, is the “endcap” type trap [79].

It is robust with a fairly uncomplicated construction and hence, is suitable for our


applications. In such a trap, the center experiences a quadrupole potential and

hence, its working principle is similar to that discussed in section 2.1. The design

of the trap, as shown in Fig. 3.4a, consists of two conical frustum shaped metallic

endcaps arranged symmetrically along a common longitudinal axis. The endcaps

are exactly alike and are made of copper. They taper towards each other from a

diameter of 16 mm to 4 mm over a distance of 6 mm. Careful alignment ensures

that the surfaces facing each other are parallel. This can be better understood with

the help of the inset of Fig. 3.4a. The spacing between the endcaps is about 4 mm.

The point of symmetry between the two endcaps is called the trap center.

The endcaps are surrounded by four pairs of bar electrodes on each side, i.e, top,

bottom, left, and right. All the electrodes are at equal distances of 12.5 mm from

the trap center. The two electrodes, constituting a pair, subtend a 600 angle at the

trap center and carry the same voltage. Each bar electrode has a diameter of 2 mm

and a length of 18.6 mm.

Figure 3.4: (a) A simulated drawing of our endcap trap. (b) An image of the realtrap.

The endcap-electrode arrangement is constructed inside a grounded aluminum

housing as shown in Fig. 3.4b. Apart from serving as a cage for mounting the

different trap components, it also shields the trap from air currents and stray elec-

tromagnetic radiations. Moreover, we also use it to mount the signal collection

optics. The bottom of the housing has a removable plastic slide (40x60 mm) with a

3.5 The Imaging System 41

1 inch glass window allowing for the pump laser to enter the trap. The slide is fre-

quently removed and cleaned to avoid accumulation of the non-trapped particles on

the glass surface. Each endcap and bar electrode has a corresponding isolated elec-

trical connection point outside the housing. Further details on the design, trapping

properties, and effects of air damping along with the simulations of the quadrupole

potential for our endcap trap can be found in literature [80].

A high voltage (Vtr) of the form Vtr(t) = Vdc + V0cos(2πft) is applied on the

endcaps to generate the trapping potential. Here, t is the time, f is the trapping

frequency, V0 is the amplitude of the trapping voltage, and Vdc is the DC offset.

The effects of these parameters on the behavior of the trap have been discussed in

section 2.1. To generate this high voltage, the Tektronix AFG3022 function genera-

tor [81] is used to produce the desired AC signal which is then amplified 2000 times

with the Trek 20/20C high voltage amplifier [82]. Additional DC voltages can be

applied across the pairs above and below the endcaps to control the vertical position

of the trapped particle. Horizontal translation of the particle can be achieved by

using a similar voltage scheme on the left and right pairs. However, we usually only

need to use vertical centering as the trapped particles are automatically centered

horizontally due to the absence of any external horizontal field. We ground the

electrode pair below the endcaps and apply the necessary voltage on the top pair

with a Keithley 247 high voltage DC power supply [83].

3.5 The Imaging System

The imaging system is a very important aspect of our experimental setup as it serves

three major functions. It is used to do coarse and precise centering of a trapped

particle between the endcaps. Also, the acquired images help in estimating the size

of the trapped entity. Moreover, during the course of measurements, it enables us

to monitor the spatial location of the microlaser. Our imaging system employs two

modes of operation with different functionalities. Since we can only afford one port

for imaging purposes due to the geometry of the trap chamber, both modes are

designed such that they can be easily switched from one to another.


The first mode employs a standard combination of a color CCD camera (Watec

Color Camera, Edmund Optics GmbH [84]) and a 10X zoom lens [84]. The images

are acquired in realtime on a PC at a frame rate of 30 Hz with the help of an IDS

Falcon framegrabber and the corresponding imaging software [85]. This system can

work at long working distances of 15-45 cm with the corresponding field of view

ranging between 0.8-28 cm, respectively. Therefore, it can simultaneously image the

whole trapping region between the endcaps. This makes the system ideal for initial

imaging, isolation and coarse centering of a single particle in the trap. However,

the magnification in this mode is not enough for precise centering and sizing of the

particle. For these purposes we switch to the second mode of the imaging system

which has a 40 X magnification.

The optical arrangement of the high magnification imaging mode is shown in

Fig. 3.5a. A two lens system consisting of a microscope objective and a plano-convex

achromat is used. Both optical components are mounted inside a lens tube (1 inch

in diameter) such that the spacing between their principal planes remains fixed at

187 mm. The lens tube is mounted on a hand controlled XYZ translational stage

for alignment purposes. The tube is removed from the XYZ stage when switching

between the two imaging modes.

The imaging port of the trap chamber is sealed by a planar glass window which

is recessed into the chamber allowing for close access to the trapped particle. The

distance of the outer surface of the glass window to the trap center is about 20 mm.

Due to constructional reasons, the value of this dimension cannot be decreased. This

limits our minimum working distance to 20 mm. Hence, we use a 24 mm working

distance microscope objective (Model: 04 OAS 006) from CVI Melles Griot [86]. The

objective has a magnification of 4 X, a focal length of 30.8 mm, and a numerical

aperture of 0.12. For a particle located at the working distance, as shown in Fig. 3.5a,

the objective forms an image a distance of 154 cm from its principal plane. This

image is 4X magnified and is located inside the mounting tube at a distance of 33 mm

from the principal plane of the achromat. The achromat has a focal length of 30 mm

and therefore, for a object distance of 33 mm, produces an image outside the tube at

3.5 The Imaging System 43

Figure 3.5: Our 40X high magnification optics. (a) A schematic showing thearrangement of the optical components (b) A picture of the actual optics showingthe microscope objective and the achromat attached in a 1 inch diameter tube. (c)The magnified image of a 25±.29 µm trapped polystyrene bead used for calibrationpurposes.

a distance of 330 mm from its principal plane. The image formed by the achromat

is focused on the chip of the Watec color CCD [84] camera. The magnification of

the achromat is the ratio of image to the object distance, i.e., 330mm/33mm=10X.

As a result, the total magnification is approximately 4 · 10 X = 40 X. An image of

the real tube lens showing the different components is depicted in Fig. 3.5b.

If there are slight deviations from the 24 mm working distance, the CCD camera

has to be moved to obtain a sharp image. In such cases, the actual magnification

might vary. Therefore, the system needs to be calibrated so that the magnification

factor does not change for different runs of the experiment. For this purpose, we

trap and center a polystyrene bead with a known diameter of 25±.29 µm and setup

the high magnification optics. This position of the CCD camera is noted and a cor-

responding image of the bead is taken. For subsequent runs with unknown particles,

the CCD camera is always relocated to this position and the tube lens is adjusted

to get a sharp image of the trapped particle. Now, the number of pixels occupied by

the unknown particle in the image is compared with that of the polystyrene bead


and hence, the particle’s size is estimated. The high magnification image of such

a 25 µm bead under uniform green illumination is shown in Fig. 3.5c. The image

shows the typical glare spots (caused by refraction) which signify the spatial extent

of the bead along the propagation direction of the illumination laser [87].

3.6 Spectral Measurements

Our spectral measurement scheme comprises of three basic components. These

are the signal collection optics, the spectrometer, and the software enabled data

acquisition.

3.6.1 Signal Collection Optics

The signal collection optics are mounted on the left port of the trap chamber. It

is a combination of two achromat lenses with focal lengths of 30 mm (Lens A) and

80 mm (Lens B), respectively. The arrangement is shown in Fig. 3.6a. Lens A is

located such that the trapped and centered particle lies in its focal point. Lens A

collects the light at a numerical aperture of 0.4 and collimates it at a beam diameter

of 25.4 cm. Lens B collects this collimated light and focuses it at a distance of 80 mm

into a multimode fiber. As shown in Fig. 3.6b, the two achromats are mounted inside

a 1 inch tube which is fixed inside a PVC cage. The cage is attached to the trap

chamber making the arrangement compact and rigid.

The multimode fiber, used here for guiding the collected signal to the spectro-

graph, is actually a fiber bundle. The individual fibers constituting the bundle are

arranged at the two ends in different fashions. At the signal collection end, the

fibers are arranged such that the individual ends symmetrically fill a 200µm wide

circular cross-section. The big area ensures better signal collection and also makes

the system less prone to misalignments. The other end of the fiber is attached to

the spectrograph. At this end, the individual fibers are arranged next to each other

in a linear fashion along the entrance slit of the spectrograph (see Fig. 3.6c).

3.6 Spectral Measurements 45

Figure 3.6: (a) Schematic of our inbuilt signal collection optics. (b) A pictureof the actual signal collection optics showing the two achromats mounted on a 1inch diameter tube inside a PVC cage. (c) A picture showing the two ends of themultimode fiber used for guiding the collected signal to the spectrograph.

3.6.2 The Spectrometer

Our spectrometer is a combination of a Czerny-Turner type spectrograph and an

EMCCD (Electron Multiplying Charged Coupled Device) camera. We use the Omni-

λ300 spectrograph/monochromator from Zolix Instruments [88]. It has one entrance

and two exit ports for the optical beam. Depending on which exit port is chosen,

the instrument can be used as a spectrograph or a monochromator. In our case, it is

used as a spectrograph to achieve single shot spectral acquisition. The spectrograph

has a f-number of 3.9 and depending on the grating, can work in the spectral range

from 185 nm to far infrared. Three different gratings, mounted on a motorized turret

for rotating and switching, are employed. Grating 1 is blazed at 500 nm and has

a groove density 300 lines/mm which provides a resolution of 0.35 nm. Grating 2

(blaze λ-750 nm) has a better resolution of 0.17 nm due to a higher groove density

of 600 lines/mm. The highest resolution (0.09 nm) is offered by grating 3 (blaze

λ-500 nm) because of its 1200 lines/mm groove density. The higher the lines/mm

the lower is the transmission efficiency.Hence, the appropriate grating for a specific

purpose is chosen based on the signal-strength and the desired resolution. The

spectral range which can be covered in a single shot spectra by grating 1, 2, and 3


are about 80, 40, and 20 nm, respectively. Note that all the resolution values quoted

above are with respect to an entrance slit width of 10 µm.

The spectral end of the fiber bundle is mounted so that it is located right against

the entrance slit (10 µm wide) of the spectrograph. We use the iXon DU-897 back

illuminated EMCCD camera from Andor Technology Ltd. [89]. It is sensitive up

to the single photon level with greater than 90% quantum efficiency of a back-

illuminated sensor. The CCD chip is thermoelectrically cooled to -70o C during

operation. The chip has a square shape with 512x512 pixels. Each pixel has an area

of 16x16 µm. The maximum pixel readout rate can be up to 10 MHz.

We use the Andor Solis 4.6 software [89] for automated spectral data acquisition.

The software can be used to control both the spectrograph and the EMCCD camera

simultaneously. For all our measurements, we acquire the data from the camera in

the “full vertical binning mode.” This means that the signal from all the EMCCD

pixels in a vertical column are added so that the whole column behaves as one pixel.

Since the grating causes a horizontal dispersion of the different wavelengths, and

we are interested mostly in the spectral characteristics, this mode is preferable as

it provides a high signal to noise ratio. However, for other applications, data can

also be acquired in the “imaging mode” where the signal from each pixel is used

to obtain an image of the entrance slit on the camera chip. The effects of the

stray background radiations can be nullified by choosing the option of “background

subtraction” during data acquisition.

The three well known narrow spectral lines of the mercury lamp, located at

546.07, 576.96, and 579.07 nm, respectively, are used for calibration purposes. The

mercury lamp is placed close to the entrance slit (10 µm wide). First, the EMMCD

is aligned by monitoring the realtime spectra to obtain the smallest linewidth of the

mercury lines. The linewidths are usually minimum for all the three gratings at the

same location of the EMCCD camera. The position of the EMCCD is then kept

fixed. However, at this position, the different gratings give different values for the

same spectral line. Therefore, the second step is to separately calibrate each of the

gratings.

3.6 Spectral Measurements 47

Figure 3.7: Calibration test of our spectrometer system for the (a) 300 lines/mm,and (b) 1200 lines/mm, gratings.

For a given grating, the shifts of the three measured mercury peaks with respect

to their expected values (quoted above) is calculated. These shifts are usually ob-

served to be positive, i.e., red shifts. The average value of the shifts is subsequently

subtracted from the wavelength axis of all the following spectra measured with the

help of that grating to achieve calibrated results. Fig. 3.7a and b show the system

calibration test for the 300 lines/mm and the 1200 lines/mm grating, respectively.

After calibration, the three mercury lines are measured to be at 545.93, 577.1, and

579.3 nm with the 300 lines/mm grating (Fig. 3.7a). These values agree well with

the expected values within the resolution (0.35 nm) of this grating. The peak at

545.93 nm is shown in the inset of Fig. 3.7a. A gaussian fit is used to obtain the

resolution limited linewidth (FWHM) for the 300 lines/mm grating to be 0.7 nm.

Similar measurements for the 1200 lines/mm grating are shown in Fig. 3.7b.

Again, the location of the measured mercury lines after calibration, shown in the

figure, are in accordance with the expected values within the resolution (0.09 nm)

of this grating. The corresponding resolution limited linewidth (FWHM), as shown

in the inset, is estimated to be 0.15 nm by fitting a gaussian curve to the peak at

546.16 nm. The 1200 lines/mm grating can only measure a spectral range of about

20 nm in a single shot. Therefore, two different scans are taken, as shown by the

spectral gap in Fig. 3.7b, to cover all the three peaks. Note that the 600 lines/mm


grating is not used in our measurements and hence, the corresponding measurements

are not shown here.

With this, most of the important aspects of our experimental setup have been

addressed. The following three chapters will discuss the use of this setup for the

development and characterization of the microscale lasers. The design and the

operation of the various experimental aspects, mentioned in this chapter, will be

frequently referred to in the following chapters.

Chapter 4

The Electrodynamically ConfinedSingle ZnO Tetrapod Laser

This chapter focuses on the development and investigation of single ZnO tetrapod

lasers using electrodynamic fields. The chapter begins with a discussion on the

preparation and structural properties of our ZnO tetrapod samples. Following this,

section 4.2 shows our novel technique to isolate and electrodynamically trap a single

ZnO tetrapod under atmospheric conditions. Section 4.3 deals with the optical inves-

tigations, such as observation of lasing, carried out on single and multiple trapped

tetrapods. The possibility of using a combination of electrodynamic and optical

fields to micromanipulate the trapped tetrapods, along with our current progress

towards the goal, is discussed in section 4.4. Finally, additional investigations of the

tetrapods on substrates (using an inverted microscope) are shown in section 4.5.

4.1 ZnO Tetrapods: Preparation and Structural

Properties

The vapor phase transport process is by far the most commonly used technique

for the preparation of ZnO based nanostructures such as nanorods, nanowires, and

tetrapods [90]. In this process, the chemical reaction for the formation of a desired

nanostructure is initiated by gas phase species. The main mechanism involves the

reaction of Zn and oxygen vapors leading to the formation of ZnO nanostructures

through a nucleation process. There are multiple ways to generate the Zn and oxygen

50 The Electrodynamically Confined Single ZnO Tetrapod Laser

vapors such as decomposition of ZnO powder and direct heating of Zn powder under

oxygen flow. More details about the mechanisms and processes of ZnO nanostructure

fabrication can be found in literature [90, 91].

Figure 4.1: SEM images of (a) a cluster of tetrapods, (b) a single tetrapod, (c) anend facet of a tetrapod leg, and (d) the center of the tetrapod.

Our ZnO tetrapod samples are prepared at the research group of Prof. Yiping

Zhao at the University of Georgia by a process similar to the vapor-solid mecha-

nism [20]. An equal mixture of Zn and graphite powder is placed inside an open

quartz tube and heated for 20 minutes to 10000C under ambient conditions at stan-

dard pressure. This leads to the formation of a cotton-like white fluffy powder

inside the quartz tube. While graphite acts as a catalyst in the reaction, the Zn and

oxygen vapor pressures are controlled to ensure that the nanostructures formed are

tetrapod-like. Scanning electron microscope (SEM) images, as shown in Fig. 4.1,

4.2 Electrodynamic Trapping of a Single ZnO Tetrapod 51

of this material reveal tetrapod samples with average leg diameters between 200-

800 nm and leg lengths between 10-30 µm. Note that for the SEM analysis, the

white tetrapod powder is sonicated in isopropanol for about 30 s and subsequently

spincoated on glass substrates.

In the past, several different models have been presented to explain the formation

of the tetrapod structure. However, the generally shared opinion is that the tetra-

pod structure initiates from a core on which the four legs preferentially grow along

the [0001] direction via nucleation [32]. The core of the ZnO tetrapod has a tetra-

hedral zinc blende crystal structure [21] while the legs grow in a hexagonal wurtzite

geometry [92, 22]. Nevertheless, there is still a lot of ongoing debate over the ac-

tual nucleation process, the growth mechanism, and the structure of the tetrapod

core [93, 94, 95]. After investigating the structural properties of the ZnO tetrapods,

we focused our efforts towards the development of the electrodynamically confined

single ZnO tetrapod laser. One of the first goals towards achieving this was to trap

a tetrapod at atmospheric conditions. The coming section discusses our technique

for the same.

4.2 Electrodynamic Trapping of a Single ZnO Tetra-

pod

We use the endcap trap to achieve the electrodynamic confinement of a single ZnO

tetrapod. The details about the design and functioning of the trap have been dis-

cussed previously (section 3.4). Most previous reports, concerning with trapping

and isolation of semiconductor nanostructures, dealt with optical trapping in solu-

tions [28, 30]. In such cases, one does not need a technique to insert the samples

into the confining field as the trapping is achieved by focusing a laser inside a solu-

tion of nanostructures. Therefore, development of an appropriate technique for the

introduction of charged ZnO tetrapods into the trap, under atmospheric conditions,

was our first challenge. We explored several methods and, as discussed below, found

electrospray ionization to be the most efficient.

Our tetrapod samples, as mentioned before, are supplied to us in the form of


a white fluffy powder. As the tetrapods are clumped together, the appearance

of the sample to the eye is very similar to powdered ZnO. Our first attempts to

introduce the tetrapod samples into the trap employed a simple procedure. The

bristles of a clean brush (similar to a painting brush) are gently brought in contact

to the sample such that some of the tetrapods stick to them. The bristles are then

slowly brought just above the aperture (diameter ∼ 3 mm) in the top port of the

trap chamber. Subsequent shaking of the brush causes the tetrapods stuck to the

bristles to fall vertically under gravity and enter the trap. Moreover, the friction in

the bristles of the brush imparts the necessary charge (required for trapping) to the

tetrapods. Now, the trap is turned on and the parameters are optimized to achieve

stable trapping. This technique is found to work very well and effectively resulted

in trapping. However, this method mostly resulted in the trapping of a cluster of

tetrapods rather than single ones. This is because the effect of the brush is not

strong enough to break the powder down to the single tetrapod limit.

Figure 4.2: High magnification (40X) image of (a) a trapped cluster of tetrapods(b) a single trapped tetrapod. The trapped particles are illuminated with a greenlaser and the scattered light is used for imaging.

The trapped tetrapod clusters are located slightly below the geometrical center

of the trap due to the gravity. Subsequently, the voltage on the surrounding bar

electrodes (Vbar) is adjusted (to compensate gravity) to bring the cluster to the

center of the trap. Approximate centering is initially done with the help of a CCD

camera. Subsequently, a high magnification setup (shown in section 3.5) is used

4.2 Electrodynamic Trapping of a Single ZnO Tetrapod 53

for precise centering of the cluster. Centering is necessary because only the central

region has a null electric field and therefore, unwanted effects of the trapping field

are eliminated. Also, at the center, the motion of the trapped particle is negligible

simultaneously in all the three dimensions. The typical values of Vbar required for

centering these clusters lie between -5 and -20 V which indicates that the clusters are

positively charged. Fig. 4.2a shows a trapped and centered tetrapod cluster under

green CW laser (1mW) illumination. The trapping parameters used here are f =

100 Hz, V0=1 KV, Vdc = 0, and Vbar = -17 V. Therefore, the employment of a brush

for sample introduction helped to demonstrate the feasibility of trapping tetrapods

in our setup. However, this method is unsuitable for our purposes as it cannot be

used to study single tetrapods.

It is soon found that electrospray ionization of a solution of tetrapods serves our

purpose if a rapidly evaporating solvent is used. The working principle of the method

is as following. We use methanol as the solvent due to its favorable evaporation

properties and non-reactivity with ZnO. A dilute, milk colored solution is prepared

by sonicating the tetrapod powder in methanol for 1-2 minutes. This helps in

dispersing isolated tetrapods in methanol. The syringe of our electrospray (described

in section 3.3.2) is filled with this solution and is placed 5 mm above the aperture in

the top port. A high voltage (∼ 3.5 kV) at the syringe tip and a gentle back pressure

to the syringe, in combination, produce a charged spray consisting of tetrapods and

methanol. As the spray enters the trap chamber, the methanol evaporates, leaving

the charged isolated tetrapods behind, which can then be trapped by optimizing the

trapping parameters. If multiple tetrapods are trapped, the process is repeated until

the trapping of a single tetrapod is achieved. Typically, tetrapods of leg lengths in

the range of 20-30 µm are trapped in our setup. Similar to the cluster, a single

tetrapod is also centered in the trap by tuning of Vbar. Fig. 4.2b shows a stably

trapped and centered single tetrapod under green CW illumination. The trapping

parameters used here are f = 300 Hz, V0=1 KV, Vdc = 0, and Vbar = -8 V. Note that

the bright green spots along the legs are not an artifact and are present in all the

trapped tetrapods. A discussion on the occurrence of these spots is more suitable


to be presented in a later part of this chapter in section 4.5.4.

Achieving stable levitation of a single tetrapod is complicated by the distribution

of charge on the sample, methanol evaporation, and multiple tetrapods which are

intertwined together. Nonetheless, with the proper parameters, the process is fairly

quick and repeatable. Moreover, the levitated tetrapods can be kept in the trap

for up to a day. Therefore, once the tetrapod is stably trapped, it can be used

for a number of optical investigations. The coming section elucidates the optical

properties of the trapped tetrapods in details.

4.3 Optical Investigations of Trapped ZnO Tetrapods

4.3.1 Photoluminescence and Raman Spectra

ZnO has a band gap of 3.4 eV (at room temperature) which corresponds to an

absorption edge at 365 nm. Therefore, UV laser sources of wavelength below 365 nm

are commonly used for optical excitation of ZnO nanostructures. We use the third

harmonic of a Q-switched Nd:YAG laser to pump the trapped tetrapods. The laser

operates at a wavelength of 355 nm and produces 10 ns pulses at a frequency of

10 Hz. The laser is focused at the center of the trap to a spot diameter of about

187 µm. The spot size is kept rather large for the uniform illumination of the

tetrapod.

Fig. 4.3a shows the fluorescence spectra of a single trapped tetrapod measured

for a single pump pulse excitation at a fluence of 2 mJ/cm2. The PL is observed to

have a peak at 388.5 nm and is about 14 nm broad (FWHM). The PL peak value

corresponds to an energy of 3.2 eV which is less than band gap of ZnO (3.4 eV). The

additional reason for this shift, apart from the excitonic PL of ZnO (section 2.2),

is most likely the heating due to the large ns pulse width of the pump laser [96].

ZnO tetrapods are also known to have a visible PL emission in the green which

is attributed to the presence of surface defects [97]. However, as the tetrapods

levitated in our trap are fairly large (leg lengths ∼ 20-30 µm), they have a low

surface to volume ratio. Therefore, the green PL behavior is found to be below

4.3 Optical Investigations of Trapped ZnO Tetrapods 55

Figure 4.3: Optical properties of a single trapped ZnO tetrapod, (a) The PL spectraand (b) The CW Raman spectra.

the noise limit in our measurements. As a guide to the eye, Fig. 4.3a also shows

the spectral location of the elastically scattered pump light at 355 nm. If desired,

the pump light can be filtered out by introducing an optical filter in the collection

optics. During measurements, the positional stability of the tetrapod is monitored

in realtime with the help of the high magnification setup (section 3.5).

Other optical properties of the tetrapods, such as the Raman spectra, are also

investigated to demonstrate the versatility of our system. Fig. 4.3b shows the CW

Raman spectra of a single trapped tetrapod. The Raman spectroscopy is carried

out with a CW frequency doubled Nd:YVO4 pump laser at a wavelength of 532 nm.

The Raman signal is found to be very weak and therefore, a high pump intensity

of 1 KW/cm2 and a long EMCCD exposure time of 30 s are used for this measure-

ment. The peaks measured in the spectra at around 330 cm−1, 380 cm−1, 437 cm−1,

575 cm−1, and at 665 cm−1 have all been reported before and accounted for based

on the crystal structure of ZnO [98]. Note that the baseline of the Raman spectra

is skewed, that is, it is higher at lower Raman shifts. This is most likely due to the

pump light leakage into the detection system thorough the filter.


4.3.2 UV Lasing in a Single ZnO Tetrapod

As mentioned in section 2.2, the legs of a ZnO tetrapod behave as individual

nanowires. This means that the legs act as efficient waveguides for the tetrapod’s

UV PL emission. Also, the legs can provide efficient Fabry-Perot type optical feed-

back by reflection at their end facets. This satisfies the necessary requirements for

lasing to occur. Therefore, when optically pumped, a single leg of the tetrapod can

act as a self-contained laser with longitudinally directional emission from its facets.

Figure 4.4: Single pump pulse excitation PL spectra of the tetrapod in Fig. 4.5a atthree different pump fluences of 2 mJ/cm2, 20 mJ/cm2, and 90 mJ/cm2.

For the lasing measurements, the pump laser and its spot size are the same

as used for the PL measurement shown in Fig. 4.3a. To start, the pump laser

at a low fluence (usually about 1 mJ/cm2) is incident on the tetrapod such that

the measured PL is just above the noise limit of the spectral measurements. The


EMCCD exposure time is chosen to be 0.09 s to make sure that the measured spectra

correspond to single pulse excitation. Now, the pump fluence is slowly increased in

small steps and the respective single pulse spectra are recorded. Fig. 4.4 shows the

emission spectra of a single trapped tetrapod for single pump pulse excitation at

three different pump fluences. At very low pump fluences ( < 2 mJ/cm2), a broad,

11 nm FWHM, PL signal centered at 389 nm is observed. By increasing the pump

fluence to 20 mJ/cm2, a narrowing of the PL spectra, emergence of sharp peaks, and

a significant increase in the overall intensity are observed which are clear indicators

for the onset of lasing. At a much higher fluence of 90 mJ/cm2, the lasing modes

become much more clear and their intensity is also considerably enhanced. Note that

the measured linewidths of all the lasing modes are limited by the resolution (0.1 nm)

of our spectral detection.

During spectral measurements, the tetrapods are simultaneously monitored in

realtime by using the high magnification setup. As we use a CCD camera for image

acquisition, the scatter of the 355 nm pump light is not detected in the imaging

system. The PL emission from the tetrapod, however, is around 390 nm and hence

can be used to acquire images. Note that for better signal to noise ratio, we usually

acquire the images above the lasing threshold fluence where the tetrapod’s emission

is considerably strong. Fig. 4.5a shows such a high magnification (40 X) image of a

single trapped ZnO tetrapod under lasing conditions at a fluence of 20 mJ/cm2. The

bright violet color agrees with the 390 nm emission observed in the corresponding

spectra. Three legs, each ∼ 24 µm in length, are clearly present in the image. During

the course of alignment we know that the forth leg is facing away from the camera

in the direction of the spectrometer. Hence the bright white spot at the center of

the image is likely a combination of scattering from the grain boundary at the base

of the tetrapod as well as directional emission from the fourth tetrapod leg [27].

The lasing modes observed in our case (ns excitation) are red-shifted compared to

similar previous experiments with fs excitation [23]. The behavior can be attributed

to the following reasons. Firstly, the typical spontaneous emission lifetime of ZnO

tetrapods is around hundreds of ps [99]. In the case of ns and fs pumping, the


Figure 4.5: High magnification (40 X) images of (a) a single trapped lasing ZnOtetrapod and (b) multiple trapped lasing tetrapod. The images are taken for a CCDexposure time of 30 ms and a pump fluence of 20 mJ/cm2.

excitation time is considerably longer and shorter, respectively, compared to the

emission decay time. This can cause differences in the evolution of the density of

excited states in the two cases. As a result, the emission gain profiles are shifted

from each other which explains the observed shift of the lasing modes [24]. Secondly,

the red-shift can also be supported by bandgap renormalization effects due to the

possible differences in the total carrier density in the two cases [60]. Moreover, the

long pulse of the ns laser also results in accumulative heating of the sample which

can also contribute to the red-shift of the excitonic emission [96].

Assuming a Fabry-Perot resonator like behavior, the free spectral range (∆λ), of

the lasing feedback cavity can be measured to be around 1 nm from the 90 mJ/cm2

spectra in Fig. 4.4. Thus the corresponding cavity length, L, can then be estimated

by, [27]

L =λ2

2∆λ(n− λdndλ

), (4.1)

where λ is the wavelength of light, the refractive index n = 2.2 and the first order

dispersion λdn/dλ = −1 for ZnO at λ = 390 nm [100]. This estimated cavity length

of 22.4 µm is close to the measured length, thus confirming previous observations

that the legs of the tetrapod tend to act as independent cavities [27].


Figure 4.6: (a) Spectral evolution of a single tetrapod, shown in Fig. 4.5a, as afunction of the fluence averaged for 200 pump pulses. (b) A plot of the lasingthreshold behavior for a single tetrapod. Each point represents the average of 20scans and their corresponding error bars and each scan covers 10 pulses. To minimizethe background PL we integrate around the lasing peak centered at 388.5 nm.


The spectral evolution of the single trapped tetrapod as a function of the pump

fluence is shown in Fig. 4.6a. For better accuracy, the spectra of Fig. 4.6a are

averaged for 200 excitation pulses. For increasing pump fluence, we observe that the

peak position of the modes remain constant while their relative intensities increase.

A typical lasing threshold behavior is shown in Fig. 4.6b, where the integrated PL

intensity is plotted as a function of the pump fluence for the lasing mode centered at

388.5 nm. The behavior is near quadratic as reported before in literature [60].The left

inset shows the spectrum at 5.5 mJ/cm2 when the first cavity modes start emerging

from the broad PL background and agrees well with previously reported threshold

values for ns pulse excitation [24]. The right inset shows the lasing spectrum at 32.8

mJ/cm2. When the pump fluence is increased beyond 32.8 mJ/cm2, the radiation

pressure of the focused pump beam makes the tetrapod unstable as shown by the

increasing size of the error bars for larger pump fluences. This instability affects the

visibility of the individual peaks of the averaged signal but the peak positions agree

well with the lasing spectra for single pulse excitation shown in Fig. 4.4. The lasing

threshold values are found to be very similar for different single trapped tetrapods.

Although these threshold values also compare well to those observed for multiple

trapped tetrapods, there are considerable differences between lasing from single and

multiple tetrapods as discussed in the next section.

4.3.3 UV Lasing in Multiple ZnO Tetrapods

In our setup, the usual trapping routine occasionally also results in the trapping

of a cluster typically comprising of 2-5 tetrapods. These tetrapods are intertwined

together and behave as one single entity in the trap. UV lasing can also be achieved

from such clusters by following the same experimental technique as used for single

tetrapods. Fig. 4.5b shows the image of a cluster of 2 tetrapods at lasing conditions

at a fluence of 20 mJ/cm2. From the image, both the tetrapods appear to have legs

of similar lengths around 16-17 µm. However, this is not the usual case as the sizes

of the tetrapods forming the trapped cluster are observed to vary considerably from

each other.


Figure 4.7: (a) Spectral evolution of multiple tetrapods, shown in Fig. 4.5b, as afunction of pump fluence averaged for 200 pulses. (b) A plot of the lasing thresholdbehavior for multiple tetrapods. Each point represents the average of 20 scansand their corresponding error bars, each scan covers 10 pulses. To minimize thebackground PL we integrate from 386 to 393 nm.


The evolution of the PL spectra for increasing pump fluence for the tetrapods in

Fig. 4.5b is shown in Fig. 4.7a. For better signal to noise ratio, these measurements

are averaged over 200 pump pulses and integrated from 386 to 393 nm. An inter-

esting feature is that the observed cavity modes are not as distinct as in the case of

a single tetrapod (Fig. 4.6a). For a single tetrapod, the legs are very similar in sizes

and therefore, their respective cavity modes have a good spectral overlap and are

evidently visible in the lasing spectra. However, due to the difference in the leg sizes

and the orientations of the two tetrapods in Fig. 4.5b, their corresponding lasing

modes are spectrally shifted with respect to each other and hence, in the overall

spectra the individual modes appear to be washed out.

The lasing threshold behavior of the tetrapods in Fig. 4.5b is depicted in Fig. 4.7b.

Each point represents the average of 20 scans and their corresponding error bars,

each scan covers 10 pulses. As expected, the increase of fluence above the threshold

value causes a narrowing of the spectral width and a sharp increase in the overall

intensity. The corresponding spectra below (3.6 mJ/cm2) and above (27.3 mJ/cm2)

the threshold are shown by the left and right insets, respectively. The lasing thresh-

old fluence is observed to be around 10 mJ/cm2 which is similar to the case of single

tetrapods. This is justified because lasing occurs in individual legs which behave as

independent cavities. Therefore, as long as the incident fluence is spatially uniform

through the extent of the trapped particle, the threshold values are expected to

be similar for single and multiple tetrapods. Note that in our case the condition

of spatially uniform pump fluence exists because the pump spot diameter is fairly

large (187 µm) compared to the sizes of the trapped particles. However, the overall

lasing intensity, at similar pump fluences, is observed to be higher in the case of

multiple tetrapods (Fig. 4.7b) as compared to a single tetrapod (Fig. 4.6b). This is

simply because of the presence of more lasing cavities (tetrapod legs) in a multiple

tetrapod cluster.

The lasing behavior of the tetrapods critically depends on their size, facet quality,

and waveguiding ability. Moreover, as the lasing from the tetrapods is very direc-

tional, the output efficiency is a function of the signal collection direction. Hence,

4.4 Micromanipulation 63

an ability to direct the emission along any desired direction can greatly enhance the

performance of the tetrapod laser. In principle, this can be achieved by transla-

tional and rotational control, or micromanipulation, of the tetrapod’s position. The

next section will discuss our experiments towards achieving micromanipulation of

the trapped tetrapods.

4.4 Micromanipulation

Micromanipulation refers to the control of a particle’s position and orientation with

microscale precision. As semiconductor nanostructures are the building blocks of

nanoscale electronic and optoelectronic devices, micromanipulating them for their

transport and assembly is of considerable interest [13, 101]. Moreover, for nanos-

tructures with directional optical emission such as nanowires or tetrapods, micro-

manipulation can be used to maximize the output efficiency and hence, considerably

increase the device performance. When nanostructures are studied on substrates,

their micromanipulation is out of question as they are rigidly attached to the surface.

However, such control has been demonstrated in liquid solutions of nanostructures

with the help of optical fields [28, 29]. Under atmospheric conditions, however, such

micromanipulation of nanostructures has not been achieved.

Our setup can also serve as an ideal system for micromanipulation because the

trapped nanostructure has all the degrees of freedom of motion. The charges present

on the trapped nanostructure make it manipulable by external electric fields. More-

over, as our setup also allows for the exposure of the trapped nanostructure to

external radiations, micromaniuplation with optical fields is also feasible. As dis-

cussed in the following sections, our approach to achieve micromanipulation of the

trapped ZnO tetrapods includes both electric and optical fields.

4.4.1 Translational Control

As discussed in section 3.4, our endcap trap is symmetrically surrounded by eight

bar electrodes. A variation in the DC voltage (Vbar) applied across these electrodes

results in a change in the local field distribution near the trap center. The charged


trapped particle experiences this change and hence, shifts from its initial trapping

position. The direction of the shift is along and its magnitude is proportional to the

change of the local electric field. Hence, the translation of a trapped tetrapod, in

our setup, can be achieved by careful tuning of the voltages applied across the bar

electrodes.

Figure 4.8: Demonstration of translational micromanipulation of a single lasingtetrapod. Three positions of the tetrapod are shown for voltages of -7 V, -9 V, and-11 V applied across the DC bar electrodes.

Fig. 4.8 demonstrates the translational micromanipulation of a single lasing ZnO

tetrapod in our setup. The DC voltage is applied across the pair of electrodes

vertically above and below the endcaps. The pair below the endcaps is grounded.

Application of a negative voltage to the pair above the endcaps results in the upward

vertical translation of the tetrapod. This is a good indication that the tetrapod is

positively charged. Fig. 4.8 is a combination of images showing the position of the

tetrapod for three different DC voltages. For a voltage of -9 V, the tetrapod is

geometrically centered in the trap. For voltages of -7 V and -11 V, the tetrapod

is positioned about 20 µm below and above the center, respectively. It can also be

seen from the figure that the positional shift is strictly translational as the rotational

orientation of the tetrapod is unaffected. Note that similar translational effects are

also observed in the horizontal direction when the DC voltage is applied across the

pairs of electrodes on either sides of the endcaps.

The maximum range of the achieved translation, without a corresponding change

in the recorded lasing spectra, is as high as 100 µm. Moreover, in this range,

the translational shifts are observed to be very reproducible and to follow a linear


relationship with the DC voltage. These results are further discussed in the next

section where they are used to estimate the number of charges on a trapped tetrapod.

4.4.2 Charge Determination

The surface charges on the tetrapods should have little effect on their optical prop-

erties since their dimensions are many orders of magnitude larger than the Bohr

exciton radius (1.8 nm for ZnO), a regime in which quantum confinement effects

would become important. To estimate the approximate order of the charging, we

use translational micromanipulation of a trapped tetrapod. Let us begin by ana-

lyzing the free body diagram of a trapped tetrapod. The mass and charge of the

tetrapod are denoted by mtp and qtp, respectively. The vertical radial direction (y)

is defined, with the center of the trap as the origin, to be positive in the vertically

upwards direction. The electric field distribution, along the y axis, corresponding

to an arbitrary voltage (Vbar) applied across the vertical pair of bar electrodes is

denoted by E(y). Note that the electrode pair below the endcaps is grounded. Now,

if a tetrapod is stably trapped at a distance of y from the center, the forces on it due

to the bar electrodes, gravitational attraction, and the endcaps must compensate

each other. Therefore, the force balance equation for such a tetrapod is given by,

qtpE(y) −mtpg + Ftrap(y) = 0, (4.2)

where Ftrap is the restoring force due to the endcaps which pulls the tetrapod towards

the center of the trap and g is the acceleration due to gravity. Note that in the above

equation, the value of Ftrap is positive or negative, respectively, for a displacement

below or above the trap center. However, at the trap center, there is no restoring

force and the value of Ftrap goes to zero. Hence, the above equation can be rewritten

for the trap center as,

qtpE(0) −mtpg = 0. (4.3)


Therefore, the charge to mass ratio of the tetrapod is given by

qtpmtp

=g

E(0). (4.4)

The estimation of the value of E(0) is not straight forward due to the complicated

arrangement of the bar electrodes. To determine the nature of the electric field

variation near the trap center, the measurements shown in Fig. 4.9 are performed.

The values of Vbar are gradually tuned and the corresponding displacements (∆y)

of the tetrapod from the trap center are measured with the help of the calibrated

high magnification imaging system. Fig. 4.9a shows the images of a tetrapod under

green CW illumination for different values of Vbar and the corresponding plot is

shown Fig. 4.9b. In the vicinity of the trap center, the variation of Vbar with ∆y

follows a linear behavior as shown by the fit. This indicates that the electric field

at the trap center has a first order variation with Vbar and can be expressed as

E(0) = C0 + C1Vbar, (4.5)

where C0 and C1 are constants of proportionality which depend on the dimensions

and geometry of the bar electrodes. For Vbar=0, there is no field at the center due to

the electrodes and hence, Eq. (4.5) gives C0=0. For our trap, C1 is estimated to be

1.41·10−2 mm−1 by simulations performed using a commercially available 3D electric

field solver [80]. Note that this simulation is carried out under the electrostatic

approximation and hence, the endcaps are considered to be grounded as they carry

only AC voltage. From the linear fit shown in the Fig. 4.9b, the magnitude of Vbar

required for the centering of the trapped tetrapod is about 8 V. Hence, the value of

E(0) can be calculated from Eq. (4.5) to be 0.113 V/mm.

Now, to estimate the charge value, the only thing yet to be determined in

Eq. (4.4) is the mass of the trapped tetrapod. From Fig. 4.9a, the leg lengths

of the tetrapod are estimated to be about 30 µm. The diameter of the legs, how-

ever, cannot be estimated from the images. Since the tetrapod under consideration

has fairly long legs, we assume that the diameters are also considerably large. As-

suming a cylindrical geometry with a diameter of about 800 nm for the legs, the


mass of the tetrapod can be calculated to be 3.4·10−13 kg. The density of ZnO used

for these calculation is 5.6 g/cm3. As a result, using these values it is estimated that

qtp=1.8·105. Note that this is only an estimate and the surface charges on various

tetrapods might vary depending on their sizes and the electrospray parameters. In

this way, we used micromanipulation to estimate the surface charges on the trapped

tetrapods.

Figure 4.9: Use of micromanipulation to determine the charge on a single trappedtetrapod. (a) Images showing the position of the trapped tetrapod for differentvoltages on the DC electrodes. (b) Plot of the tetrapod distance from the trapcenter as a function of the voltage on the DC electrodes.

The next aspect of micromanipulation is rotational control which, for absorbing

microparticles, can be achieved by polarized optical fields. Based on this concept,

the next section discusses our idea to achieve rotational control over the tetrapod

along with related experiments.


4.4.3 Towards Rotational Control

The optical torque effects produced by circularly/elliptically polarized light beams

are known since the mid 1930s [102]. The general idea behind optical torques can be

explained with the help of basic mechanics. If circularly/elliptically polarized light is

incident on an absorbing particle, the absorption leads to a loss of the incident angu-

lar momentum. According to the law of conservation of angular momentum, this lost

angular momentum must be imparted to the absorbing particle. Consequently, the

particle increases in mechanical angular momentum and thus experiences a torque.

Apart from the absorptivity and size of the particle, the generated optical torque

depends on the wavelength, power, spatial profile, and polarization of the incident

beam. Detailed information and calculations on optical torques can be found in

literature [103]. In most cases, optical torques are negligible due to their small mag-

nitude. However, these effects become observable for highly absorbing particles of

microscopic sizes [104].

If a rotating microscopic particle is located in a viscous medium, like gases or liq-

uids, it experiences a damping torque which is proportional to its angular speed [105].

Hence, a particle inside a viscous medium will accelerate till the damping torque

equals the incident optical torque and subsequently, will rotate constantly at the

acquired angular speed. The sense of rotation (clockwise or counterclockwise) of the

particle is the same as the incident angular momentum. Therefore, variation of the

incident optical torque can be used for controlled rotation of microscopic absorbing

particles. Previous experiments have demonstrated such control over microparticles

by manipulating the polarization properties of an elliptically polarized gaussian laser

beam. The observed angular speeds are typically in the range of 1-25 Hz [106].

Our idea of achieving rotational control over the trapped tetrapod employs the

above mentioned concept. The tetrapods are micron sized particles and are strongly

absorbing for our pump wavelength of 355 nm. Therefore, in principle, by controlling

the polarization characteristics of the pump beam, it should be possible to rotate the

trapped tetrapod. The pump beam as generated by the Q-switched laser is linearly

polarized and has a gaussian intensity profile. Note that all the measurements shown


in this chapter till now are performed with the linearly polarized pump beam. To

make the pump beam elliptically polarized, a linear polarizer and a λ/4 plate, both

designed to work at 355 nm, are respectively installed in the beam path. First, the

linear polarizer is introduced and its pass axis is adjusted and fixed exactly along

the polarization of the pump light. This is done to precisely determine the pump

laser polarization and also to remove any possible randomly polarized stray light.

Subsequently, the λ/4 plate is introduced in the beam with its fast axis overlapping

with the pass axis of the polarizer. Note that the polarizer and the λ/4 plate

are mounted on calibrated rotational mounts. Now, by rotating the λ/4 plate with

respect to the pass axis of the polarizer, the ellipticity of the pump laser polarization

can be manipulated.

Subsequently, the experiments to attempt to rotate the trapped tetrapods are

carried out with the elliptically polarized pump beam. However, it is discovered

that our Q-switched laser is not a feasible source to achieve rotational control in

our setup. For the rotation of fairly large microparticles like tetrapods, a large

pump fluence is required to generate sufficient optical torque. However, before such

high fluences are reached in our setup, the impulse of the radiation due to the high

peak powers of the pump pulses makes the trapping unstable. A fluence of up to

100 mJ/cm2 can be sustained by the tetrapods after which they are kicked out of

the trap by the beam. Up to this fluence value, no apparent rotation of the tetrapod

is observed.

As a solution to this problem, we propose that CW lasers (absorbed by the

tetrapods) be used in future experiments. This is because the optical torque is pro-

portional to the average power of the laser and not to the peak power. Compared

to a CW laser, for the same average power, a pulsed laser delivers a much higher

impulse to the particle because of the short pulse width. Therefore, if CW lasers

are used, the tetrapods can be exposed to higher average powers without creating

sufficient impulse to make the trapping unstable. And hence, considerable optical

torque required for controlled rotation of the tetrapod could be achieved. We, how-

ever, could not attempt this yet because of the unavailability of the required high


power CW source at our disposal. Note that most previous studies demonstrating

rotational control also employed CW lasers in their experiments [104, 106].

At this point, most of our significant experiments performed on the trapped

tetrapods have been discussed. The next experiments, as shown in the coming sec-

tion, are performed on glass substrates with an inverted microscope. These measure-

ments help us in better understanding of the tetrapods and enable the comparison

between their optical properties in the trap versus on the substrate.

4.5 Study of ZnO Tetrapods on a Glass Substrate

In the past, several experiments have studied ZnO nanostructures such as nanowires

and tetrapods on substrates [60, 14]. Such setups use high NA optics which improves

the signal collection efficiency and the imaging resolution. Moreover, sample prepa-

ration on substrates is fairly sinple (spin-coating is typically used) and therefore,

several samples can be investigated in quick succession. Additionally, in our case,

such measurements enable us to compare the properties of the tetrapod laser in the

trap versus the substrate. If there is an influence of the charges or the substrate ma-

terial on the optical characteristics of the tetrapods, this comparison should clearly

point it out. We investigate our tetrapod samples on glass substrates with the help

of a home-built inverted microscope. The details of the experimental setup are given

in the coming section.

4.5.1 Experimental Setup

The schematic of the inverted microscope setup is shown in Fig. 4.10. The salient

features of the setup are as following. The system operates on a 100 X microscope

objective (Olympus MPlan) with a numerical aperture (NA) of 0.95 and a working

distance of 210 µm. The option of using a 50 X objective (of similar kind) with a nu-

merical aperture of 0.55 also exists. For optical alignment purposes, the objective is

mounted with a combination of a gimball mount (for rotation) and a hand-controlled

XYZ stage (for coarse translation). A piezo controlled 3D XYZ stage (Tritor 102,

Piezo Systems Jena), with a resolution of 10 nm over a range of 80 µm in all the

4.5 Study of ZnO Tetrapods on a Glass Substrate 71

three dimensions, is used for precise positioning of the sample. The piezo stage

is mounted on a hand-controlled XY translational stage for coarse control of the

sample position. For imaging, the samples can be illuminated by employing a fiber

lamp as a white light source.

Figure 4.10: The schematic of the inverted microscope setup. The inset shows theimage (under white light illumination) of a 12±.25 µm bead used for calibration ofthe system.

The linearly polarized (TM) pump beam is focused on the substrate with a lens

(f = 25 cm) from the side at an angle θ = 100. The spot size diameter for the 355 nm

beam at the focus of the 25 cm focal length lens is about 187 µm. However, due to

the angle θ between the substrate and the pump beam, the spot on the substrate

becomes elliptical in shape. The short axis of the ellipse remains as 187/2=93.5 µm

while the long axis becomes 187/(2sinθ) = 1.1 mm. The area of such an ellipse

is 0.16 mm2 which will be used to estimate the effective pump fluences. Note that

pumping in the confocal geometry is not favorable for this application. This is

because for such a high NA, it results in a sub-micron spot size on the substrate

and hence, does not lead to the uniform illumination of the tetrapods. Moreover,


pumping from vertically above the substrate is also avoided because then the intense

pump laser is directly along the signal collection path and therefore, might be hard

to completely filter out.

Our microscope objectives are infinitely corrected. Therefore, the light collected

signal is collimated as shown by the violet beam in the Fig. 4.10. The collimated

signal is then focused through a pinhole with the help of Lens-1 (f = 10 cm).

Such a scheme enables the selective study and imaging of different parts of a single

tetrapod. Depending on the requirement, the diameter of the pinhole can be chosen

to be from 25 to 200 µm. Lens-2 (f = 10 cm) is used to collect and refocus the light

either on to a CCD or in to a multimode fiber by adjusting a flipper mirror. The

CCD signal is used for imaging purposes and the multimode fiber is attached to the

spectrograph (with EMCCD) for spectral measurements. Such a geometry ensures

that the image acquired and the measured spectra correspond to each other.

The samples are prepared on glass cover slips (thickness = 170 µm, area = 2.5 x

2.5 cm2). The system is calibrated with the help of polystyrene beads of known size.

A solution of 12±.25 µm diameter beads in isopropanol is sonicated and subsequently

spin-coated on the cover slip. The inset of Fig. 4.10 shows the CCD image of such

a bead under white light illumination. Hence, the value of an unknown dimension

can be found out by comparing the number of pixels it occupies on the CCD image

with the corresponding value for the 12 µm bead.

The tetrapod samples are also prepared by spin coating their dilute solution

in methanol on the cover slips. The methanol evaporates quickly which results in

isolated tetrapods dispersed on the substrate. The samples are then placed on the

piezo stage for measurements.

4.5.2 Lasing on Substrate vs Lasing in Trap

The inverted microscope setup can be used to simultaneously image and spectrally

investigate the tetrapods. For lasing measurements, a suitable single tetrapod is

located on the substrate using the CCD camera as shown in Fig. 4.11a. Note that

the tetrapod is fairly large and therefore, only one of the legs is shown in the im-


age. Now, once the tetrapod is chosen, the substrate position is kept unchanged.

Subsequently, the excitation laser is turned on to illuminate the tetrapod and mea-

surements similar to section 4.3.2 are performed. Note that the measurements shown

here are performed with the 50 X microscope objective without the pinhole.

Figure 4.11: The magnified images (50 X objective) of a tetrapod on a glass sub-strate acquired with the inverted microscope setup under a) white light illumination,and b) lasing conditions.

The image of the lasing tetrapod is shown in Fig. 4.11b and the lasing threshold

behavior is given by Fig. 4.12. The spectra are averaged for 200 excitation pulses

and integrated in the range between 385-393 nm. The threshold fluence value for

the onset of lasing is observed to be around 25 mJ/cm2. The insets at the top

left and bottom right corners represent the spectra below and above threshold,

respectively. The lasing mode structure (bottom right inset) agrees well with the

length of one leg measured from Fig. 4.11b according to the Eq. (4.1). At high

fluences above 45 mJ/cm2, the linearity of the threshold curve is disturbed. This is

most likely because of the observed positional vibrations of the tetrapod (induced

by pump radiation pressure). Moreover, at low fluences, the spectra is expected to

be dominated by the leg facing the camera. However, at high fluences, the lasing

signals from other legs might also become significant leading to the disturbance in

the linear threshold behavior.

For comparison, it is interesting to see that the lasing properties for single


tetrapods on substrate, namely the threshold and the mode structure, are very

similar to those in the trap. The threshold values are about 10 mJ/cm2 (Fig. 4.6)

and 25 mJ/cm2 (Fig. 4.12) for the trap and the substrate, respectively. Although

we always observe slightly higher thresholds on the substrate, this does not neces-

sarily mean that it is an intrinsic effect. The differences in the experimental setups,

the collection optics, the orientation and sizes of the tetrapods are most likely the

cause for the different threshold behavior in the two cases. In addition, the scat-

tering losses due to the substrate also contribute in raising the threshold. However,

the observed mode structure on substrates, just like the trap, still indicates that

individual legs acting as independent cavities.

Figure 4.12: The lasing threshold behavior for the tetrapod of Fig. 4.11. Thespectra are averaged for 200 excitation pulses and integrated in the range between385-393 nm. These measurements are performed without a pinhole.

With these measurements, we conclusively show that the charging and the ex-


ternal electrodynamic fields do not hamper the optical properties of ZnO tetrapods.

We believe that this also holds true for semiconductor nanostructures of different

geometry and composition as long as the regime of quantum confinement is not

reached. Therefore, our approach based on electrodynamic trapping also has great

potential to carry out fundamental and applied studies even on other nanomaterials.

In addition, an electrodynamic trap offers several advantages over substrates. For

example, it can possibly serve as an efficient tool for micromanipulation, transport

and assembly of such structures.

4.5.3 Q Factor Estimation of Lasing Modes

Due to the presence of surface PL, possible signals from multiple legs, and potential

intercavity coupling effects, the lasing measurements in the trap are not ideal for

calculating the Q factor of the modes. The leakage of the longitudinal cavity modes

from the leg of a tetrapod is highly directional. The output is maximum along the

direction perpendicular to the end facets. Therefore, to estimate the Q factor of

the lasing modes, it is desirable to collect the signal only from the end facets of

one of the legs. This helps in efficient collection of the modes from a single cavity

and also eliminates the contribution of the surface PL. Our inverted microscope can

acquire isolated signals from different parts of the tetrapod by selective imaging

(with the pinhole) and hence, is used for measuring the Q factor values. Note that

the measurements shown in this section are performed with the 100 X microscope

objective.

Using the inverted microscope setup, a tetrapod is located (on the glass sub-

strate) such that one of its legs points along the longitudinal axis of the microscope

objective. In other words, the end facet of such a leg is parallel to the substrate.

Subsequently, through selective imaging, the PL signal from the end facet is iso-

lated. Fig. 4.13a shows the PL spectra (above lasing threshold) of such a facet

of a tetrapod with leg length (L) of about 25 µm. The spectra clearly shows five

cavity modes with almost uniform spectral separation (∆λ∼ 1 nm). The measured

L and the observed ∆λ are in accordance with Eq. (4.1) and hence, reconfirm the


Fabry-Perot type behavior. Compared to Fig. 4.4, the background PL is consider-

ably reduced making the modes very distinct. Also, the selective imaging ensures

that the measured signal does not consist of contributions from other legs of the

tetrapod.

Figure 4.13: (a) The PL spectra of a 25 µm leg length tetrapod acquired with theinverted microscope. (b) The zoomed in spectra for the mode at 388.7 nm used forthe Q factor estimation.

To estimate the Q factor, the mode centered at 388.7 nm is chosen as it has the

highest intensity. Using a gaussian fit, as shown in Fig. 4.13b, the linewidth (∆λ)

of the mode is calculated to be 0.31 nm. Therefore, the value of the Q factor of

the lasing mode can be calculated to be Q = λ/∆λ ∼1250. It is important to note

that the measured ∆λ from Fig. 4.13a is limited by the resolution of our spectral

equipment. Hence, the actual Q factor is expected to be higher than 1250.

4.5.4 Transverse Whispering Gallery Modes on the Tapered

Legs

An interesting aspect observed, but not discussed, in the high magnification images

of the trapped tetrapods is the presence of local regions of high intensities along the

legs. For example, Fig. 4.2b shows the bright regions of the green scattered light

along all the visible legs of the tetrapod. Also, similar behavior is observed for a

lasing tetrapod (leg pointing towards the top right corner) as shown in Fig. 4.5a.


Initially, this characteristic is thought to be due to the scattering from structural

imperfections in the tetrapod. However, as more samples are investigated, it is found

that this is a characteristic behavior of a trapped tetrapod. This is a good indication

that the bright regions are not an artifact but rather an intrinsic characteristic of

the tetrapod structure. However, due to the small NA of our imaging optics, it is

not possible to extract enough information from images (like Fig. 4.2b and 4.5a) to

explain the bright regions. With the inverted microscope, these technical limitations

can be overcome and therefore, the effects at hand can be investigated.

Figure 4.14: (a) A magnified image (100 X objective) of an optically pumpedtetrapod leg showing the WGMs along the taper. The inset is a similarly magnifiedimage of the same leg under white light illumination (b) Comparison of the WGMbehavior observed in (a) with theoretical simulation.

Fig. 4.14a shows the magnified image acquired with the 100 X objective, of a

leg of an optically pumped tetrapod. It shows the leg tapers as it moves away

from the tetrapod core and along the taper, there are almost equally spaced bright

regions. Each bright region consists of two spots which are located at each end of the

transverse cross-section. This is a clear signature of the WGM type behavior and has

been reported recently in ZnO tetrapods [107]. As shown is Fig. 4.1b, the legs of our

tetrapod samples tend to taper. Typically, the diameters can be a few microns near

the core and can taper below 100 nm at the end facet. Now, for certain diameter


values in this range, the emitted PL (around 390 nm) satisfies the WGM resonance

condition. Hence, the PL in the regions around these diameters is enhanced similar

to that in the case of an optical microdisk cavity [10]. The presence of the two bright

spots at each region is due to the tangential leakage, along the direction of signal

collection, of two counterpropagating modes. Note that parts of Fig. 4.14a appear

to be defocused because all the WGMs do not lie in the same horizontal plane due

to the tapering of the leg.

Figure 4.15: (a) The schematic of a hexagonal tetrapod waveguide taken fromliterature (see text for reference). The white arrows inside the hexagon representthe propagating WGMs. (b) The SEM image of the end facet of one our tetrapodsshowing its hexagonal shape.

The cross-section of the ZnO tetrapod legs is hexagonal in geometry. Fig. 4.15a

shows the schematic of a hexagonal tetrapod waveguide taken from literature [108].

The white arrows inside the hexagon represent a circulating WGM. The SEM image

of the end facet of one of our tetrapods, as shown in Fig. 4.15b, indicates that our

samples also have a hexagonal cross-section. From theory [108], the WGM resonance

condition for such a hexagonal cavity is given by

Ri =λ

6n[N +

6

πarctan(β

√3n2 − 4)], (4.6)

where, Ri is as shown in Fig. 4.15a, λ is the wavelength of light, n is the refractive

index of ZnO, and N is the mode number. The factor β depends on the polarization


of the pump light and is given by n and n−1 for TE and TM (our case) polarizations,

respectively. Using Eq. (4.6), the corresponding mode numbers are calculated for

the resonant diameters measured from Fig. 4.14a. Moving towards the core, the

mode numbers (rounded off to their nearest integer) consecutively range from 12

to 20. Subsequently, these integral mode numbers are plotted with respect to their

corresponding measured diameters as shown in Fig. 4.14b. For comparison, a simu-

lation carried out with the help of Eq. (4.6) is also shown. The figure shows that for

the same mode number, the measured and the simulated diameter values are in close

accordance with each other. Hence, this conclusively shows that the bright regions

are caused by a WGM like behavior on the tapered ZnO leg. Note that the above

analysis is carried out for n = 2.2 and λ = 390 nm. Also note that the leftmost

bright spot at the end facet (in Fig. 4.14a) is not considered as it corresponds to the

leakage of the longitudinal cavity mode of the leg.

To investigate the spectral characteristics of the WGMs, the isolated signal from

single/few bright regions are studied (with no contribution from other parts of the

tetrapod). Fig. 4.16a shows the image of an optically pumped tetrapod (above

threshold) used for this purpose. The fourth leg of this tetrapod points into the

plane of the image. Fig. 4.16b, showing the WGMs, is the image of one of the legs

of the same tetrapod. Three different locations to be investigated are chosen on the

leg and are denoted by region 1, 2, and 3. By selective imaging, the intrinsic spectra

of the center and the three regions are acquired and are shown in Fig. 4.16c. The

selective image of the corresponding regions is shown as insets in the graphs. For

comparison, the graphs are normalized and plotted on the same wavelength axis.

Moreover, fine grid lines are drawn as a guide to the eye.

The obtained spectra for regions 1, 2, and 3 appear to be similar as most modes

spectrally overlap within the resolution limit (0.1 nm) of our system. Moreover,

these modes are also detected in the corresponding spectra taken for the center of

the tetrapod. Now, the center of the tetrapod does not support WGM type behavior

due to its complicated geometry. Therefore, the modes measured at the center only

arise from the longitudinal Fabry-Perot type cavity of the different legs. However,


since most modes in regions 1, 2, and 3 are also present at the center, it indicates

the leakage of the Fabry-Perot modes into WGMs.

Figure 4.16: (a) Image of the center (core) of an optically excited tetrapod. (b)Image of the WGMs on one of the legs of the tetrapod shown in (a). (c) Normalizedspectra (acquired by selective imaging) of different regions of the leg in (b). Theinsets in each spectra show the image of the corresponding region.

For a uniformly excited tetrapod, the Fabry-Perot type cavity of the legs is the

most efficient. Hence, the density of the photons corresponding to the feedback of

this cavity dominates inside the tetrapod. Due to slight structural imperfections or

scattering centers, this dominant light is prone to leakage into other possible cavities.

Now, the legs of the tetrapod taper smoothly and hence, the diameter changes

continuously over a range of 100s of nanometers. This allows for the dominant light

to always find the corresponding resonant diameters which satisfy the Eq. (4.6).

Therefore, at these diameters the local density of the Fabry-Perot mode leakage is

4.6 Summary of the Chapter 81

enhanced by WGM type coupling. The longitudinal spatial spread of a single bright

region (in Fig. 4.14a and 4.16b) is expected because the leaked light has a multimode

structure (not monochromatic). Hence, for a given mode number, the corresponding

resonant diameter value has a small spread.

4.6 Summary of the Chapter

To sum up, this chapter presented our results involving the trapping, lasing, and

micromanipulation a single ZnO tetrapod. The chapter started with a brief dis-

cussion on the preparation and structural properties of our ZnO tetrapod samples.

Following this, our technique to electrodynamically trap a single ZnO tetrapods

under atmospheric conditions was demonstrated. Later, Fabry-Perot type UV las-

ing around 390 nm achieved from such trapped tetrapods (single and multiple) was

shown. Also, precise translational micromanipulation of the tetrapods was demon-

strated and the possibility of achieving rotational control was discussed. The charges

on a single tetrapod, using translational micromanipulation, were estimated to be

about 1.8·105. In the end, the tetrapods were optically investigated on substrates

using an inverted microscope setup. The lasing characteristics on substrates are

found to be similar to that in the trap. In addition, the quality factor of the tetra-

pod lasing modes was estimated to be larger than 1250. Moreover, WGMs were

observed, and theoretically explained, on the tapered legs of the tetrapods.

Electrodynamic trapping is a versatile tool and hence, is ideal for fundamental

and applied studies of a variety of microscale particles. If the trapped microscale

particles are of liquid state, they serve as efficient optical microcavities due to their

spherical shape caused by the surface tension of the liquid. Such drops also act as

microscale lasers in the presence of linear or non-linear optical gain. Along this line

of thought, the next chapter will discuss the development and characterization of a

microdrop laser where the optical gain comes from Raman scattering in glycerol.


Chapter 5

Raman Lasing inElectrodynamically TrappedGlycerol Microdrops

In this chapter, the Raman lasing characteristics of pure glycerol microdrops are

discussed. Our technique of trapping and isolating a single glycerol microdrop is

shown in section 5.1. In section 5.2, the continuous-wave (CW) Raman spectroscopy

of a trapped glycerol microdrop is presented. Subsequently, section 5.3 shows our

observations of Raman lasing at around 630 nm from the trapped microdrops. The

temporal evolution, the interpretation, and the possible manipulation of the on/off

behavior of the Raman lasing are presented in section 5.4. Finally, section 5.5

discusses our experiments attempted to improve the lasing performance by doping

the glycerol microdrops with Ag nanoaggregates.

5.1 Trapping of a Single Pure Glycerol Microdrop

Glycerol (Propane-1,2,3-triol) is a colorless, odorless and water soluble liquid with a

chemical composition of C3H5(OH)3. It is also fairly viscous at room temperature.

One of the main reasons for the high viscosity of glycerol is the presence of three -OH

groups (in the glycerol molecule) which leads to strong intermolecular forces due to

the formation of hydrogen bonds. For comparison, the viscosity of glycerol (1.5 Pa·s)is more than 1000 times higher than the corresponding value for water (1 mPa·s)at room temperature. For such highly viscous liquids, electrospray ionization is a

84 Raman Lasing in Electrodynamically Trapped Glycerol Microdrops

widely used technique for microdrop generation. However, for these liquids, a stable

cone jet mode operation (discussed in section 3.3.1) of the electrospray is hard to

achieve. As a result, a reproducible control over the diameter range of the spray is

difficult to achieve through manipulation of the electrospray parameters. Detailed

experiments demonstrating the electrospray characteristics of glycerol are reported

in literature [109].

In our setup, the selection and trapping of a glycerol microdrop of a desired

size is rather achieved by careful tuning of the trap parameters. The arrangement

and the geometry of our endcap trap is discussed in section 3.4. The top port

of the trap chamber, which has a removable grounded aperture of 3 mm in its

center, is used for sample introduction into the trap. The syringe of our electrospray

(described in section 3.3.2) is filled with commercially available 99.5 % pure glycerol

(Manufacturer: Carl Roth GmbH). It is mounted vertically above the trap chamber

such that the tip of its metallic needle is approximately 1 cm above the aperture. The

needle is connected to the positive terminal of an external high voltage DC power

supply. The endcap trap is then turned on with no voltage on the DC bar electrodes.

Finally, application of a high voltage (2.5-3 kV) to the needle, combined with a gentle

back pressure on the syringe, results in the introduction of charged microdrops into

the chamber. Depending on the trap parameters, multiple microdrops (of fairly

similar sizes) are subsequently trapped between the endcaps.

The typical diameter sizes of the drops studied in our experiments lie between

10-50 µm. The behavior of our endcap trap is discussed previously in section 3.4.

In our trap, V0 is kept constant at 1 kV. However, selective trapping of drops of

different sizes is achieved by tuning of the parameters f and Vdc. For trapping

bigger drops (diameter ∼ 40-50 µm), f = 100-150 Hz and Vdc = 0 are favorable

parameters. However, if f = 450-500 Hz and Vdc = -60 to -80 V are chosen, the

trapping of smaller drops (diameter ∼ 10-20 µm) is preferred. Drops of intermediate

sizes (diameter ∼ 20-40 µm) are usually trapped by choosing f and Vdc between the

above indicated two extremes.

A green CW laser (1 mW) is employed to illuminate the trapped drops and the

5.1 Trapping of a Single Pure Glycerol Microdrop 85

scattered light is used to image the drops. In a typical trapping routine, multiple

drops of similar sizes are trapped. The next critical task is the isolation of one

single microdrop in the trap. As the trapped drops are similar but not exactly of

the same size, their respective regimes of stable trapping (discussed in section 2.1)

slightly differs. Therefore, precise tuning of f and Vdc is done to gradually shift the

stability regime of the trap which enables the elimination of individual microdrops

in succession. The tuning is stopped when a single microdrop is left in the trap. For

example, as f is decreased, the smaller drops become unstable and if Vdc is reduced,

the bigger drops tend to leave the trap. When a single drop is isolated, it is trapped

vertically below the geometrical center of the trap due to the gravitational attraction.

Hence, the DC voltage on the surrounding bar electrodes (Vbar) is adjusted to bring

the microdrop to the center of the trap and compensate gravity. Centering the

drop in the trap is extremely important because at the center, the amplitude of the

drop motion is zero in all the three dimensions, therefore, it can be considered as

a stationary microcavity. Moreover, the center point experiences zero electric field,

thus, extraneous effects due to the trapping field are negligible.

Figure 5.1: A trapped 45 µm diameter glycerol microdrop.(a) CCD image of thegreen light scattered from a trapped and coarsely centered microdrop (b) High mag-nification (40X) image of the same microdrop after precise centering.

Coarse centering is initially done with the CCD camera image (as shown in

Fig. 5.1a) and then precise centering is achieved with the help of the high magnifi-


cation setup (discussed in section 3.5). Fig. 5.1b shows the high magnified image of

a precisely centered 45 µm drop. It shows the typical three glare spots (mentioned

in section 3.5) of the scattered green light which suggest that the drop is stationary.

These spots appear to be lines if the drop is not perfectly centered which indicates

that the drop is in motion. As expected, bigger drops require a higher DC voltage

for gravity compensation as compared to smaller drops. Due to the fact that the

drops are positively charged [75], typical (Vbar) values are negative and lie between

-10 V to -80 V. Once a single drop is trapped and centered, the system is ready for

measurements and investigations.

5.2 CW Raman Spectroscopy of a Trapped Glyc-

erol Microdrop

One of the first measurements performed on the trapped glycerol microdrop is CW

Raman spectroscopy. More details about the theory of Raman scattering have been

discussed previously (section 2.4). Apart from elucidating the various Raman bands

present in glycerol, this measurement also makes sure that our spectral detection

scheme is sensitive enough to measure the Raman signal from the drop. After

centering the trapped microdrop, the trap chamber is sealed to eliminate any dis-

turbances due to air currents. The pump laser is then focused on the microdrop

and the elastically scattered light is filtered (with a notch filter) in the spectral de-

tection. The pump laser used here is the high power (0.2-5 W) CW Nd:YVO4 laser

at a wavelength of 532 nm (see section 3.2).

For a better signal to noise ratio, high pump intensity (500-1500 W/cm2) and

long exposure time of the EMCCD (1-10 s) are chosen in our measurements. At

such high powers, the radiation pressure of the beam might slightly displace the

microdrop vertically from the trap center. Therefore, the high magnification images

are continuously monitored and Vbar is tuned accordingly to maintain the centered

position of the drop. Fig. 5.2 shows the Raman spectra (Stokes Shift) measured

from a 45 µm drop (the same drop as in Fig. 5.1), where the pump intensity is 1000

W/cm2 and the exposure time of the EMCCD is 5 s.

5.2 CW Raman Spectroscopy of a Trapped Glycerol Microdrop 87

Figure 5.2: Normalized CW Raman Spectra of a 45 µm pure glycerol microdrop.

CW Raman spectra are recorded for several drops in the size range of 10-50 µm

for comparison. It is found that all the drops, irrespective of their sizes, exhibit the

same peaks (as in Fig. 5.2) in their CW Raman spectra. No dominant peaks are

observed corresponding to the microdrop cavity feedback which suggests bulk like

behavior.

Raman scattering characteristics of bulk glycerol have been investigated in detail

in literature [110]. By comparison with previous reports, assignment of the respec-

tive chemical bonds to most of the peaks observed in Fig. 5.2 is done and is shown in

Table 5.1. However, the peaks at 729 cm−1, 1603 cm−1, and 2368 cm−1 (in Fig. 5.2)

cannot be accounted for and are believed to be present due to the impurities in the

99.5 % pure glycerol. Note that Anti-Stokes Raman spectroscopy was also carried

out (on the drops) but no detectable Raman bands were observed towards the blue

side of the pump laser.


Band Center (cm-1

) Contributing Bond

475 CCO Rock

541 CCC Deformation

910 CH2 Rock

970 CH2 Rock

1114 CO Stretch

1312 CH2 Twist

1515 CH2 Deformation

2776 CH Stretch from C-2

2930 Symmetric CH Stretch from CH2

2971 Antisymmetric CH Stretch from CH2

3405 Antisymmetric OH Stretch

Table 5.1: Bond assignment to the different Raman peaks observed in Fig. 5.2.

After examining the CW Raman characteristics of the microdrops, we inves-

tigated their Raman lasing behavior. These investigations along with the related

discussion follow in the coming section.

5.3 Observation of Raman Lasing Near 630 nm

The basic requirements for the development of a laser, as discussed before, are: an

optical feedback mechanism, a gain medium, and a pump source to excite the gain

medium. A trapped stationary glycerol microdrop is spherical in shape and therefore

serves as an efficient optical microcavity. It can also provide non linear optical gain

through the Raman scattering in glycerol. Therefore, the only other requirement

5.3 Observation of Raman Lasing Near 630 nm 89

left is the pump laser. Use of pulsed pump lasers (as pump sources) is favorable

because their high peak powers produce a considerable density of Raman photons

in the microcavity for stimulated emission to happen (see section 2.4). We use a

Q-switched Nd:YAG laser (frequency doubled) at a wavelength of 532 nm as the

pump source. It produces 10 ns broad pulses at a repetition rate of 10 Hz. The laser

is focused to a spot size of 360 µm (FWHM) on the trapped glycerol microdrop.

The spot size is measured with the help of the knife-edge technique.

Figure 5.3: Raman lasing spectra of electrodynamically trapped glycerol micro-drops. Black curve -Raman spectrum of glycerol drops between 612-662 nm underCW (532 nm) excitation. Blue curve - multimode lasing at 628.4 nm and 630.9 nm,respectively, from a 44.7 µm drop. Red curve (scaled up 5 times) - Single modelasing at 629.6 nm from a 10.3 µm drop. The blue and red curves are measured forsingle pulse Q-switched (532 nm) excitation of the drop.

To start, the pump laser is incident on the particle and the elastically scattered

light into the detection system is filtered. The exposure time of the EMCCD is set

to 0.09 s to record single pulse excitation spectra. A low pump fluence of around

100 mJ/cm2 is initially chosen such that the detected Raman signal is just above


the noise limit. The pump fluence is then gradually increased in small steps and the

corresponding single pulse Raman spectra are recorded. As the fluence exceeds the

threshold value, the Raman gain exceeds the cavity losses and lasing is observed.

Among the several peaks in the CW Raman spectra of glycerol (Table 5.1),

the symmetric (2930 cm−1) and anti-symmetric (2971 cm−1) -CH stretch bands

(from -CH2) have the highest Raman gain. Consequently, during stimulated Raman

scattering, the density of Raman photons corresponding to these peaks dominates

heavily in the cavity. Hence, only these bands give rise to lasing and overshadow

the contribution of other Raman bands. As our pump source is at 532 nm, these

bands are observed to be centered at 628.3 nm and 630.2 nm, respectively.

Typical room-temperature single and multimode lasing along with the CW Ra-

man spectra from pure glycerol microdrops in the spectral range of 612- 662 nm are

shown in Fig. 5.3. As a guide to the eye, the black curve (dotted) shows the CW

Raman spectra of the glycerol drops. As mentioned before, the bands at 628.3 nm

and 630.2 nm are the ones that lead to lasing. The broad band at 648 nm is the con-

tribution of the OH stretching modes in glycerol. The blue curve shows multimode

lasing at 628.4 nm and 630.9 nm from a 44.7 µm diameter drop for a single pump

pulse excitation at a fluence of 490 mJ/cm2. The two modes are observed to be

spectrally separated by 2.5 nm. Assuming that these modes are of the same order

and polarization, the wavelength separation (∆λ) between them can be theoretically

estimated [61] and is given by

∆λ =λ2 arctan(m2 − 1)1/2

2πr(m2 − 1)1/2, (5.1)

where λ is the mean wavelength of the two modes, m is the refractive index of the

medium, and r is the radius of the cavity. For the drop in discussion, λ=629.7 nm,

2r=44.7 µm, and the refractive index of glycerol (m) = 1.47. Using these values, ∆λ

can be estimated to be 2.2 nm which is close to the experimental value of 2.5 nm.

The other important aspect of the blue curve in Fig. 5.3 is the presence of only two

lasing modes and not more. This is because the high Raman gain region (black

curve) is ∼ 5 nm in bandwidth (FWHM) and the intermodal separation (∆λ) for

the corresponding drop is ∼ 2.5 nm. Hence, the gain region can only overlap with


a maximum of two modes giving rise to the observed spectra.

Figure 5.4: (a) High magnification (40X) image of a trapped 37 µm pure glyceroldrop under green CW illumination showing the 3 typical glare spots. (b) Thesame drop, as in (a), under lasing conditions (pump light filtered) exhibiting thecharacteristic red lasing spots. (c) Schematic to explain the occurrence of the pairof red lasing spots.

Eq. (5.1) shows that for a given λ, as the drop size (r) decreases, the intermodal

separation (∆λ) increases. Hence, if r decreases to a value such that ∆λ is more

than the FWHM (5 nm) of the gain (black) curve, only single mode lasing will

be supported by the microdrop cavity. In our experiments, single mode lasing is

typically observed for drops of diameters less than 20 µm. This is justified as,

according to Eq. (5.1), ∆λ for such a drop can be estimated to be about 4.9 nm.

The red curve (scaled up by a factor of 5 for better visualization) shows single mode

lasing at 629.6 nm from a 10.3 µm diameter drop for single pump pulse excitation

at a fluence of 490 mJ/cm2. Note that in Fig. 5.3, for the same fluence, Raman


lasing is weaker for the smaller drop. This is mainly because the smaller drop

experiences a smaller cross-section of the pump beam. Also note that the linewidths

(FWHM) of all the lasing modes in Fig. 5.3, limited by the resolution of our spectral

measurements, are measured to be about 0.7 nm.

Fig. 5.4a shows a magnified (40X) image of a 37 µm glycerol drop under green

CW illumination. The three typical glare spots (mentioned in section 3.5) caused by

the scattered green light are clearly visible. Fig. 5.4b is the image of the same drop

(pump laser filtered out) under lasing condition at a pump fluence of 490 mJ/cm2.

The pair of characteristic red lasing spots [111] caused by tangential leakage of the

counter-propagating lasing modes can be seen in the image. The red color of these

spots agrees well with the multimode lasing observed around 629 nm in simultaneous

spectral measurements. The occurrence of these spots can be briefly explained with

the help of Fig. 5.4c.

In Fig. 5.4c, the microdrop is depicted by the thick circle and the pump laser

is shown by the vertical arrows (green). As the spot-size of the pump laser is 360

µm, considerably larger than the diameters of our microdrops, the pump beam is

considered as a plane wave. Due to the curvature of the microdrop and the difference

in the refractive indices of glycerol and air, refraction occurs (shown by tilted green

arrows) and the intensity of the pump beam is locally enhanced at two spots inside

the drop. These are depicted by the red ellipses. The enhancement of the local

intensity at these spots can be up to 10 times for the spot at the illuminated face

and up to 100 times for the spot at the shadow face [111]. These spots, due to

the high local pump intensity, experience a much higher Raman gain compared

to other parts of the microdrop. Therefore, the lasing threshold is first reached

by the counterpropagating WGMs which pass through these spots. Due to the

spherical symmetry of the microdrop, these modes can lie in every plane (inside the

microdrop) that includes both the high intensity spots. When the drop is observed

at a 900 angle to the incident pump beam, as in our case, most of the lasing signal

comes from the tangential leakage of the counterpropagating modes in the plane

parallel to the observation direction. Hence, the image of the drop appears to be


two bright spots (their color corresponds to the lasing wavelength), as shown in

Fig. 5.4b. Note that our spectral measurement scheme and the imaging optics are

setup in the same plane, as shown in Fig. 5.4c, to make sure that the obtained image

and the measured spectra correspond to same lasing modes.

Figure 5.5: Raman lasing threshold behavior of a 35 µm pure glycerol drop. Thecorresponding spectra above and below threshold are shown by the insets at the topand bottom, respectively.

The threshold values for Raman lasing for the microdrops are also investigated.

Fig. 5.5 shows such a measurement for a 35 µm drop where the threshold is observed

around 325 mJ/cm2. The bottom and top insets show the normalized lasing spectra

below (190 mJ/cm2) and above (370 mJ/cm2) threshold, respectively. As the fluence

increases above threshold, considerable increase in the lasing intensity (more than 2

orders of magnitude) and narrowing of the mode linewidths are good indicators for

the onset of lasing.


The Raman lasing for all the drops is found to be temporally fluctuating. In

other words, the lasing shows an “on/off” behavior even when the system parameters

are kept unchanged. Therefore, our threshold measurements are performed quickly

during an “on” period of lasing. This hinders precise measurements and hence, the

threshold curve in Fig. 5.5 only has a limited number of data points. Moreover,

two different “on” periods are observed to have different lasing intensities for the

same pump fluence, hence making the reproducible threshold curves very difficult.

Therefore, the threshold measurements can only provide us with an estimate of the

threshold fluence rather than its exact value. For all the drops in our size range (10-

50 µm), the threshold pump fluences are measured to lie between 200-390 mJ/cm2.

To better understand and explain the “on/off” behavior, detailed measurements of

the temporal fluctuations of the lasing intensity are carried out and are discussed in

the following section.

5.4 The On/Off Behavior of Raman Lasing

For a laser to find practical applications, its stable operation is one of the first

and foremost requirements. In the case of our glycerol microdrops, it is found that

the output lasing intensity fluctuates temporally (on/off) and stable operation is

extremely hard to achieve. When the trapped drops are exposed to a steady pump

fluence (above threshold), at different times the Raman lasing intensity is observed

to differ by up to a factor of 105 to 106. These fluctuations were initially thought to

be totally random as any apparent correlation between the operating parameters and

the on/off behavior was hard to find. To ensure that this was not an artifact of our

system, the average pump power constancy, positional stability of the microdrop,

and uninterrupted spectral measurements were reconfirmed. After performing these

checks, it is concluded that the fluctuations are not due to any external influences

but are an intrinsic characteristic of the glycerol microdrop Raman laser. Therefore,

we decided to investigate the Raman lasing signal from the microdrops in details in

an attempt to understand the reason for the fluctuations and to possibly find a way

to control them.

5.4 The On/Off Behavior of Raman Lasing 95

0 200 400 600 800 1000 1200 14000.0

5.0x105

1.0x106

1.5x106

2.0x106

Inte

grat

ed R

aman

Las

ing

Inte

nsity

Time (seconds)

Figure 5.6: Temporal evolution of the Raman lasing intensity for a 44.7 µm pureglycerol drop at a pump fluence of 490 mJ/cm2.

Long-term lasing intensity measurements are carried out on pure glycerol drops.

We record a series of lasing spectra (for single pump pulse excitation) up to 15000

pulses (25 minutes) for several drops of different sizes. Subsequently, using MAT-

LAB, lasing intensity is integrated between 624-634 nm for each spectrum and plot-

ted against the time of incidence of the corresponding pulse to obtain the temporal

evolution of lasing. Fig. 5.6 shows such a measurement for a 44.7 µm pure glycerol

drop at a pump fluence of 490 mJ/cm2. One of the first things that can be seen from

this figure is that the fluctuations of the Raman lasing are not completely random.

Lasing occurs in temporally separated, nearly symmetric bursts. In other words,

the lasing turns “on” during the occurrence of a burst and turns “off” afterwards.

Three bursts centered at 150 s, 420 s and 1180 s, respectively, are observed during

the measurement time window of 1500 s. The average duration of these burst is

about 265 s.


Another interesting aspect of Fig. 5.6 is the profile of the individual bursts. For

all the bursts, lasing builds up and decays in time in a symmetric fashion. Moreover,

the peak lasing intensities of the bursts are significantly higher than the intensities

during the “off” periods. The peak lasing intensities for the bursts at 150 s, 420 s,

and 1180 s are about 8·104, 7·105, and 1.2·106 respectively, whereas the “off” periods

intensities are close to zero. These features, in combination, are very good indicators

that the lasing bursts are associated with some type of resonance of the system. In

fact, the “Double Resonance Effect” can qualitatively explain the existence and

characteristics of these lasing bursts, and is discussed in details in the following

section. Note that the fluctuations of the pulse to pulse lasing intensity within each

burst are most likely due the slight deformations of the trapped drop which lead to

directional leakage of the lasing modes [112, 113]. This leakage may not be in the

direction of the collection optics for every pulse, hence resulting in the intraburst

fluctuations. Moreover, tiny fluctuations (pulse to pulse) of the pump laser fluence

may also contribute to this behavior.

5.4.1 Interpretation: The Double Resonance Effect

To explain the Raman lasing intensity fluctuations of our glycerol microdrops with

help of the “Double Resonance Effect” (DRE), it is important to first discuss the

DRE. As the name suggests, the DRE for a microcavity laser refers to a condition

where both the pump light and the lasing mode/modes are resonant with the WGMs

of the cavity. A resonance of the pump light with a WGM is called an “input

resonance.” Similarly, a resonance of the light produced by the gain medium with a

WGM is referred to as an “output resonance.”

For a simplified understanding of the input resonances, let us consider the picture

shown in Fig. 5.7. The concentric circles A, B, and C represent three microcavities

(same refractive indices of “m”) with diameters of d−∆d, d, and d+∆d, respectively,

where ∆d << d. The vertical green arrows represent the plane wave monochromatic

pump light at a wavelength λ. Consider that the value of d is such that the micro-

cavity B has an available WGM precisely at a wavelength of λ. As d is changed by


Figure 5.7: Schematic to explain the concept of “input resonance.” The verticalgreen arrows represent the plane wave monochromatic pump light. The circles, A,B, and C depict three microcavities with similar diameters and the same refractiveindex.

a small value ∆d, the WGMs shift and therefore, A and C will not have a WGM

exactly at λ. Now, if each of the microcavities A, B, and C individually experiences

the same pump fluence, the density of the pump photons will be considerably higher

for cavity B. This is because in cavity B, the pump photons couple to the available

WGM and spend a longer time in the cavity, thereby enhancing the overall pump

density inside the cavity. This condition is called an “input resonance.” This effect

is not observed for cavities A and C as there is no available WGM in the cavity for

the pump photons to couple to. Hence, for a fixed λ and a constant m, the input

resonances are determined by the size of the microcavity.

The concept of “output resonance” is very similar to that of the input resonance.

Excitation of the gain medium inside a microcavity (by the pump light) leads to

the generation of photons at a wavelength different than that of the pump. If the

size of the microcavity is such that these newly generated photons couple to one

of its WGMs, then this condition is known as an “output resonance.” When the

input and output resonances occur simultaneously for a microcavity, the condition

is called a double resonance. However, in our case, the high gain Raman emission

from glycerol is not monochromatic and is about 5 nm in bandwidth (FWHM).


For glycerol microdrops of diameters above 20 µm, the spectral spacing between

adjacent WGMs is less than 5 nm. Therefore, most microdrops in our diameter

range always have one or more WGMs that spectrally overlap with the Raman

gain, hence satisfying the output resonance condition. Our pump laser, however, is

spectrally much narrower (compared to the Raman emission) and satisfies the input

resonance condition only for certain drop sizes. Therefore, in our case, it can be

concluded that the occurrence of a double resonance is mainly determined by the

input resonances (as the output resonance condition is always satisfied for drops of

diameter greater than 20 µm). For drops between 10-20 µm, however, we repeat

the trapping process until a drop which satisfies the output resonance condition is

found.

Occurrence of a burst of lasing is closely related to the double resonance effect.

In general, since we cannot precisely control the sizes of our trapped microdrops,

the input resonance condition is initially not satisfied. Therefore, the microdrop

behaves like cavity C in Fig. 5.7. However, as the microdrop is pumped above

threshold, the excess heat generated causes it to evaporate and change slowly in

size. As the evaporating drop reaches a critical size (cavity B), where the input

resonance condition is satisfied, the density of pump photons inside the drop is

strongly enhanced. As a result, the increased Raman gain can overcome the cavity

losses resulting in lasing. With further evaporation, the drop behaves like cavity

A, and hence, lasing is ceased. Lasing repeatedly resumes as the evaporating drop

reaches other critical sizes, where the input resonance condition is again satisfied.

As the initial sizes of the trapped drops are different, the lasing fluctuation behavior

is also different for each drop.

In Fig. 5.6, the three bursts correspond to different sizes of the evaporating mi-

crodrop where the input resonance condition is satisfied at 532 nm. The varying

durations and amplitudes of the bursts can be attributed, respectively to the differ-

ent linewidths and scattering efficiencies of the corresponding input resonances [61].

The nearly symmetric profile of the bursts is a good indication that the drop evap-

orates at a fairly constant rate. Note that the small change in the size of the drops,


during the course of measurements, cannot be detected by our size measurement

technique. However, from theory [61], the adjacent input resonances (at 532 nm )

for a 44.7 µm pure glycerol drop correspond to a diameter change of 0.13 µm. In

Fig. 5.6, the average inter-burst separation is around 515 s. Hence, assuming that

the two alternate bursts correspond to adjacent input resonances, the rate of change

of the drop diameter can be estimated to be 0.25 nm/s, which is comparable to the

previously reported evaporation rate of glycerol microdrops [39].

This interpretation of the lasing fluctuations qualitatively explains all of our

experimental observations and results. However, to ensure its correctness, it is im-

portant to show that the lasing fluctuations are a function of the drop’s evaporation

rate. The following section will discuss this aspect in details.

5.4.2 Effect of Microdrop Evaporation on the On/Off Be-havior

Our interpretation of the Raman lasing fluctuations, based on the DRE, explains

the features of the on/off behavior very well. In the past, similar interpretations

have been reported to explain the bursts in the stimulated Raman spectra of glycerol

microdrops [39, 114, 115]. However, in a recent publication, an alternate explanation

to these fluctuations is proposed. It is reported here that the fluctuations are rather

due to the thermally induced random density fluctuations in the microdrop [41]. To

resolve this ambiguity, we carried out further investigations to conclusively ensure

the correctness of our interpretation.

If our interpretation is correct, the lasing bursts should be manipulable by al-

teration of the drop’s evaporation rate. An increase in the evaporation rate should

simultaneously increase the burst frequency and decrease the average duration. We

demonstrate this behavior by increasing the evaporation rate of the drop in two

ways; by addition of water in the drops and by increasing the pump fluence. As

water has a larger vapor pressure than glycerol under ambient conditions, addition

of water leads to higher evaporation rates in drops. Measurements are performed

at a constant fluence (above threshold) on several glycerol/water drops for glycerol


Figure 5.8: Temporal evolution of the Raman lasing intensity for (a) a 45.3 µm70 % glycerol drop at a pump fluence of 490 mJ/cm2 (b) a 46.5 µm 40 % glyceroldrop at a pump fluence of 490 mJ/cm2.


concentrations varying from 40 to 100 % and the results agree well with the ex-

pected trend. For example, Fig. 5.8 shows the temporal evolution of the Raman

lasing intensity at a fluence of 490 mJ/cm2 for two drops of similar sizes but with

different glycerol concentrations. Fig. 5.8a corresponds to a 70 % glycerol drop with

a diameter of 45.3 µm. Fig. 5.8b, however, shows the behavior of a 46.5 µm diameter

40 % glycerol drop. Note that the corresponding behavior of a 100% glycerol drop

has been shown in Fig. 5.6. By comparison of these three curves, it can be seen

that the bursts become more frequent (or the interburst separation decreases) as

the concentration of glycerol goes down. The average interburst separation is only

about 100 s for the 40 % drop, 236 s for the 70 % drop, and as high as 515 s for the

pure glycerol drop. Moreover, the average duration of the bursts, which is about

265 s for 100 % glycerol, also decreases to 103 s and 27 s for 70 % and 40 % glycerol,

respectively. Note that drops with less than 40 % glycerol content were too unstable

to be levitated in the trap for long periods, and hence are not studied.

Increasing the pump fluence, without changing the glycerol concentration, also

increases the evaporation rate due to higher heating of the drop. As shown in

Fig. 5.9a and 5.9b, the temporal evolution of lasing for similar sized drops is altered

in the expected manner. Fig. 5.9a (pump fluence ∼ 490 mJ/cm2) and Fig. 5.9b

(pump fluence ∼ 785 mJ/cm2) correspond to similar pure glycerol drops with diam-

eters of 43.8 µm and 45.6 µm, respectively. The drop at the higher pump fluence

(785 mJ/cm2) produces about 6 lasing bursts, twice of that at the lower pump flu-

ence (490 mJ/cm2), during the same measurement period. Also, the average burst

duration decreases from 107s to 64s with increase in the pump fluence. Hence, these

results, in combination with the results for varying glycerol concentrations, show

that the evaporation of the drop has a strong effect on its lasing on/off behavior.

Therefore, our interpretation that the lasing fluctuations are caused by the double

resonances in the evaporating glycerol drop is conclusively supported.

After understanding the lasing fluctuation behavior, our next task was to at-

tempt to reduce these fluctuations. Achieving this would considerably enhance the

performance of the laser as the output would then be fairly stable and reproducible.


Figure 5.9: Temporal evolution of the Raman lasing intensity for (a) a 43.8 µmpure glycerol drop at a pump fluence of 490 mJ/cm2 (b) a 45.6 µm pure glyceroldrop at a pump fluence of 785 mJ/cm2.

5.5 Doping the Glycerol Microdrop with Ag Nano-aggregates 103

As the main reason for the fluctuations is the evaporation of the drop due to heating

by the pump laser, achieving lasing at a lower pump fluence would lead to less heat-

ing and hence, decreased evaporation. Moreover, this will also increase the output

efficiency of the laser. Following this idea, we tried to lower the Raman lasing thresh-

old of our microdrops by enhancing the Raman gain of glycerol through “Surface

Enhanced Raman Scattering (SERS).” The next section will briefly describe the idea

of SERS and will discuss our related experiments. Note that the other possibility to

lower the lasing threshold is the direct evanescent coupling of the pump light to the

microdrop WGMs. This, however, is unfeasible in our setup as putting an external

coupling element (like a tapered fiber or a prism) near the trapped microdrop might

drastically effect the electrodynamic properties of the trap.

5.5 Doping the Glycerol Microdrop with Ag Nano-

aggregates

5.5.1 Background: Surface Enhanced Raman Scattering

Surface Enhanced Raman Scattering (SERS) refers to the enhancement of Raman

scattering (up to factors of 1015) from molecules in the close vicinity of a metal

surface. SERS was first discovered by chance in the early 1970s [116] and was soon

recognized and established as a useful phenomenon [117, 118]. Since then, several

studies have highlighted SERS as a powerful tool for investigating the properties

of single/few molecules and nanoparticles in solutions [119, 120] and also on sub-

strates [121, 122]. A brief explanation of the working principle of SERS will follow in

this thesis and more details along with an extensive review of the past experiments

can be found in literature [123, 124].

For a discussion related to SERS, it is necessary to start with the concept of

“Surface Plasmons (SPs).” “Plasmons” are electromagnetic waves that refer to a

collective excitation of the electron gas in a metal. If the plasmon is confined close

to and propagates parallel to the metal’s surface, it is called a “Surface Plasmon.”

SPs have shown a lot of promise in the fields of subwavelength optics, bio-photonics,


data storage, and microscopy and therefore, are of considerable interest to physicists,

biologists, material scientists and engineers [125]. A detailed description of the

theory of SPs and their important applications is available is literature [126].

An excitation of a SP of a metal is known as a “Surface Plasmon Resonance

(SPR).” SPRs can be excited on smooth surfaces via evanescent coupling of the

SPR frequency radiation. For optical frequencies, the commonly used technique for

this purpose is called “prism coupling”, where the evanescent field is generated by

the total internal reflection of a laser at one of the faces of a prism [127, 128]. SPRs

can also be generated locally, called “Localized Surface Plasmon Resonances (LP-

SRs)”, by scattering on rough surfaces where the order of roughness is much lower

than the wavelength of the scattered radiation [129, 130]. The surface roughness is

usually created by nanoscale corrugation of a metallic surface or by depositing metal

nanoparticles on an otherwise smooth surface. In either case, the strong plasmon

resonances of the nanoscale metallic entities lead to the generation of LSPRs [131].

When SPRs are excited on a surface (smooth or rough), the corresponding elec-

tromagnetic field is greatly enhanced. This enhancement is local in the case of

LSPRs. If a molecule is adsorbed on such a surface, it experiences this huge local

field and hence, the intensity of the Raman scattering from the molecule is consider-

ably enhanced. This phenomenon is known as surface enhanced Raman scattering.

Even greater enhancement can be achieved if the Raman scattered light also ex-

cites a SPR of the system. In this case, the overall enhancement scales roughly as

E4 [124], where E is the local electromagnetic field. Therefore, even a local field

enhancement of about 103-104 leads to giant enhancement factors in the range of

1014-1015 for the Raman scattering, which are otherwise very hard to achieve. In

liquids, as in our case, SERS is commonly achieved by generation of LSPRs with

the help of metallic nanoparticles [119, 120].

The physics behind the mechanism of LSPRs in metal nanoparticles and aggre-

gates is widely available in literature [132]. The general idea follows here. Let us

consider a spherical metal particle with a radius much smaller than the wavelength

of the incident light. For such a particle, the field of the incident light (Elight) ap-


pears to have a spatially constant amplitude and a time dependent phase. This is

known as the quasi-static regime. As the solutions of pure electrostatics also apply

very well in the quasi-static regime, the field induced on the surface of the particle

(Einduced) is given by [124, 132]

Einduced =ǫp(ω) − ǫmǫp(ω) + 2ǫm

Elight, (5.2)

where, ω is the frequency of light, ǫp(w) is the frequency dependent dielectric func-

tion of the metal, and ǫm is the dielectric constant of the surrounding medium. It is

obvious from Eq. (5.2) that Einduced has a singularity at a frequency (ωp) for which

Re[ǫp(wp)] = −2ǫm. The satisfaction of this condition at the frequency ωp, known as

the plasmon resonance frequency, constitutes a LSPR. Note that the above analy-

sis also explains most phenomena observed with non-spherical metal nanoparticles

and nanoaggregates. However, in these cases, the factor of 2 in the denominator of

Eq. (5.2) varies depending on the nanostructure.

Eq. (5.2) shows that the dielectric function of the constitution of the nanopar-

ticles is a key component in determining the LSPR bands. Metals such as silver,

gold and copper are commonly used because their dielectric functions allow for the

LSPR bands to lie in the visible region. In addition, for nanoscale particles, the

dielectric function also depends on the size of the particle, hence giving the LSPR

bands a size dependence [133]. As a result, a significant effect of the particle size is

observed on the bandwidth and the peak position of the LSPR. The peak position

can either “blue-shift” or “red-shift” with decreasing particle size depending on the

properties of the particle and the surrounding medium. The bandwidth, however,

is usually observed to vary inversely with the particle size [134].

At this point, we can come back to the idea (mentioned in the previous section)

of lowering the lasing threshold of our microdrop Raman laser. In principle, if

SERS can be used to enhance the Raman signal from glycerol, then lasing should

occur for considerably lower pump fluences. These low pump fluences will then

reduce the evaporation of the drop and improve the performance of our laser. We

tried to achieve this in the trapped glycerol microdrops by doping them with silver


(Ag) nanoaggregates. The reasons for using these nanoaggregates along with their

properties are discussed in the coming section.

Figure 5.10: (a) A near field scanning optical microscopy image of a typical Agnanoaggregate taken from literature (see text for reference) (b) SEM image of asingle Ag nanoparticle (c) SEM image of a Ag nanoaggregate (d) SEM image ofseveral Ag nanoaggregates.

5.5.2 Ag Nanoaggregate Properties

For the case of lasing in glycerol microdrops, the Raman signal comes from the

-CH stretch bonds (from -CH2) which have a large stokes shift (∼ 3000 cm−1). For

such high spectral separation of the Raman signal from the pump laser (∼100 nm

in our case), SERS is relatively weak when metal nanoparticles are used. This is

because metal nanoparticles can only support LSPRs for a small range of frequencies

and therefore, simultaneous resonances for the pump and the Raman signal are


hard to achieve [124]. However, this can be achieved by using metal composites

with nanoscale features (nanoaggregates). These nanoaggregates also provide giant

optical responses (like nanoparticles) and they can do so for a comparatively broader

range of frequencies [135]. Therefore, in our experiments, we preferred to use the

nanoaggregates to achieve SERS in the glycerol microdrop.

300 400 500 600 700 800

0.0

0.2

0.4

0.6

0.8

1.0

No

rma

lize

d A

bs

orp

tio

n

Wavelength (nm)

Ag nanoparticles

Ag nanoaggregates

Pump

Wavelength

Raman Lasing

Wavelength

Figure 5.11: Measured absorption spectra of the Ag nanoparticles (black dottedcurve) and aggregates (red solid line). The green mark at 532 nm shows the pumpwavelength and the red mark around 630 nm shows the Raman lasing wavelength.

The nanoaggregates support spatially localized surface plasmons along their

structure which leads to nanoscale spatial regions, known as the “hot spots”, where

the linear and non-linear optical responses are greatly enhanced [136, 137, 138].

A direct image of a Ag nanoaggregate obtained by near field scanning optical mi-

croscopy at a wavelength of 1 µm , taken from literature [139], is shown in Fig. 5.10a.

The nanoaggregate is shown in the X-Y plane. The Z-axis shows the local field en-


hancement factors which are on the order of 105 at the hot spots. Note that even

increased optical enhancements are expected when the nanoaggregates are located

inside a microcavity, as in our case, due to the cavity’s resonant feedback [139].

We used commercially available Ag nanoparticles (in a clustered powder form)

from IoLiTec GmbH with an average diameter of 40 nm. A colloidal solution of

these samples is subsequently prepared to investigate their properties. To form the

nanoaggregate colloid, a few milimoles of the powder is mixed in 10-30 centiliters

of water and immersed in an ultrasound bath (for 2-5 minutes). This leads to the

formation of a mud colored liquid which is then spin coated on silicon substrates

for scanning electron microscopy (SEM) analysis. The SEM images, for example

Fig. 5.10b, c, and d, reveal that the nanoaggregate colloid consists of a variety

of particles with sizes ranging from a single particle up to a few microns. If the

nanoaggregate colloid is further diluted (5-10 times) and immersed in the ultrasound

bath for longer periods (up to 30 mins), the nanoparticle colloid is formed, where

the average particle size of the colloid is close to the single nanoparticle limit.

In colloids, the optical absorption is well known to be induced by LSPR excita-

tions [140]. In other words, the absorption spectra of a colloid also represents its

LSPR bands. Therefore, absorption spectroscopy is an effective technique to find

out the LSPR bands of a given colloid. Fig. 5.11 shows the normalized absorption

spectra (measured with a Varian CARY 50) of the nanoparticle (dotted black curve)

and the nanoaggregate (solid red curve) colloids. The pump wavelength (532 nm)

and the Raman lasing wavelength (630 nm) are shown with the help of green and

red lines, respectively. Although the peak value of the LSPR band for both the

samples are similar, the major difference is the emergence of the extended tail in

the case of the nanoaggregates. Due to the variety of sizes and shapes present in

the nanoaggregate colloid, the optical response of the “hot spots” is spread over a

broader range of frequencies, hence giving rise to this tail like structure. This re-

sult is a demonstration of the previously mentioned notion that the nanoaggregates

can support LSPRs for a broader spectral range as compared to the nanoparticles.

For the nanoparticles, the relative strength of the plasmon resonance at the pump


laser wavelength and the Raman lasing wavelength of glycerol are 0.45 and 0.17,

respectively. However, these values are increased to 0.8 and 0.7, respectively, in

the case of nanoaggregates. This makes the simultaneous LSPRs for the pump and

the Raman signal more efficient and therefore, higher enhancement factors in SERS

with the nanoaggregates are expected. However, in spite of this promising approach,

lowering of the lasing threshold turned out to be very difficult in our experiments.

This is because we discovered that Raman lasing is practically prohibited in such

doped drops to begin with. We investigated this unexpected result in details and

found that there are several reasons, as discussed in the next section, which cause

this behavior.

5.5.3 Effects of Nanoaggregate Inclusion

To study doped glycerol microdrops, a colloidal solution with a required concen-

tration of Ag nanoaggregates is prepared, as discussed before, with pure glycerol

instead of water. Note that the absorption spectroscopy of pure glycerol indicates

that glycerol has almost no absorption in the spectral region of interest. Therefore,

the properties of the Ag nanoaggregate colloid in water, shown in Fig. 5.11, are

expected to remain similar in glycerol. Subsequently, this solution is used to trap a

drop of a desired size according to the procedure described in section 5.1. Glycerol

drops with different sizes (10-50 µm) and a range of Ag nanoaggregate concentra-

tions, from 1 µM to 50 mM, are studied. Solutions with more than 50 mM Ag are

not investigated because higher concentrations are found to result in solubility satu-

ration of glycerol. A high magnification (40 X) image of a 35 µm pure glycerol drop

doped with Ag nanoaggregates under green CW illumination is shown in Fig. 5.12a.

Apart from the three glare spots, the several bright green spots along the body of

the microdrop can be seen which indicate the presence of the nanoaggregates. These

nanoaggregates appear to be in a convective motion when observed in realtime. Note

that only the micron sized nanoaggregates are seen in the image as the scattering

from the smaller ones is not strong enough to be detected by the CCD camera. Also

note that in the corresponding image for a non-doped drop, the body of the drop


appears to be dark with only the three glare spots visible (Fig. 5.4a).

The investigations on the presence of SERS are started off by comparing the CW

Raman spectra of similarly sized doped and non-doped glycerol microdrops for the

same pump intensity. For Ag molarities below 5 mM, the Raman spectra of doped

and non-doped drops are found to be similar in intensity. This is understandable

because in SERS, only the signal from the molecules adsorbed on the nanoaggre-

gates is enhanced. And therefore, if the concentration of the nanoaggregates is low,

then the signal from the non-adsorbed molecules dominates heavily making an en-

hancement in the overall intensity very hard to detect. However, if the Ag molarities

are increased to be between 20-50 mM, the collective Raman signal from the doped

drop is observed to be slightly enhanced than the non-doped drop. However, this

enhancement fluctuates randomly in time and is not very reproducible under similar

conditions. This is most likely because the enhancement properties depend on the

system geometry which, in this case, changes with time as the aggregates are not

stationary in the microdrop. The typical enhancement factors observed by us lie

between 2 and 10. Fig. 5.12b shows one of our best results for a 25 mM Ag doped

drop where the collective enhancement is about 10 times. The figure shows the cor-

responding spectra for two 35 µm diameter drops, where the black (solid line, left

Y-axis) and the green (dotted line, right Y-axis) spectra refer to the non-doped and

doped drops, respectively. The corresponding CW pump intensity for both these

measurements is about 1000 W/cm2.

Another feature that is observed from Fig. 5.12b is the reduction of the quality

factor of the microdrop cavity. For clarity, the spectral region around 650 nm is

zoomed-in and shown in Fig. 5.12c. For pure glycerol drop, although the cavity

feedback is not very strong but it is still observable. This is shown by highlighting

the WGMs with the help of the dotted red circles. However, for the doped drop,

these modes appear to be completely washed out. This is because of the excess

scattering losses experienced by the circulating WGM due to the presence of the

nanoaggregates. Similar effects in microdrops doped with latex nanoparticles have

been reported before [141, 142]. Therefore, the first complication of our system


is the following. To get SERS we need a high nanoaggregate concentration which

unfortunately destroys the feedback properties of the cavity.

Figure 5.12: (a) A high magnification (40 X) image of a 35 µm pure glycerol dropdoped with Ag nanoaggregates under green CW illumination. (b) Comparison ofthe CW Raman spectra of the drop in (a) with a similar drop of pure glycerol. Boththe spectra are recorded at the same pump fluence for an EMCCD exposure timeof 30 s. (c) Zoomed in spectra of Fig. 5.12b around 650 nm. The dotted red circlesare used to highlight the measured WGMs.

The second unexpected problem is the response of the doped drops towards the

high peak-powers of the Q-switched pulses. The doped drops are found to be much

more sensitive to the radiation pressure as they suffer from severe motional insta-

bility even at moderate pump fluences of around 250-300 mJ/cm2. This instability

can even be so intense that the drop is pushed out of the trap. We attribute this


behavior to mainly two effects; increased elastic scattering and increased optical

absorption inside the microdrop due to the presence of the nanoaggregates. Each

nanoaggregate acts as a local scattering center and therefore, the overall elastic scat-

tering from the drop is enhanced. Since the radiation pressure is a function of the

scattered light [143], the effective radiation pressure experienced by the microdrop

is also increased leading to instability. Moreover, as the nanoaggregates have con-

siderable absorption at the pump wavelength of 532 nm, as shown in Fig. 5.11, their

presence also increases the heating rate of the microdrop. This causes random fluc-

tuations in the elastic scattering pattern of the drop by speeding up the convective

motion of the nanoaggregates. As the enhanced radiation pressure follows these fluc-

tuations, it results in further instability of the trapped drop. In conclusion, because

of the quality factor degradation and the increased radiation pressure sensitivity, it

is realized that SERS with the help of metal nanoaggregates is prohibited in trapped

microdrops. And therefore, our efforts to enhance the lasing performance did not

produce much success.


This chapter focused on the Raman lasing (from the WGMs) and the corresponding

blinking characteristics of electrodynamically trapped glycerol microdrops. Starting

with our technique of trapping and isolating a single pure glycerol microdrop, CW

Raman spectra of such a drop were later shown. Under Q-switched excitation, size

dependent single or multimode Raman lasing were achieved in the microdrops. The

threshold pump fluences were reported to lie between 200-390 mJ/cm2. Following

this, the on/off behavior of the Raman lasing was investigated in detail for varying

glycerol concentrations and pump fluences. It was shown that the on/off behavior

is due to the double resonances in the evaporating glycerol microdrop and that it

can be manipulated by controlling the drop’s evaporation. Finally, the chapter was

concluded by showing our efforts to enhance the lasing performance of the microdrop

Raman laser by doping it with Ag nanoaggregates.

The ideas mentioned in this chapter are very relevant even in other microdrop


Raman laser systems. In fact, the basics of microcavity lasing are quite similar even

if the optical gain comes from processes other than Raman scattering. The next

chapter discusses such a microcavity laser where semiconductor quantum dots pro-

vide the necessary gain for lasing. Therefore, the concepts and techniques discussed

in this chapter will be quite useful for the experiments presented in the next chapter.


Chapter 6

The Quantum Dot MicrodropLaser in an Electrodynamic Trap

In this chapter, we present the lasing characteristics of electrodynamically trapped

microdrops doped with CdSe/ZnS colloidal quantum dots. The chapter starts with

a discussion on the structure and properties of our CdSe core/shell colloidal quan-

tum dots. In section 6.2, the whispering gallery mode type feedback properties of

the trapped microdrops are demonstrated. Drop-size dependent single or multimode

lasing, from the quantum dot gain, is shown in section 6.3. Here, detailed inves-

tigations of the lasing threshold are presented and the observed blue shift of the

lasing modes (with increasing fluences) is discussed. Finally, section 6.4 compares

the quantum dot concentrations, in our lasing microdrops, with the theoretically

predicted behavior. It is found that lasing can occur at concentrations almost 2

orders of magnitude lower than expected. The chapter concludes with a discussion

of the possible reasons for such a behavior.

6.1 The CdSe/ZnS Core-Shell Quantum Dots

Nowadays, the general opinion is that semiconductor nanocrystals (or colloidal quan-

tum dots) hold the promises for future photonic devices. As discussed in Chapters 1

and 2, the quantum confinement effect in colloidal quantum dots enables them to

possess precise bandgap tunability. Moreover, they can be stably dispersed in a

variety of colloidal solutions due to their friendly molecular coupling characteristics.

116 The Quantum Dot Microdrop Laser in an Electrodynamic Trap

Due to their nanoscale sizes, these particles have a large surface-to-volume ratio.

Hence, their surface characteristics strongly influence their optical behavior. In ad-

dition, their surface has vacant states which make them extremely damage prone

by readily reacting with the surroundings. Also, these particles are very sensitive

to light and can be easily photo-oxidized and photo-bleached. Therefore, passiva-

tion or capping of the surface states is an indispensable requirement for practical

applications of colloidal quantum dots. Moreover, appropriate changes in surface

chemistry can also considerably reduce the non-radiative transitions (between the

energy levels) in the quantum dot (QD) and hence, increase their quantum yield

(QY).

One way to passivate the surface is by bonding of organic groups, such as phenyl,

thioglycolic acid (TGA), or Cysteamine (MA), to the vacant surface states. Depend-

ing on the attached group, the nanocrystals can be dispersed into a variety of organic

solvents like toluene and hexane. Such nanocrystals have good photoluminescence

(PL) properties with a quantum yield up to 10 % at room temperature. The or-

ganic capping, however, leads to a slight red-shift of the fluorescence along with an

extended (µs scale) photoluminescence lifetime [144, 46]. Also, such QDs have a

low optical damage threshold. Since lasing experiments, in general, require consid-

erable pump fluences, these QDs are not appropriate for our purposes. As a first

test, we acquired organically capped CdTe QDs (peak emission λ ∼ 620 nm) from

Dr. Andrey Rogach (LMU Munich) for preliminary testing. As expected, the QDs

(in trapped drops) quickly bleached and showed fairly negative response to lasing

attempts for both CW and pulsed pumping.

Inorganic coupling is another approach for surface passivation which involves

growing a coating of a different semiconductor material over the nanocrystal core.

Nanocrystals prepared in this way are known as “core/shell” quantum dots. These

nanocystals can tolerate fairly high optical fields, compared to their organically

capped counterparts, and hence are more suitable for our experiments. The core

(in such particles), which solely decides the PL emission, has a lower bandgap than

the shell to create a “quantum well” like exciton confinement. By choosing the

6.1 The CdSe/ZnS Core-Shell Quantum Dots 117

appropriate core/shell combination, their carrier dynamics can be modified to obtain

better PL properties [145]. Moreover, the semiconductor shell allows for better

electrical connectivity of the core as compared to that of organic coupling.

Figure 6.1: (a) A pictorial representation of the structure of a CdSe/ZnS quan-tum dot taken from literature (Photo Courtesy: Evident Technologies Inc.). (b)Photoluminescence and absorption properties of our CdSe/ZnS quantum dots.

The choice of an appropriate shell for a given core depends on the structural,

electrical, and chemical compatibility of the two semiconductors. The core/shell

combination of CdSe and ZnS serves this purpose very well [146]. CdSe and ZnS are

both direct bandgap semiconductors of tetrahedral structures. The bandgap of ZnS

(3.2 eV) is larger than that of CdSe (1.7 eV). However, their lattice constants, of

0.608 nm (CdSe) and 0.54 nm (ZnS), are fairly close which supports epitaxial growth

of one over another. We use the water soluble CdSe/ZnS core/shell quantum dots,

bought from Evident Technologies Inc., in our lasing experiments [147]. The CdSe

core has a radius of 2.6 nm which results in a PL centered at 625 nm. The shell

is coated with functional lipids for further stability and dispersivity. The specified

QY of these quantum dots is about 59 %. A pictorial representation, taken from

literature [147], of such a single quantum dot is shown in Fig. 6.1a.

The measured optical properties of our quantum dot samples are shown in

Fig. 6.1b. The dotted curve (blue) (measured with a Varian CARY 50) repre-


sents the absorption spectra. The absorption sets in around 675 nm and increases

with decreasing wavelengths. Rather than distinct absorption peaks, an absorption

continuum is observed as this is an ensemble measurement. The green line depicts

the pump wavelength of 532 nm. The observed PL, shown by the solid black curve,

is centered at 625 nm and is about 33 nm in width (FWHM). Now, the figure clearly

shows that there is a fair overlap between the absorption and the emission spectra.

The possible region of optical gain is, therefore, decided by a compromise between

the two curves [148]. Although PL is maximum at the peak wavelength, the corre-

sponding effective photon density is decreased due to absorption. Hence, the overall

spectral region of possible optical gain is not located at the center. It is red-shifted

from the peak wavelength to a spectral region where the absorption is minimal and

the PL is still reasonably efficient. Such a region, depending on the sample, is typ-

ically 20-30 nm wide and is shown by the red colored box in Fig. 6.1b. As shown

in the coming sections in this chapter, the lasing peaks lie in this spectral window

although the WGMs are present throughout the PL spectra.

6.2 Whispering Gallery Modes in the Quantum

Dot Doped Microdrop

As discussed in the previous chapters, the first step in our experiments is to trap

the microdrop/microparticle. Our CdSe/ZnS quantum dots are water soluble and

are delivered as a highly concentrated colloidal solution. We dilute (in water) these

samples heavily (∼ few µM) for our purposes. Since water evaporates rapidly at

room temperature, it is not favorable to trap drops of this diluted solution. Hence,

it is mixed with glycerol (30-50 % vol.) to obtain reduced evaporation rates.

This glycerol-water-QD solution is injected, and microdrops are trapped, in a

manner similar to that discussed in section 5.1. However, instead of the electrospray,

a home-built piezo-driven microdrop generator is used. This device consists of a

tapered glass capillary with a hollow cylindrical piezo around its outside surface.

The capillary is typically tapered down to inside diameters between 20-100 µm at

the exit tip. The expansion and compression of the piezo, with an external high AC

6.2 Whispering Gallery Modes in the Quantum Dot Doped Microdrop 119

voltage, creates time dependent pressure variations inside the capillary and results

in a stream of microdrops. Application of a high DC voltage (∼ 2-3 kV) to the liquid

(inside the capillary) provides the necessary charges to the microdrops for trapping.

More details about this device can be found in [80, 149]. This technique, although

slightly more involved, is advantageous over the electrospray for the handling of

quantum dots. First, the sizes of the drops can be well controlled by using capillaries

of different diameters. Second, the resulting microdrop stream is more directional

(than electrospray) and can be aimed directly at the center of the trap causing very

little liquid wastage. This decreases the QD usage per run and hence, makes the

experiment more cost effective. However, this technique shows operational problems

for high glycerol concentrations, above 50 %.

To start with the measurements, a QD doped microdrop is trapped with similar

trapping parameters as those of section 5.1. Subsequently, the optical feedback

properties of the microdrop are investigated with a CW laser. Fig. 6.2a shows the

spectra of a 9 µm drop (30 % glycerol) with an overall QD concentration of 1.13 µM.

This drop will be referred to as the “9 µm drop” from now on. The 1 mW green

laser (see section 3.2), at an intensity of ∼ 90 mW/cm2 is used here. The EMCCD

exposure time is chosen to be 5 s. The overall PL is similar to that of the QD

solution (shown in Fig. 6.1b). However, distinct signature of the microdrop resonant

feedback, represented by the almost periodic sharp peaks, is observed here. The

average spacing between the adjacent peaks, near 625 nm, is about 6 nm. According

to the theoretical model (Eq. (5.1)), the spectral spacing between similarly polarized

modes is expected to be 11.2 nm. Since the separation between two alternate modes

(∼12 nm) is close to the theoretical value, it is a good indication that any two

adjacent modes in Fig. 6.2a are of different polarizations (TE or TM). In other

words, a TE mode is surrounded by TM modes on each side and vice versa.

A similar measurement for a drop of 34 µm diameter (30 % glycerol), with a

QD concentration of 0.57 µM, is shown in Fig. 6.2b. The zoomed in spectra around

625 nm is shown in the corresponding inset for clarity. This drop will be referred

to as the “34 µm drop” from now on. Again, clear WGM type features can be


Figure 6.2: Observation of Whispering Gallery Modes from the quantum dot dopedmicrodrops of sizes, (a) 9 µm, and (b) 34 µm. The zoomed-in spectra of (b) around625 nm is shown in the corresponding inset.

seen in the PL spectra. However, due to the large background PL and the closely

spaced WGMs, the peaks are not as pronounced as in the case of Fig. 6.2a. Again,

the alternate mode separation, around 625 nm, is about 2.9 nm which is close to

the value of 2.96 nm predicted for similarly polarized modes (by Eq. (5.1)). Hence,

similar to the 9 µm drop, every second mode has the same polarization (TE or

TM). Moreover, the area under the curve (proportional to the number of emitted

photons) for Fig. 6.2b is about 28 times higher than that for Fig. 6.2a. Interestingly,

the number of quantum dots (molar density · drop volume) in the bigger drop is

27 times that of the smaller drop. This closeness of the ratios demonstrates the

expected linear behavior of the spontaneous PL and explains the difference in the

heights of the two curves.

After investigating the WGM resonances, we attempted to obtain CW lasing

in the trapped microdrops. For this, we used the high power CW millenia laser

(section 3.2) as the pump source. However, no stimulated emission could be ob-

served. The PL showed a linear increase up to a certain pump intensity after which

photobleaching of the QDs is observed. This is indicated by the decrease in the

PL emission with further increase of pump intensities. However, as shown in the

next section, use of a pulsed pump source can avoid bleaching as the time between

6.3 Lasing from Quantum Dots in the Trapped Microdrop 121

pulses can be used for excited state relaxations in the QDs. Hence, the threshold

for stimulated emission can be reached without destroying the QDs and lasing can

be achieved.

6.3 Lasing from Quantum Dots in the Trapped

Microdrop

As discussed previously (see Fig. 6.1), the QDs can only provide optical gain in a

finite spectral window. Depending on the feedback properties of the microdrop, the

number of modes lying inside this window are different. Moreover, the gain expe-

rienced by individual modes inside the window also varies based on the emission-

absorption characteristics at the corresponding wavelength. Typically, the gain pro-

file has a single-peak like structure inside the gain window. In other words, it has a

maximum at a point (inside the window) from which it decreases (in both directions)

towards the edges of the window. Therefore, for a given quantum dot sample, the

lasing mode structure can be manipulated by controlling the resonant feedback of

the microdrop. There could be many ways to achieve this, such as refractive index

or temperature variations. However, the easiest approach is to use microdrops of

different sizes which is demonstrated in the coming section.

6.3.1 Single and Multimode Lasing

The protocol for observing lasing with the QD doped droplet is similar to that dis-

cussed in section 5.3. The 532 nm Q-switched pump laser, for these measurements,

is focused at the trap center to a spot size of about 1.3 mm. Since the intermodal

spacing of WGMs is inversely proportional to the drop size (Eq. (5.1)), the num-

ber of lasing modes is expected to reduce for decreasing drop diameters. In our

experiments, single mode lasing is typically observed for drop diameters less than

10 µm. Fig. 6.3a shows the lasing spectra of a 9 µm drop (30 % glycerol) with a

QD concentration of 1.13 µM, i.e., the 9 µm drop. The pump fluence and the EM-

CCD exposure time, for this measurement, are 56.25 mJ/cm2 and 5 s, respectively.

The lasing intensity is dominated by a single mode at 638.2 nm. Comparison with


Fig. 6.2a, which is the CW spectra for the same drop, indicates that the observed

lasing mode does not exactly overlap with any CW WGM features. In fact, the

closest CW WGM (at 638.78 nm) is 0.58 nm red-shifted to the lasing mode. Such

a shift could be a result of microdrop evaporation caused by the Q-switched laser.

This effect is discussed later in this chapter (section 6.3.3).

Figure 6.3: Lasing from the quantum dot doped microdrops of sizes (a) the 9 µmdrop and (b) the 34 µm drop. The QD concentration in drops shown in (a) and (b)are 1.13 µM and 0.57 µM, respectively. The corresponding pump fluences are 56.25and 75 mJ/cm2, respectively.

Another interesting feature of Fig. 6.3a is the weak side mode located at 642.65 nm.

The separation of this mode from the lasing mode is about 4.45 nm. This value is

considerably smaller than the spacing between adjacent CW WGMs (∼6 nm). More-

over, the lasing modal separation is expected to be more that 6 nm if evaporation

effects are considered. Therefore, this suggests that the lasing side peak is a higher

order mode which is hidden (in the background PL) in the CW measurements.

Multimode lasing, in our experiments, is typically observed for drops of diameters

above 10 µm. Fig. 6.3b shows the lasing spectra of a 34 µm drop (30 % glycerol)

with a QD concentration of 0.57 µM, i.e., the 34 µm drop. The pump fluence and the

EMCCD exposure time, for this measurement, are 75 mJ/cm2 and 1 s, respectively.

The overall profile of the lasing modes, the envelope that could be obtained by


joining the peaks of the modes, represents the effective gain curve of the quantum

dots. There are eleven distinct modes present which have an average intermodal

separation of about 1.54 nm. Therefore, the observed spacing between every second

mode is again 3 nm, as in Fig. 6.2 which indicates the presence of alternating TE

and TM modes.

6.3.2 Threshold Measurements

The lasing threshold, for the 9 µm drop, is observed to be around 53 mJ/cm2.

However, due to its small size, the pump laser causes a significant radiation pressure

instability which makes the threshold behavior difficult to characterize. The bigger

drops, like the 34 µm drop, can comparatively tolerate much higher fluences and

thus, can be better studied. Fig. 6.4a shows the spectral evolution, at different

fluences, for the 34 µm drop. Each spectra is an average over 200 pump pulses.

The overall evolution can be qualitatively separated into three distinct regions. The

first region (not shown in Fig. 6.4a) is the characteristic QD PL emission at very

low fluences of less than 10 mJ/cm2(similar to the black curve in Fig. 6.1b). For

moderate fluences (10-50 mJ/cm2), the initial lasing features start to emerge in the

gain region while the background PL can still be seen. This is shown by the spectra

for 30 mJ/cm2 which is multiplied by a factor 100 for better visibility. At high

fluences (above 50 mJ/cm2), the lasing modes dominate the emission. In such cases,

the background PL becomes so weak that it appears to be non-existent in comparison

with the lasing modes, as seen in the spectra for 97.7, 150, and 180 mJ/cm2.

The corresponding lasing threshold behavior is shown in Fig. 6.4b. 20 spectral

scans, each containing 10 pump pulses, are carried out at varying pump fluences.

Each of the 20 scans are integrated over the lasing region of 630-660 nm. Subse-

quently, the mean and the standard deviation of the integrated areas are plotted

against their respective fluence values to obtain the threshold curve. Up to a thresh-

old fluence of around 50 mJ/cm2, the integrated area is very low with a small slope.

At the threshold, a rapid transition to a higher slope can be seen which is a char-

acteristic of the lasing behavior. The curve shows a linear increase up to a fluence


of 83 mJ/cm2 after which the behavior becomes slightly chaotic. Moreover, with

increasing fluences, the error bars also increase in magnitude. These two effects are

results of the higher positional instability of the drop (induced by radiation pres-

sure) with increasing fluences. Since the lasing intensity critically depends on the

coupling of the pump light, even slight variations of the drop position can drastically

effect the emitted lasing intensity.

Figure 6.4: (a) Spectral evolution of the 34 µm drop as a function of the increasingpump fluence. Note that the spectra at 30 mJ/cm2 is multiplied by 100 for bettervisibility. (b) The lasing threshold behavior of the 34 µm drop. (c) The CCD imageof a lasing microdrop of diameter 40 µm. Except for the size, this drop is similar tothe 34 µm drop in every other respect.

At very high fluences, above 160 mJ/cm2, the curve shifts to flat profile indicating

a gain saturation effect. This is most likely because at such high fluences, all the QDs

(which contribute to lasing) are in the excited state and therefore, an increase in

the pump fluence will appear to be transparent to the drop and will not increase the

lasing signal. Another possibility is that such high fluences lead to the degradation of


the quantum dots. At fluences above 200 mJ/cm2, the drops are usually kicked out

of the trap by the radiation pressure of the pump beam. We also investigated lasing

threshold behaviors for drops of different sizes and QD concentrations. Changing the

concentration from 0.57 mM to 1.13 mM, for two drops of similar sizes, did not lead

to a considerable shift of the threshold value. However, for the same concentration,

the threshold appeared to be a bit higher for the smaller drops. Unfortunately,

the threshold can be affected by other parameters such as evaporation and heating

effects.

The lasing thresholds (∼50 mJ/cm2) are considerably lower compared to pre-

viously reported values (∼1200 mJ/cm2) in liquids [50]. This is most likely due

to the configuration of our microdrop laser. The QDs are present throughout the

volume of the microdrop cavity. Therefore, the higher order modes, where most of

the intensity is distributed inside the drop, can also be excited. Hence, more QDs

are excited leading to increased optical gain. Note that only low order modes can

be efficiently excited when the gain material is located outside the microcavity [52].

Moreover, low thresholds can also result due to the high Q-factors of our spherical

and smooth surfaced microdrops. Such cavities can have Q-factors up to 106 [150].

However, the resolution limit of our spectra measurements only allow us to infer

that our Q-factors are greater than or equal to 6.5x103.

Fig. 6.4c shows the CCD image (pump filtered) of a 40 µm lasing drop. Except

for the size, this drop is similar to the 34 µm drop in every other respect. The two

characteristic lasing spots, explained previously in Fig. 5.4c, caused by the tangential

leakage of the counterpropagating lasing modes can be clearly seen in the image.

The red color of the spots matches very well with the simultaneous multimode lasing

observed around 645 nm.

6.3.3 Microdrop Evaporation Effects: Blue Shift of LasingModes

Another notable feature observed in the lasing behavior, of the QD microdrop laser,

is the spectral blue shift of the lasing modes and the gain profile at high pump


fluences. Fig. 6.5a is the normalized multimode lasing spectra for the 34 µm drop at

three different fluences of 75, 112.5, and 150 mJ/cm2. All three fluences are below the

saturation limit (see Fig. 6.4b). It can be seen that with increasing pump fluences,

the individual modes and the overall gain profile, both undergo a considerable blue

shift. The modal blue shift, which can be attributed to the slight evaporation of the

microdrop, is shown in detail in Fig. 6.5b. It is well known that the evaporation of

microdrops and the corresponding shifts in the WGMs are related [151]. If ∆a is

the change in the drop radius (a) then

∆a

a=

∆λl,s

λl,s

, (6.1)

where λl,s is the peak wavelength of a mode of lth number and sth order and ∆λl,s

represents its spectral shift.

Figure 6.5: The blue shift of the lasing modes with increasing pump fluences. (a)Blue shift of the overall spectra, (b) Blue shift of individual modes, (c) Blue shift ofthe gain region. Note that the center position, in (c), is determined by a gaussianfit of the gain region.

In our case, the modal blue shift, which is almost linear with the pump fluence,

has a maximum observed value of 2 nm. Using the above equation, the correspond-

6.4 Low Quantum Dot Density in the Lasing Microdrop 127

ing change in the drop diameter can be estimated to be 0.105 µm. Such a small

change, not detectable by our size measurement technique, can be easily caused by

evaporation due to heating of the drop by the pump laser. Hence, the modal blue

shifts are explained. Note that the drop evaporation will also change the separation

between two modes of adjacent mode numbers (λl,s and λl±1,s). However, for large

drops like the 34 µm drop with tiny size changes (0.105 nm), λl±1,s − λl,s is about 3

orders of magnitude smaller than ∆λl,s and hence, can be neglected for all practical

purposes [151].

Fig. 6.5c shows the overall blue shift of the gain region with increasing pump

fluences. The center position of the gain region, for each fluence, is determined by

fitting a gauss envelope to the lasing modes. Up to moderate pump fluences of about

100 mJ/cm2, the gain region appears to be fairly stationary with a negligible shift.

With subsequent increase in the fluence, above 100 mJ/cm2, the blue shift becomes

much more noticeable. An overall shift of about 3.2 nm is observed at a fluence

near 190 mJ/cm2. These shifts are non reversible and are most likely due to the

permanent photo-oxidization of the QDs by the pump radiation [152]. The intense

pump light starts degrading the CdSe core from the outside. Hence, the effective

size of the active core decreases. Since, smaller QDs have larger effective bandgaps

(see section 2.5), the overall PL spectra shifts to the blue. The gain region shifts

with the PL spectra, hence, explaining the observed behavior.

6.4 Low Quantum Dot Density in the Lasing Mi-

crodrop

Another interesting aspect of our microdrop laser is the occurrence of lasing at very

low QD concentrations. In fact, the QD concentrations at which lasing occurs can be

almost 2 orders of magnitude lower than those predicted by theory. Several previous

reports, mostly by V. I. Klimov et. al., have proposed the theory of optical gain and

stimulated emission in nanocrystal QDs [48, 153]. Gain in nanocrystal QDs is com-

plicated by the competing radiative and non-radiative processes. Surface trapping

states are the first cause of non-radiative processes. However, there contribution can


be minimized, as in our case, by efficient passivation of the QD surface. The other

major cause, which is hard to overcome, is Auger recombination. In this process,

the energy released by an exciton recombination is used up to excite a secondary

electron to a higher level and hence, no energy is emitted as radiation.

To achieve optical gain, the buildup time of stimulated emission (τs) must be

smaller than the Auger recombination lifetime (τ2). In other words, the condition

for stimulated emission must be reached before the excited state energy is lost to

Auger recombinations. τ2 scales as the third power (R3) of the dot radius and hence,

becomes significantly short (picosecond scale) to compete with stimulated emission

for colloidal QDs. τ2 is not a function of the spatial arrangement of the QDs and can

be expressed as β ·R3 where, β is a constant for a given material system. τs, on the

other hand, depends on multiple parameters and can be mathematically expressed

as [153]

τs =4πR3nr

3ζσgc, (6.2)

where nr is the refractive index, c is the speed of light, and σg is the gain cross-

section. Also, ζ is defined as the volume fraction which is the ratio of the volume

occupied by the QDs to the total volume of the system. Therefore, the requirement

τs < τ2 gives

ζ > ζmin, ζmin =4πnr

3cβσg

. (6.3)

ζmin is defined as the minimum volume fraction required for stimulated emission

to overtake Auger recombination. For CdSe QDs of radius 1.3 nm, this analysis

proposes that ζmin = 0.002 [153]. The ζ for our 0.57 µM drops, however, has a value

of approximately 2.5·10−5, which is about 80 times smaller than ζmin. This clearly

violates the theoretical prediction.

We believe that the explanation to this observed behavior must lie in Eq. (6.3).

One of the possible reasons could be that the gain cross-section (σg) for our quantum

dots is higher which would decrease the effective value of ζmin. Our quantum dots

have a radius of 2.6 nm which is twice as much of those used to estimate ζmin = 0.002.

Assuming that σg scales as R3 [48], this accounts only for a factor of 8 reduction in

the value of ζmin. Even if this were true, the reduction is still not close to the overall


factor of 80.

Another scenario could be that the Eq. (6.3) is valid only for solid state systems,

i. e., where the QDs are stationary. In our liquid state microdrops, the QDs are free

to move around and rearrange accordingly. Therefore, it could happen that the QDs

migrate to the surface of the microdrop which would increase the effective ζ in the

lasing mode volume (first order). Let us assume the best case where all QDs inside

the microdrop have migrated near the surface. To account for the remaining factor

of 10, this would mean that all the QDs are distributed within a radial fraction

of 3.5 % from the drop surface. This thickness value for the 34 µm drop would

correspond to about 0.6 µm. Such a compact rearrangement of the quantum dots

seems physically very questionable. In conclusion, although we are certain that we

do see lasing in our microdrops at much lower QD concentrations than expected,

more experimental data is required for a rigorous understanding of the same.


In this chapter, a WGM microdrop laser based on CdSe/ZnS colloidal quantum

dots was developed in an electrodynamic trap. Beginning with the properties of

the CdSe/ZnS core/shell quantum dots, coupling of the microdrop’s WGMs to the

quantum dot PL was demonstrated. Under pulsed excitation, single and multimode

lasing from quantum dot doped drops of different sizes were demonstrated. Typical

lasing thresholds were reported to to around 50 mJ/cm2. With increasing fluences,

blue-shifts of the lasing modes and the overall gain region were observed and, re-

spectively, explained. In the end, our quantum dot concentrations were compared

with previous reports and were found to be more than 2 orders of magnitude lower

than expected.


Chapter 7

Conclusion

7.1 Summary of the Thesis

The work presented in this thesis focused on the study of lasing phenomena in mi-

croscale particles using electrodynamic levitation. Three different lasers, which are

the ZnO tetrapod laser, the glycerol microdrop Raman laser, and the CdSe/ZnS

quantum dot microdrop laser, were developed and investigated. The charged mi-

crolasers were studied in an end-cap type trap where they were confined in all three

dimensions by electrodynamic fields. The trapped particles were optically excited

with the second or the third harmonic (depending on the required pump charac-

teristics) from a 10 Hz Nd:YAG Q-switched laser (pulse width∼10 ns). A sensitive

spectral acquisition scheme, based on a spectrograph and an EMCCD camera, was

used to record the optical emission of the particles. The dimensions of the trapped

particles were estimated by a calibrated high magnification imaging system.

The development of the tetrapod laser involved the demonstration of a novel

approach to electrodynamically isolate a single ZnO tetrapod under atmospheric

conditions. Such a technique removes the undesirable interference from the substrate

material and, thus, provides an effective method to study the intrinsic properties

of such nanostructures. Fabry-Perot type UV lasing, from the nanowire legs of

trapped single and multiple tetrapods, was observed. Clear threshold signatures

were also shown to support the lasing behavior. The viability of micromanipulating

trapped nanostructures with electric fields was also established by showing precise

132 Conclusion

translation of a single tetrapod. In addition, a comparison of the lasing properties

of the tetrapods in the trap and on a glass substrate was carried out. Although the

threshold was slightly higher on the substrate, no significant difference was found.

However, as the measurements on substrates involved higher magnification, WGM

type cavity effects were observed on the tapered tetrapod legs.

Subsequently, Raman lasing characteristics of glycerol drops with diameters in

the range of 10-50 µm were examined. Size dependent single and multimode lasing

near 630 nm, accompanied by the lasing threshold behavior, were observed. Long

term measurements of the output intensity showed that lasing was temporally inter-

mittent. This behavior was investigated in detail for different glycerol concentrations

and pump fluences which strongly suggested that the intermittency was due to dou-

ble resonances in the evaporating droplet. In addition, to explore the possibility of

Surface Enhanced Raman Scattering (SERS) in the glycerol microdrop, which would

lower the lasing threshold, the drop was doped with silver nanoaggregates. However,

the observed reduction in the drop’s quality factor and the increased heating effects,

caused by the Ag nanoaggregates, made this approach unfavorable.

Finally, the CdSe/ZnS quantum dot microdrop laser and its properties were

presented. The structural and optical characteristics of our CdSe/ZnS (core/shell)

quantum dots were illustrated. Also, the red-shift of the optical gain window, with

respect to the photoluminescence (PL) peak, was explained. The cw PL spectra

of the doped microdrops demonstrated the coupling of the WGMs to the quan-

tum dot emission. Under pulsed excitation, low-threshold single and multimode

lasing (around 640 nm) were observed from 10-50 µm diameter drops. With in-

creasing pump fluences, an almost linear blue shift of the lasing modes was observed

which was justified based on the drop’s evaporation. Additionally, a non-reversible

blue-shift of the quantum dot gain region occurred due to the permanent photo-

oxidization of the dots by the pump radiation. Finally, an analysis of the quantum

dot density in the drop indicated that our concentrations were more than two or-

ders of magnitude lower than expected from theory. A brief discussion showed that

a compact rearrangement of the quantum dots near the drop’s surface, although

7.2 Future Outlook 133

unlikely, is a possible explanation to this observation.

7.2 Future Outlook

The microscale lasers, examined in this study, have a lot of room for improvement

in their efficiency, performance, and controlled operation. Future efforts, in general,

should be to develop these lasers to the point where they could be actively employed

in practical applications. Our views on some of the favorable directions to take this

work forward are discussed below.

To begin, although we demonstrated translational micromanipulation of the

trapped tetrapod, the rotational control remains extremely desirable. This could

pave the way for directional and selective probing of different legs with ease, some-

thing which is extremely difficult to achieve on a substrate. Hence, the quality

factor can be greatly improved by maximizing the signal collection efficiency along

the lasing emission direction. Moreover, rotating the sample would also assist in ob-

taining the lasing signal from different legs which may give the device some spectral

tunability. Rotational control of absorbing microparticles could be achieved with

optical torque of a strongly focused, polarized beam. As discussed in section 4.4.3,

our efforts to achieve this with a Q-switched laser were not fruitful. However, we

believe that a high power cw UV laser, which is absorbed, could generate the neces-

sary optical torque to rotate the trapped tetrapod. For example, the third (355 nm)

or the fourth (266 nm) harmonic of a diode pumped cw Nd:YAG laser would be

ideal. In addition, the possibility of achieving rotation by scattering of a circularly

polarized radiation, from the tetrapod, may also be explored.

In addition, studying ZnO tetrapods and other nanomaterials in different electro-

dynamic trap configurations is another interesting possibility. For example, a linear

Paul trap [154] could be used to trap and study multiple nanostructures. Moreover,

such traps, similarly used to guide ions in mass spectrometers, would also enable effi-

cient transport of the trapped nanostructures. As a preliminary step, we constructed

a linear octupole (eight poles) trap as shown in Fig. 7.1a (trap diameter=1.5 cm,

electrode diameter=3 mm, electrode length=4 cm). Successful operation of the trap

134 Conclusion

was assured by levitation of chalk dust particles (sizes∼ 50-100 µm). Fig. 7.1b shows

a CCD image of such trapped chalk particles, between the poles, in an almost lin-

ear fashion. The particles are illuminated with a green laser for imaging purposes.

Investigations of ZnO tetrapods and other nanostructures, based on similar ideas,

are planned in the near future.

Figure 7.1: (a) A picture of our linear octupole trap. (b) A CCD image of chalkdust under green illumination trapped in our linear trap.

Similarly, several challenging tasks remain open in the case of the glycerol mi-

crodrop Raman laser. As discussed in section 5.4, evaporation causes changes in the

coupling between the pump beam and the microdrop’s WGMs and hence, leads to

unwanted fluctuations of the lasing intensity. First, the effect of evaporation on the

blinking properties can be better described by extensive measurements with drops

of different sizes, glycerol concentrations, and pump fluences. Such measurements

could help in theoretical modeling of the process and hence, the blinking properties

of any given drop could be predicted. Another desirable task would be to control the

evaporation to maintain the input resonance condition. This would lead to much

more efficient and stable laser operation. A humidity chamber, which can control the

sizes of microdrops with nanometer precision [155, 156], could be installed around

the trap for this purpose. In such a setup, precise Raman lasing threshold values

can be found for better characterization of the laser. Another approach could be to

use a tunable pump source and gradually shift the pump wavelength, in accordance

7.2 Future Outlook 135

with the drop’s evaporation, such that the input coupling is maintained at all times.

In addition, since the spectral location of the Raman gain depends on the pump

wavelength, the possibility of obtaining Raman lasing over the whole transparency

window of glycerol can be explored by using different pump sources.

The quantum dot microdrop laser also has opened the prospects of many in-

teresting experiments. To start, the lasing behavior and the threshold values can

be better studied with respect to different quantum dot concentrations and drop

sizes. These measurements would help better characterize the CdSe/ZnS quantum

dot laser and may even help in theoretical formulation of the lasing process. In addi-

tion, the lowest limit of quantum dot concentration which still supports stimulated

emission should be found out. Secondly, the stimulated emission cross-section for

our CdSe/ZnS quantum dots should be experimentally determined and the factor

of 8 (see section 6.4) compared to [153] should be ensured. Also, the distribution of

the quantum dots inside the microdrop, possibly by high magnification white light

microscopy, should be interesting to investigate. These measurements, in combina-

tion, can help in confirming that our measured concentration is indeed lower than

expected. In that case, the existing theory which predicts the lowest QD concentra-

tion for lasing [153] would need revision.

Effects of deforming the microdrop, in both the Raman and the quantum dot

laser, with external influences could also be investigated. Controlled deformation

could lead to lasing emission along a preferential direction and could hence, improve

the device efficiency. In our setup, the effects of gravity on the trapped microdrop

are cancelled by an external DC field. In such a situation, an external AC electric

field can cause controlled shape oscillations of the spherical droplet. The angular

frequency (fn) of these vibrations for small amplitudes, in vacuum, is given by [157]

f 2n =

n(n− 1)(n+ 2)γ

4ρπ2R3, (7.1)

where n in the number of nodes in the oscillation, and R, ρ, and γ are the radius,

density, and the surface tension of the droplet, respectively. Therefore, for any given

drop, the application of an additional low aplitude quadrupole field, apart from the

136 Conclusion

trapping field, at a frequency of fn can be a way to achieve a deformation of the

order n. For example, for a glycerol drop of radius 10 µm, the above equation can

be used to calculate f2 (n = 2) to be about 100 KHz.

In conclusion, we believe that the present work helps in the advancement and bet-

ter understanding of the field of microscale lasers. Our novel technique of levitating

the lasing microparticles, under atmospheric conditions, enables complete isolation

from external influences (like substrates or solutions). We demonstrated the versa-

tility of this technique by investigating three different material systems, namely, the

ZnO tetrapod laser, the glycerol microdrop Raman laser, and the CdSe/ZnS quan-

tum dot microdrop laser. However, the scope of the this technique is far-reaching

and can be used to study a variety of other micro and nanomaterials. In addition,

biological specimen, such as DNA molecules, living cells, micro-organisms, etc., can

also be investigated. For example, intrinsic physical properties, such as elasticity

and UV damage threshold, of a trapped microscale DNA molecule can be investi-

gated. Also, biological cells (typically 10 µm in size), can be studied in trapped

microdrops. Artificial cell fusion, previously demonstrated with optical traps [158],

could be a novel experiment to be performed in our setup. As the trapped drop

evaporates, the average separation between the cells would decrease which might

result in the fusion of two or more cells. In addition, similar drops containing a sin-

gle cell can be used to investigate the cell damage process (due to air exposure and

charging) near the completion of droplet evaporation. Furthermore, experiments

with micro-organisms in the trapped drops, such as tracking bacterial motion and

studying viral reproduction, can be of considerable interest.

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Publications

Journal Articles

1. R. Sharma, J. P. Mondia, J. Schaefer, Z. H. Lu, and L. J. Wang, “Effect of

evaporation on blinking properties of the glycerol microdrop Raman laser ”, Jour-

nal of Applied Physics 105, 113104 (2009).

2. R. Sharma, J. P. Mondia, J. Schaefer, W. Smith, S.-H. Li, Y. P. Zhao, Z. H.

Lu, and L. J. Wang, “Measuring the optical properties of a trapped ZnO tetrapod”,

Microelectronics Journal 40, 520 (2009).

3. J. P. Mondia, R. Sharma, J. Schaefer, W. Smith, Y. P. Zhao, Z. H. Lu, and

L. J. Wang, “An electrodynamically confined single ZnO tetrapod laser”, Applied

Physics Letters 93, 121102 (2008).

4. J. Schaefer, J. P. Mondia, R. Sharma, Z. H. Lu, A. S. Susha, A. L. Rogach,

and L. J. Wang, “Quantum Dot Microdrop Laser”, Nano Letters 8, 1709 (2008).

5. J. Schaefer, J. P. Mondia, R. Sharma, Z. H. Lu, and L. J. Wang, “Modular

Microdrop Generator”, Review of Scientific Instruments 78, 066102 (2007).

Conference Contributions

6. R. Sharma, J. P. Mondia, J. Schaefer, Z. H. Lu, and L. J. Wang, “Microdrop Ra-

man Laser and Plasmon Enhancement Effects”, CLEO 2008, San Jose, USA (May

2008). (Oral)


and L. J. Wang, “Quantum Dot Microdrop Laser”, CLEO 2008, San Jose, USA

(May 2008). (Oral)

8. R. Sharma, J. P. Mondia, J. Schaefer, W. Smith, S.-H. Li, Y.-P. Zhao, Z. H.

Lu , and L. J. Wang, “Optical Properties of levitated ZnO nanowires” WRA-LDSD,

Nottingham, UK (April 2008). (Oral)


and L. J. Wang, “Emission Properties of Quantum Dots in a Levitated Microdrop”,

CLEO 2007, Munich, Germany (June 2007). (Poster)

Documents

Electrodynamically Confined Microscale Lasers · Rachit Sharma aus Bhilai, Indien Max-Planck-Institut fu¨r die Physik des Lichts Erlangen, 2009. Als Dissertation genehmigt von den