Mikroskopische Methoden & Anwendungen · • Ole Christensen Rømer (1644 - 1710) Calculation...

1

Mikroskopische Methoden

& Anwendungen WS 12/13

MLS Master 1. Semester

Strukturanalytik, Teil D

Prof. Rainer Duden, Prof. Alfred Vogel, Dr. Gereon Hüttman

Lichtquellen und Detektoren

Sir Isaac Newton Christian Huygens Augustin Fresnel James Maxwell Albert Einstein

What is Light?

The question of the nature of light has puzzled philosophers

and scientists for more the 2000 year and is connected with

main discoveries in physics.

History of Optics • Ancient philosophers in Greece (Pythagoras, Demokrit,

Empedokles, Plato, Aristoteles)

What is light? Basic geometrical optics, refraction of light, primitive

lenses (ice), mirrors

• Middle ages: arabic scientists e.g. Alhazen (1000 n. Chr.)

reflection, spherical and parabolic mirrors, anatomy of human eye

• Roger Bacon (1215-94)

Lens for correction of sight → first glasses

• Leonardo da Vinci (1452-1519)

describes the „Camera obscura“

• Galileo Glailei (1564 - 1642)

First telescope

• Willebrod Snellius (1591 - 1621)

Law of refraction

• Robert Boyle (1626 - 1691)

Robert Hook (1635 - 1703)

Diffraction, Interference, colours form thin layers

• Isaac Newton (1642 - 1727)

Colours of white light 1666, light as particles

• Ole Christensen Rømer (1644 - 1710)

Calculation of the velocity of light by the shading of

the moons of Jupiter

• Christian Huygens (1629 - 1695)

Light as waves in the aether

• Thomas Young (1773 - 1829)

Interference principle

• Jean Fresnel (1788 - 1827)

Description of light propagation by diffraction and

interference

• Michael Faraday (1791 - 1867)

Experimente zur magnetischen Induktion und

Drehung der Polarisation

• James Clark Maxwell (1831 - 1879)

Theory of electromagnetic waves

• Heinrich Hertz (1857 - 1894)

Generation and detection of electromagnetic waves

Light is an Electromagentic Wave…

Spectrum of Electromagnetic Waves

m/s103 8 cf

Light is quantitized!

h = 6,6 10 Js -34 c = 3 108 m/s

/chhE

The photoelectic effect showed the

quantization of light. For a certain

metal only below a certain

wavelength light can remove

electrons, irrespectively of the

irradiance.

Experimental set up

• Albert Einstein (1879 –1955)

quantisation of light

Sun

H

Na

Cu

CN

CO2

solid or fluid

• Niels Bohr(1885 - 1962)

quantisation of energy state

• Albert Einstein (1879 –1955)

Stimulated Emission

All these 50 years of conscious brooding have brought me no nearer to the answer to the question „What

are light quanta?“. Nowadays every Tom, Dick and Harry thinks he knows it, but he is mistaken.

(Einstein)

The first working laser, a ruby laser,

constructed by Theodore Maiman in 1960

In the second half of the 20th century applied optics flourished:

• Communication (Laser (1960), optical fibers, ...)

• Image projection (Displays, laser TV...)

• Cameras (surveillance , IR-cameras, CCD, photography,...)

• Thin film technology (dielectric mirrors, filter,...)

• Laser (communication, medicine, data recording, bar-code

scanner,CD, material processing,...)

• Optical crystals (opt. modulators, LCD-Displays, LED,...)

• Holography (non-destructive testing, credit cards,...)

• Quantum computer

• .......

The 21st century as the century of the photon?

Sources of Optical Radiation

1. Sunlight, skylight

2. Incandescent sources

a) Blackbody sources

b) Nernst glower and globular

c) Tungsten filiament

3. Discharge lamp

a) Spectral sources (Hg, D2, He)

b) High-intensity sources (Hg, Xe)

c) Flash lamps

4. Fluorescent lamps

5. Light emitting diodes (LED)

6. Lasers

7. Synchrotron sources

T

Tungsten filiament (Black Body Radiation)

Gas Discharge Atoms or gas molecules are excited by

an electrical discharge. On return to their

ground state they emit light of a

characteristic wavelength (photon

energy)

Gas discharge lamp for microscopy

Emission von Halbleitern (LEDs)

Eg: band gap EA: Excitation energy of doped atom

External voltage lifts Fermi-energy level in n-region

creates inversion in transition zone

Electron-hole recombination creates photons (or phonons)

High conversion efficiency electric energy light (30-50%)

holes

free electrons

Light Emitting Diode (LED)

Comparision of Gas Discharge Lamp with LEDs (1)

Comparision of Gas Discharge Lamp with LEDs (2)

LASER: Light Amplification by Stimulated Emission of Radiation

- Nobel Price 1964 (CT) and 1981 (AS) -

Key properties of laser irradiation

• Narrow spectral bandwidth

(“monochromacity”)

• Small divergence

• Coherence

High (spectral) brightness

What is this good for?

Charles H. Townes

*1915, age 93

Arthur Schawlow

1921-1999

laser physics 27 SS 2008 nach R. Hibst: Laser, Laser/Material-Interaction, and Applications

background

radiative transfer

between energy

levels

H - atom

energ

y 13.6 eV

0 eV

+

-

Lyman Balmer Paschen

spectral series

DE = h n

laser physics 28 SS 2008 nach R. Hibst: Laser, Laser/Material-Interaction, and Applications

“keeping Einstein´s track”

two level system

idealized material:

assembly of Ntot atoms

with just two energy levels

Ntot = N1 + N2

radiative transfer between two levels

is allowed: h n21 = E2 – E1

E2

E1 N1, g1

N2, g2

atoms are in thermal equilibrium

with radiation field

laser physics 29 SS 2008 nach R. Hibst: Laser, Laser/Material-Interaction, and Applications

“keeping Einstein´s track”

two level system E2

E1 N1, g1

N2, g2

what will happen?

laser physics 30 SS 2008 nach R. Hibst: Laser, Laser/Material-Interaction, and Applications

emission and absorption

(induced) absorption E2

E1 N1

N2

(g1=g2=1)

laser physics 31 SS 2008 nach R. Hibst: Laser, Laser/Material-Interaction, and Applications

emission and absorption

spontaneous emission

E2

E1 N1

N2

laser physics 32 SS 2008 nach R. Hibst: Laser, Laser/Material-Interaction, and Applications

emission and absorption

stimulated (induced) emission

E2

E1 N1

N2

photons / field in phase: coherent

Probabilities for absorption and stimulated

emission are equal!

laser physics 33 SS 2008 nach R. Hibst: Laser, Laser/Material-Interaction, and Applications

emission and absorption

two level system rate balance

E2

E1 N1

N2

laser physics 37 SS 2008 nach R. Hibst: Laser, Laser/Material-Interaction, and Applications

elements of a laser

1. Amplifying medium

2. Pump

3. Resonator

laser physics 38 SS 2008 nach R. Hibst: Laser, Laser/Material-Interaction, and Applications

pumping mechanisms

• Optical excitation (laser, arc-

lamp, flash-lamp)

• Electrical gas discharge (gas

laser)

• Radio frequency excitation of

gases

• Chemical reaction (F + H HF*

+ H)

• Injection of charges (diode laser)

• Acceleration of electrons in

amagentic field (free electron

laser)

laser physics 39 SS 2008 nach R. Hibst: Laser, Laser/Material-Interaction, and Applications

laser process

laser physics 40 SS 2008 nach R. Hibst: Laser, Laser/Material-Interaction, and Applications

pump energy

many atoms, ions, or molecules excited in laser medium

laser process

laser physics 41 SS 2008 nach R. Hibst: Laser, Laser/Material-Interaction, and Applications

high

reflecting

mirror

outcoupling

mirror

laser process

laser physics 42 SS 2008 nach R. Hibst: Laser, Laser/Material-Interaction, and Applications

lasing condition

Amplification > Loss

pump powerpump power

resonator losses

inversion

photon density

resonator losses

inversion

photon density

Electronic oscillator

Laser oscillator

laser physics 43 SS 2008 nach R. Hibst: Laser, Laser/Material-Interaction, and Applications

different laser types

The first laser build

Historic Overview

•1917 Postulation of the stimulated

emission Einstein

•1928 Experimental proof of the

stimulated emission Ladenburg,

Kopfermann

•1950 Experimental proof of an

inversion Purcel, Pound

•1951/1955 Suggestion to use stimulated Fabrikant, Weber,

emission for amplification Basov,Prochorov

•1954 NH3-Maser Townes

•1958 Suggestion to use stimulated

emission for amplification in the

optical reagion Schawlow, Townes

•1959 Suggestion to build

a gass laser Javan

•1960 First laser build (Ruby laser) Maiman

•1961 First HeNe-laser Javan,Benett, Herriott

•1962 First semiconductor laser Nathan,Duncke,

Burns, Dill, Lasher

laser physics 44 SS 2008 nach R. Hibst: Laser, Laser/Material-Interaction, and Applications

fundamental (TEM00 ) mode

phase front

2

w0

(confocal resonator L=R)

z

stable cavity,

(e.g. confocal resonator)

0w

laser physics 45 SS 2008 nach R. Hibst: Laser, Laser/Material-Interaction, and Applications

Die Schlüsseleigenschaften der Laserstrahlung?

Schlüsseleigenschaften der Laserstrahlung

• Schmale spektrale Bandbreite

(“Momochromasie”)

• Geringe Divergenz

⇒ Kohärenz

Hohe (spektrale) Strahldichte Le

Bsp. - 1 mW Laserpointer, 650 nm Le = 2.37 • 105 W/cm²

- 100 W Glühbirne W = 4 , w0 = 3 cm

Le = 7.2 • 10-3 W/cm²

Unterschied von 3 • 107 !

22

0

2

0

22

0

W

eL

Monochromacity, small divergence spectrometry

Enormous gain in

spectral brightness by

• Small divergence

of laser light source

• Tunable, narrow

spectral bandwidth

“Brightness” (Radiance)

Semiconductor lasers (3)

Gain guided Gain and refractive index guided

laser diode stack (for optical pumping) Individual laser diode

auf Reflektor

Semiconductor

lasers (4)

Laser diode

material

(active region

/ substrate)

Typical emission

wavelengths Typical application

InGaN / GaN,

SiC

380, 405, 450,

470 nm data storage

AlGaInP / GaAs 635, 650, 670 nm laser pointers, DVD players

AlGaAs / GaAs 720–850 nm CD players, laser printers

InGaAsP / InP 1000–1650 nm optical fiber

communications

Advantages

• Small

• Inexpensive

• High efficiency (30-50%)

• High power (kW)

• Can be modulated up to 10 GHz

• Long lifetime

Disadvantages

• Asymmetric beam profile

(e. g. θx=10°, θy=30°)

• High divergence

• Low pulse energies

High-power

diode lasers

Band gap determines laser wavelength

Modes of Light Detection

Optical detectors

Photon Tube

Outer electrooptical effect

Quantum Efficiency

Responsivity (Sensitivity)

Spectral Sensitivity

A/Wm000.806R

A/Wm000.806/

000

PPhc

ePRIP

Spectral Sensitivity

Photomultiplier Tube Detector

Anode

• High sensitivity at

low light levels

• Cathode material

determines spectral

sensitivity

• Good signal/noise

• Shock sensitive

The Photodiode Detector

• Wide dynamic range

• Very good

signal/noise at high

light levels

• Solid-state device

Θ Θ

Spektrale Empfindlichkeit

Photomultiplier (PMT) Photodiode (PD)

ElektronenneI Photonenn

hc

hc

enn

hc

eIR

nn

PhotonenElektronen

PhotonenElektronen

// :hkeitEmpfindlic

/ :beuteQuantenaus

Θ Θ Θ

Θ Θ

Θ Θ Θ Θ Θ Θ

Avalanche Photodiode

Schematic Diagram of a

Photodiode Array

• Same characteristics

as photodiodes

• Solid-state device

• Fast read-out cycles

CMOS Sensor

(Complementary Metal-Oxide-Silicon)

Charge Coupled Device (CCD)

Die CCD-Typen

L – lichtempfindliche Pixel,

T – Transfer-Register,

A – Ausleseverstärker.

Types of CCD-Sensors

FF CCDs = Full Frame CCDs

Application: astronomy, spektroskopy,

Advantages: high resolution, large arrays are possible

(7000x9000 Pixel by Philips)

Draw-back: mechanical shutter necessary

FT CCDs = Frame Transfer CCDs

Advantages: high resolution, high aperture, internal

shutter possible, homogeneous sensitivity

Draw-back: strong smearing effects, large chip areas

IL CCDs = Interline CCDs

Application: video cameras, low cost

Advantages: small chip

Draw-back: small aperture (can be increased by on-chip

lenses), strong smearing effects

Einchip Farb-CCD-Kameras

3-Chip Farb-CCD

Wieviele Pixel sind notwendig?

Struktur

Pixel

Wieviele Pixel sind notwendig?

Struktur

Pixel

Wieviele Pixel sind notwendig?

Struktur

Pixel

Wieviele Pixel sind notwendig?

Struktur

Pixel

Wieviele Pixel sind notwendig?

Struktur

Pixel

Wieviele Pixel sind notwendig?

Die kleinsten auflösbaren Strukturen sollten mit mindestens

4 Pixeln abgetastet werden!

Abtastung der Bilder

Kenngrößen für CCD- Kameras • Die Quantenausbeute, also die Wahrscheinlichkeit, dass ein einfallendes Photon ein

Elektron auslöst. Die Quantenausbeute von CCDs hängt von der Wellenlänge des Lichts

ab und kann über 90 % betragen (Fotografischer Film zum Vergleich: 5 bis 10 %).

• Der Dunkelstrom der lichtempfindlichen Zellen. Der Dunkelstrom ist stark

temperaturabhängig und führt aufgrund seiner statistischen Eigenschaften zu

Dunkelstromrauschen. Er ist für alle Pixel individuell verschieden und eine Quelle des

Bildrauschens. Weiter können einzelne „hot pixels“, also Pixel mit besonders hohem

Dunkelstrom auftreten.

• Die Anzahl der Ladungen, die in einem Pixel gespeichert werden können (engl. full well

capacity oder well depth'').

• Das Verhalten, wenn durch Überbelichtung in einzelnen Pixeln mehr Ladung erzeugt wird,

als gespeichert werden kann. Tritt die Ladung in benachbarte Pixel über, spricht man von

„Blooming“. Viele CCD-Kameras vermeiden diesen Effekt, indem die überschüssigen

Ladungen abgeleitet werden („anti-blooming gate“), dadurch kann aber auch schon

Ladung verloren gehen, bevor ein Pixel wirklich voll ist. Der Zusammenhang zwischen

Lichtmenge und Ladung ist dann nicht mehr linear, und genaue Messungen sind nicht

mehr möglich.

• Die Effizienz des Ladungstransports zum Ausleseverstärker (Charge Transfer Efficiency).

• Das Rauschen des Ausleseverstärkers (Ausleserauschen, engl. readout noise).

Empfindlichkeit

104

Bildrauschen

=

Ausleserauschen

+

Photonenrauschen

Typische Parameter für CCD-Kameras

Korrektur der Bilder

Electron Multiplying CCD

Photon Detectors

Semicondutor Vaccum tube

internal

ampli.

no

internal

ampli.

Radiometrische Größen

Lichtquelle bestrahlte

Fläche Raumwinkel

Größe Symbol Einheit

Strahlungsenergie (Radiant energy) [J] eQ

3

Energie einer Anzahl von Photonen

Strahlungsdichte (Radiant energy density) / [J/m ] volumetrische Energiedichte

Bestrahlung (Radiant exposure)

e ew dQ dV

2/ [J/m ] pro Fläche empfangene Energie

Strahlungsfluss (Radiant flux) / [W] Strahlungsenergie pro Zeit

spezifische

A

e e

e e

H dQ dA

dQ dt

2

2

ustrahlung (Radiant exitance) / [W/m ] Strahlungsfluss pro Emitterfläche

Bestrahlungsstärke (Irradiance) / [W/m ]

e e

e e

M d dA

E d dA

Strahlungsfluss pro Empfängerfläche

Strahlstärke (Radiant intensity) / [W/sr] Strahlungsfluss pro Raumwinkel

Strahldichte (Radiance)

e eI d d W

2 / cos [W/(sr m )] Strahlungsfluss pro Raumwinkel pro Emitterfläche

e eL dI dA

Photometry

Photometry measures the brightness of light

Relavive spectral sensitivity of the eye for

photopic (V) and scotopic (V‘) vision

photometric unit = 685 lm/W V() radiometric unit

Spectral Radiometry and

Photometry

For polychromatic light all quantities can be related to the

wavelength :

dds /)()(

ds

2

1

)(

Total quantities are calculated by integration :

Radiometrische Größen Größe Symbol Einheit

Strahlungsenergie (Radiant energy) [J] eQ

3

Energie einer Anzahl von Photonen

Strahlungsdichte (Radiant energy density) / [J/m ] volumetrische Energiedichte

Bestrahlung (Radiant exposure)

e ew dQ dV

2/ [J/m ] pro Fläche empfangene Energie

Strahlungsfluss (Radiant flux) / [W] Strahlungsenergie pro Zeit

spezifische

A

e e

e e

H dQ dA

dQ dt

2

2

ustrahlung (Radiant exitance) / [W/m ] Strahlungsfluss pro Emitterfläche

Bestrahlungsstärke (Irradiance) / [W/m ]

e e

e e

M d dA

E d dA

Strahlungsfluss pro Empfängerfläche

Strahlstärke (Radiant intensity) / [W/sr] Strahlungsfluss pro Raumwinkel

Strahldichte (Radiance)

e eI d d W

2 / cos [W/(sr m )] Strahlungsfluss pro Raumwinkel pro Emitterfläche

e eL dI dA

Größe Symbol Einheit

Lichtmenge (Luminous energy) [lm s] vQ

2

gewichtet Energie einer Anzahl von Photonen

Belichtung (Luminous exposure) / [lm s/m ] pro Fläche empfangene Lichtmenge

Lichtstrom (Luminous

v vH dQ dA

2

flux) / [lm] Lichtmenge pro Zeit

Spezifische

Lichtausstrahlung (Luminous exitance) / [lm/m ] Lichtstrom pro Emi

v v

v v

dQ dt

M d dA

2

tterfläche

Beleuchtungsstärke (Illuminance) / [lm/m ] Lichtstrom pro Empfängerfläche

Lichtstärke (Luminous intensity) /

v v

v v

E d dA

I d d

W

2

[lm/sr], [cd] Lichtstrom pro Raumwinkel

Leuchtdichte (Luminance ) / cos [cd/m ] Lichtstrom pro Raumwinkel pro Emitterfläche

v vL dI dA

Photometrische Größen

What limits the irradiance we can

achieve?

It is the Radiance (Strahldichte)

of the light source!

2211

2211

WW

AA

HH

/ cose eL Q t A W