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Physik Department
Upgrade of an Attosecond Beamline
for >1 mJ/<5 fs Driver Pulses
Setup and Attosecond Experiments
Master's Thesis
by
Clemens Jakubeit
October 2013
Max-Planck-Institut für Quantenoptik
Technische Universität München
Prof. Dr. Reinhard Kienberger
Physik Department
Upgrade of an Attosecond Beamline
for >1 mJ/<5 fs Driver Pulses
Setup and Attosecond Experiments
Master's Thesis
by
Clemens Jakubeit
October 2013
Max-Planck-Institut für Quantenoptik
Technische Universität München
Prof. Dr. Reinhard Kienberger
Zusammenfassung
Mit wachsender Genauigkeit physikalischer Untersuchungen steigen die technischen
Anforderungen an die nötigen experimentellen Versuchsaufbauten. Für erfolgreiche
moderne Forschung ist häu�g das Zusammenspiel vieler ausgefeilter Instrumente
notwendig. Diese Arbeit beschreibt eine derartige komplexe Einrichtung, eine At-
tosekundenbeamline am Max-Planck Institut für Quantenoptik in Garching. Der
Aufbau wurde entwickelt um ultra schnelle physikalische Prozesse zu untersuchen. In
Kapitel 2 geben wir eine Einführung in die relevanten physikalischen E�ekte. Kapitel
3 beschreibt den verwendeten Femtosekundenlaser. Während der Erstellung dieser
Arbeit wurde der Laser mit einer weiteren Verstärkerstufe ausgestattet, um die ver-
fügbaren Pulsenergien zu erhöhen. Der Prozess des Einbaues und dafür nötige Unter-
suchungen sind in Sektion 3.2 dargestellt. Den Aufbau der eigentlichen Beamline für
die Erzeugung von XUV Attosekundenpulsen und experimentelle Untersuchungen
mit diesen beschreibt Kapitel 4. Da für die vorgesehenen Experimente eine Vaku-
umumgebung notwendig ist, müssen die beweglichen Teile des Aufbaues motorisiert
sein. Die dafür notwendigen Bauteile wurden ausgetauscht und ihre Steuerung neu
erstellt. Sektion 4.2 beschreibt die durchgeführten Änderungen und Verbesserungen.
Die vorgestellte Infrastruktur wurde für zwei Experimente verwendet. In Kapitel 5
werden zeitaufgelöste Messungen der UV-Absorption und der daraus resultieren-
den Spaltung von Ozon präsentiert. Kapitel 6 beschreibt Untersuchungen der durch
hochintensive Femtosekundenpulse ausgelösten nichtlinearen optischen Antwort von
Siliziumdioxid.
I
Abstract
With increasing detail of the investigations of physical phenomena the technical
requirements for the necessary experimental setups become more demanding. For
modern research often a long chain of sophisticated instruments has to be com-
bined. This thesis describes such a complex facility, an attosecond beamline and
the associated femtosecond laser at the Max-Planck-Institute of Quantumoptics in
Garching, designed for the investigation of ultra fast physics. In chapter 2 we give a
brief theoretical introduction to the e�ects involved. Chapter 3 presents the driving
femtosecond laser system. During the performance of this thesis the laser involved
was upgraded with an additional ampli�er to increase the available pulse energies.
The process of the installation and necessary considerations therefor are described
in section 3.2. The setup of the beamline required for the generation of XUV pulses
with attosecond durations and subsequent experiments is presented in chapter 4.
Due to the necessity of vacuum for the intended applications, the moveable parts
in the beamline need to be motorized. The respective hardware was exchanged and
the associated computer control rewritten. Section 4.2 gives a detailed explanation
of the changes made. The described infrastructure was applied to two experiments.
In chapter 5 time resolved measurements of the UV absorption and subsequent dis-
sociation of ozone are presented. Chapter 6 treats the investigation of the nonlinear
optical response of silicon dioxide during the transition of high intensity femtosecond
laser pulses.
III
Contents
Zusammenfassung I
Abstract III
1 Introduction 1
2 Theory 3
2.1 Short laser pulses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2 Nonlinear optics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2.1 Perturbative regime . . . . . . . . . . . . . . . . . . . . . . . . 8
2.2.2 High harmonic generation . . . . . . . . . . . . . . . . . . . . 12
2.2.3 From pulse trains to isolated attosecond pulses . . . . . . . . . 15
2.3 Pulse characterization . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.3.1 Autocorrelation . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.3.2 FROG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.3.3 Streaking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3 Laser FP3 21
3.1 Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.1.1 Oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
3.1.2 Chirped pulse ampli�er . . . . . . . . . . . . . . . . . . . . . . 23
3.1.3 Booster stage . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.1.4 Spectral broadening . . . . . . . . . . . . . . . . . . . . . . . . 24
3.1.5 Compression . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3.1.6 Carrier envelope phase stabilization . . . . . . . . . . . . . . . 26
3.2 Installation of the booster stage . . . . . . . . . . . . . . . . . . . . . 27
3.2.1 Spectra at di�erent locations . . . . . . . . . . . . . . . . . . . 28
V
VI Contents
3.2.2 Pulse optimization for compression via feedback . . . . . . . . 29
3.2.3 Spectral broadening . . . . . . . . . . . . . . . . . . . . . . . . 31
3.2.4 Final compression . . . . . . . . . . . . . . . . . . . . . . . . . 31
4 Beamline AS2 33
4.1 Beamline setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
4.1.1 High harmonic generation . . . . . . . . . . . . . . . . . . . . 34
4.1.2 Interferometer . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
4.1.3 Experiment chamber . . . . . . . . . . . . . . . . . . . . . . . 44
4.2 New software control system for the beamline . . . . . . . . . . . . . 47
4.2.1 Programmatic structure of the control system . . . . . . . . . 47
4.2.2 Access control . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
4.2.3 Server program . . . . . . . . . . . . . . . . . . . . . . . . . . 49
4.2.4 Low level functions available for further development . . . . . 49
4.2.5 Programs developed or extended . . . . . . . . . . . . . . . . 50
5 Ozone 53
5.1 Basic measurement principle . . . . . . . . . . . . . . . . . . . . . . . 54
5.2 Production of ozone . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
5.3 Pressure control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
5.3.1 Automation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
5.4 Experimental results and outlook . . . . . . . . . . . . . . . . . . . . 58
6 Silicon dioxide 61
6.1 Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
6.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
7 Conclusion 65
A Bibliography i
B Acknowledgments v
Chapter 1
Introduction
Research today is often driven by the desire to understand nature in more detail.
In the �eld of microscopy spatial resolution has increased to an extent where single
atoms [1] and even electron distributions in molecules [2] can be resolved directly.
Besides the spatial characteristics, also the temporal evolution of events gets studied
in ever more detail. Processes like the wing beat of a �y or the �ring of a bullet, tak-
ing place in the regime of milli- or microseconds respectively, can still be recorded
by conventional camera systems. Events in modern day electronics happen on a
timescale of nano- and picoseconds while the investigation of molecular motion ex-
tends into the regime of femtoseconds. The observation of the electrons within atoms
and molecules �nally requires resolutions in the regime of attoseconds. The inves-
tigation of processes always requires probes with properties similar to the studied
objects. With conventional cameras the shutter speed has to be as fast as the event
to be recorded, and nano- and picosecond durations are measurable by sophisticated
electronic devices. For femto- and attosecond resolution, events among the fastest
controllable today have to be employed, ultra short light pulses.
The generation of laser pulses with sub 100 femtosecond duration became possi-
ble with the discovery of passive Kerr-lens mode locking in Ti:Sa laser systems
in 1991 [3]. Advances in dispersion control soon led to pulses as short as �ve fem-
toseconds [4]. By the introduction of spectral broadening [5] and subsequent com-
pression these durations could be reduced even further. With these pulses available
as a driver it became possible to generate sub-femtosecond pulses via a conversion
1
2 1. Introduction
process. The basic principle of high harmonic generation was already predicted in
1992 [6] and experimentally con�rmed in 1993 [7]. The challenge to isolate single
pulses from the train of pulses generated by this process was �nally managed in
2001 [8], pioneering attosecond metrology. The availability of these tools allows the
investigation of ultrafast processes in unprecedented detail. With sub femtosecond
resolutions new insights in the dynamics of electrons in solids, molecules, or even
atoms are possible.
Chapter 2
Theory
2.1 Short laser pulses
The endeavor to investigate physical processes in ever more detail requires probes
in the regime of the observed objects. In the same way as with the pursuit to in-
creased spatial resolutions, new, quantum mechanical e�ects have to be considered,
increasing the temporal resolution requires to account for e�ects that can, in most
other applications, be neglected. A prime tool for the investigation of ever faster
molecular and atomic processes are laser pulses in the few femtosecond and attosec-
ond regime. With decreasing pulse duration, dispersion gets a deciding factor for the
generation and handling of such pulses. This section gives a brief introduction to
the model used for the description of ultra short laser pulses and their propagation
in dispersive media.
A general, scalar, mathematical description of pulses is given by
U(t) = A(t) · eiω0t = |A(t)| · eiϕ(t) · eiω0t (2.1)
with A(t) the complex envelope, ω0 the central angular frequency, |A(t)| and ϕ(t) the
amplitude and phase of the envelope respectively. This description is only considering
the temporal evolution. The spatial distribution and propagation of the pulses is
neglected. For the complete characterization of a pulse at a �xed location it is
su�cient to evaluate the central frequency and the envelope's amplitude and phase.
3
4 2. Theory
As the phase can have complex structures, it is a common method to expand it in
a power series.
ϕ(t) = ϕ(0) + ϕ(1) · t+1
2· ϕ(2) · t2 +
1
6· ϕ(3) · t3 + ...+
1
n!ϕ(n) · tn (2.2)
with ϕ(n) the nth derivative ϕ(n) = ∂nϕ∂tn
This structure allows the separation of di�erent temporal variations in the distribu-
tion of frequencies throughout the pulse. An intuitive quantity for the description
of pulses is the instantaneous frequency which is given by
ωi =∂φ(t)
∂t=
∂
∂t(ω0t+ ϕ(t)) = ω0 +
∂ϕ(t)
∂t(2.3)
A linear change in the instantaneous frequency is called a linear chirp, which is
caused by a non-zero second order phase in equation 2.2. According to the sign of
ϕ(2) linear chirps are classi�ed as positive or negative respectively. Figure 2.1 shows
the �eld of chirped pulses. With time, the instantaneous frequency increases (a) or
decreases (b) linearly, increasing the pulses duration simultaneously.
Up to this point we only considered the temporal evolution of a pulse at a �xed
position in space. To be able to utilize and manipulate such pulses the e�ects of
propagation through materials are important.
Though in many applications the refractive index and therefore the phase velocity
vph(ω) = c0n(ω)
is considered a material speci�c constant, it is in fact a frequency
dependent function called dispersion. As pulses can be described as a superposition
of plane waves of di�erent frequencies, dispersion has important implications. These
are most easily treated in the spectral domain. Not di�erent from the cases of longer
pulses or continuous waves, the spectrum V (ω) of ultra short pulses can be obtained
2.1. Short laser pulses 5
−5 0 5
−1
0
1
time (arb. units)
U(t)
(arb
. uni
ts)
−5 0 5time (arb. units)
a) b)
Figure 2.1: Chirped pulses. A positive(a)/negative(b) second order phase ϕ2 causes
an increase(a)/decrease(b) in the instantaneous frequency over time
by the Fourier transform of the wave form U(t) 2.1.
V (ω) = F(U(t)) =1√2π
∫ ∞−∞
dt · U(t) · e−iωt (2.4)
Similar to the description in the time domain in equation 2.1, this function can be
separated into amplitude and phase.
V (ω) = |V (ω)| · ei·ψ(ω) (2.5)
The amplitude |V (ω)| represents the spectrum of the pulse, that can be measured by
a common spectrometer. To determine the spectral phase ψ(ω), more sophisticated
methods have to be applied, see 2.3. The transformation into the frequency domain
does not lose any information, so this description still is a complete characterization
of the pulses properties. Again it is convenient to expand the phase ψ(ω) into a
6 2. Theory
power series to separate di�erent e�ects in the behavior of the pulse
ψ(ω) = ψ(0) + ψ(1) · ω +1
2ψ(2) · ω2 +
1
6ψ(3) · ω3 + ...+
1
n!ψ(n) · ωn (2.6)
with ψ(n) the nth derivatives.
The propagation of plane waves through a distance L of a dispersive medium causes
a phase shift of
∆ψ(L, ω) = k(ω)L =ω · n(ω)
c0
· L (2.7)
with k the wavenumber, c0 the vacuum speed of light, n(ω) the frequency dependent
refractive index. While in the temporal domain this is only valid for plane waves,
it is a complete description in the spectral domain, as the frequency dependence is
accounted for. The pulse after the distance L can be written as
V (ω, L) = |V (ω, 0)| · ei·ψ(ω,L) =
= |V (ω, 0)| · ei·(ψ(ω,0)+∆ψ(ω,L)) = V (ω, 0) · ei·∆ψ(ω,L)(2.8)
This is a simple equation to describe the e�ect of the propagation of a pulse through
a dispersive medium. The e�ect is represented by an addition in the spectral phase.
Going back to the expansion of the phase 2.6, the e�ects are classi�ed after their or-
der. To retrieve material speci�c but length independent factors they can be divided
by the length of the material. First order
∂ψ
∂ω= L · ∂k
∂ω=L
vg(2.9)
is called group delay and is the inverse group velocity multiplied by the length. Due
to the nature of Fourier transforms, �rst order translates into the time domain as
a delay. Therefore �rst order is often assumed to be zero, as the zero mark for the
time scale can in most cases be chosen accordingly. Second order
GDD =∂2ψ
∂ω2= L · ∂
2k
∂ω2= L ·GVD (2.10)
2.2. Nonlinear optics 7
is called group delay dispersion (GDD) and the length independent measure, group
velocity dispersion (GVD). Second order is the �rst to change the pulse's form. A
non-zero GVD translates into the already described linear chirp and at the same
time increases the pulses duration. To form short pulses in time, a large spectral
bandwidth as well as a �at phase are required. This leads to the fact, that even
small amounts of dispersive material and higher orders of dispersion can have a sig-
ni�cant impact on the pulses form and duration. In the optical regime only materials
with positive dispersion are available. Negative dispersion, as e.g. necessary for the
compression of positively chirped pulses, has to be introduced by di�erent optical
path lengths via e.g. prisms, gratings, or chirped mirrors.
2.2 Nonlinear optics
In the regime of low intensity optics the polarization response of matter to the
electro-magnetic �eld is linear in its behavior. With increasing intensities this changes,
nonlinear, higher order polarization has to be considered. As a whole range of new
e�ects and possibilities gets available, the description gets more complicated.
The ongoing e�orts to investigate new properties of matter are often limited by
the available probes. While the range of laser active materials is wide, the available
frequencies are still limited mostly to the visible and infra red spectral range. Fortu-
nately it is possible to extend that range signi�cantly by using frequency conversion
techniques made possible by nonlinear optical e�ects. Two regimes for the e�ects
treated in this theses have to be distinguished. While the behavior in the perturba-
tive regime can be derived from Maxwell's equations, at even higher intensities the
process of high harmonic generation requires a semiclassical approach, considering
the actual motion of electrons in an electric �eld.
8 2. Theory
2.2.1 Perturbative regime
The starting point for the description of the perturbative regime is the one dimen-
sional wave equation for non magnetic materials, derived fromMaxwell's equations
∂2E∂z2− 1
c20
∂2E∂t2
= µ0∂2P∂t2
(2.11)
with E the electric �eld, z the spatial coordinate, c0 the vacuum speed of light,
µ0 the vacuum permeability, P the polarization of the material. In this equation
the polarization P can be viewed as a source, driving the oscillations of the electric
�eld. With increasing �eld intensities the classical approximation of a linear response
P = ε0χE is not valid any more. Higher, non linear, orders of polarization have to
be considered. The polarization can be written as
P = ε0 · (χ(1) · E + χ(2) · E2 + χ(3) · E3 + ...) (2.12)
with χ(i) the ith order electric susceptibility, which in the general case is an (i+ 1)th
order tensor. Following the structure of this expansion, the nonlinear e�ect are clas-
si�ed according to the order i of the involved susceptibility χ(i).
Looking exemplarily at the second order polarization, it can be seen how this for-
malism describes the generation of new frequencies. Considering the polarization as
a source for waves, according to equation 2.11, light with the according frequencies
follows directly. With E1,2 the electric �elds, A1,2 the complex amplitudes, A∗1,2 their
complex conjugates, ω1,2 the angular frequencies, cc. the complex conjugate
P(2) = ε0χ(2)E1E2 =
= ε0χ(2)(A1 · eiω1t + A∗1 · e−iω1t) · (A2 · eiω2t + A∗2 · e−iω2t)
= ε0χ(2)(A1A2e
i(ω1+ω2)t + A1A∗2ei(ω1−ω2)t + cc.)
(2.13)
This example shows the two e�ects of sum frequency generation (SFG) (ω3 = ω1+ω2)
and di�erence frequency generation (DFG) (ω3 = ω1−ω2). As this description does
not exclude the case of ω1 = ω2 it also includes second harmonic generation (SHG)
(ω3 = 2 · ω1) and static polarization ω3 = 0. Phenomena using the same frequency
2.2. Nonlinear optics 9
as input multiple times are called degenerate.
Apart from the mentioned second order e�ects, third order phenomena are important
for the understanding of this thesis.
Third harmonic generation
While with increasing order of the e�ect the e�ciency of the frequency conversion
decreases, third harmonic generation (THG) (ω4 = 3 · ω1) is an important tool.
Besides the obvious, to be able to generate the third harmonic, there is an additional
reason to use it instead of SHG. Looking at the underlying molecular processes, it
can be shown that even order phenomena can not take place if the used medium is
centrosymmetric, excluding e.g. gases.
Optical Kerr e�ect
The so called optical Kerr e�ect, simply "Kerr e�ect" in this thesis, describes the
change of the refractive index of an optical medium by the intensity of the passing
electromagnetic wave. It is to be distinguished from the optoelectric Kerr e�ect,
where the refractive index varies with the square of the �eld strength of an exter-
nally applied static electric �eld. The frequency mixing of the optical Kerr e�ect is
degenerate and of the third order. It can be written as:
ω4 = ω1 − ω1 + ω1 = ω. (2.14)
Going back to equation 2.12, the total polarization of frequency ω in absence of
second order is given by
1
ε0P tot(ω) = χ(1)E(ω) + 3 · χ(3)|E(ω)|2 · E(ω) (2.15)
The factor of three comes from the permutations of the frequency mixing. With
some transformations, it can be shown that this leads to the expression for the total
10 2. Theory
refractive index
n(t) = n0 + n2 · I(t) (2.16)
with n0 the usual, static refractive index and n2 the nonlinear refractive index
n2 =3χ(3)
4n0
(2.17)
[9].
The intensity dependent refractive index changes the propagational behavior of high
intensity beams and pulses signi�cantly. Especially intensity gradients lead to new
e�ects that have to be considered. They can be separated into spatial and temporal
e�ects.
Self-focusing Self focusing describes the nonlinear e�ect of an inhomogeneous
spatial intensity distribution and the resulting change of the refractive index. The
spatial distribution of the intensity of a laser beam can often be approximated by a
Gaussian beam.
I(z, r) = I0 ·(
w0
w(z)
)2
· exp
(− 2r2
w2(z)
)(2.18)
with I0 the intensity at the beam center at the beam waist, w0 the beam waist, r the
radial distance from the beam center, w(z) the beam size at position z. Equation
2.16 shows that this leads to a proportional distribution of the refractive index in
space. The radial di�erence of optical thickness results in the material acting as an
e�ective lens. The nonlinear response to the pulse changes the wavefront curvature
of the incident beam and focuses it.
Self-phase modulation Self-phase modulation describes the spectral nonlinear
e�ect of an inhomogeneous temporal intensity distribution and the resulting change
in the refractive index[10]. The change in the refractive index generates a time
dependent phase, causing a variation in the instantaneous frequency of the pulse.
2.2. Nonlinear optics 11
−1 0 1
0
time (arb. units)
Inte
nsi
ty (
arb
. u
nits
)
−1 0 1
0
∆ ω
i (a
rb. u
nits
)
Pulse envelope∆ ω
i
Figure 2.2: Self-phase modulation. Red: Temporal intensity distribution of the
pulse. Blue: Change in the instantaneous frequency, ∆ωi, caused by self phase mod-
ulation.
Looking at a transform limited pulse (ϕi(t) = 0) after passing through a medium,
the instantaneous frequency is given by
ωi =∂
∂t(ω0t− kL) =
∂
∂t(ω0t− k0 · n(t) · L) (2.19)
with the length of the material L, the total time-dependent refractive index n(t), and
the vacuum k vector k0. With equation 2.16 and a Gaussian intensity distribution
this leads to
ωi =∂
∂t(ω0t− k0 · L · (n0 + n2 · I0 · exp
(− t
2
τ 2
)) (2.20)
= ω0 +2n2 · k0 · L · I0
τ 2· t · exp
(− t
2
τ 2
)= ω0 + ∆ωi(t) (2.21)
12 2. Theory
Figure 2.2 shows schematically the temporal intensity pro�le of a Gaussian pulse
and the resulting change in the instantaneous frequency ∆ωi, caused by SPM. The
leading edge of the pulse is shifted to lower frequencies, while the trailing edge
experiences a shift to higher frequencies. Important about SPM is the fact that this
change is not only a temporal redistribution of spectral components, like it might be
caused by dispersion, but causes the generation of new spectral components. This
makes SPM a valuable tool for spectral broadening and the generation of super
continua.
A complete description of the mentioned phenomena exceeds by far the possibilities
of this thesis. More details can be found in "Nonlinear Optics" by Robert W. Boyd
[9]
2.2.2 High harmonic generation
The generation of high harmonic radiation (HHG) from ultra intense laser pulses is
a highly nonlinear process. In contrast to low order nonlinear e�ects, like the ones
introduced earlier, HHG can not be described by perturbative theory.
Starting from intensities of approximately 1012 W/cm2 the perturbative approach
fails because the electric �eld reaches the magnitude of the binding potential of
electrons in atoms. The atoms start to ionize in the �eld. This is caused by two main
processes: Multi photon ionization (MPI) and tunnel ionization. A �rst attempt of a
description was developed by Keldysh in 1965 [11] and re�ned by Ammosov, Delone,
and Krainov in 1986 [12]. An important parameter to distinguish between the two
processes is the so called Keldysh parameter
γ =
√Ip
2UP=
ω
e · Emax·√
2IP ·m (2.22)
with IP the ionization potential of the considered medium, ω the angular frequency
of the laser �eld, Emax the maximum of the electric �eld, e the elementary charge,
m the electron mass, and UP the ponderomotive energy
UP =e2E2
max
4mω2(2.23)
2.2. Nonlinear optics 13
describing the time averaged kinetic energy of an electron in an oscillating electric
�eld[13]. For values γ < 1 tunnel ionization predominates, for γ > 1 MPI.
Figure 2.3: Three step model of high harmonic generation. a) Deformation of the
binding potential and tunneling, b) electron acceleration, c) recollision [14]
The process of HHG in an atomic gas can be described in the quasi classical ap-
proximation of the so called three step model [13]. In the �rst step, tunneling, the
binding potential in the atom is deformed strongly enough for the electron to tun-
nel through the Coulomb barrier and the atom being ionized (Figure 2.3 a). In the
second step the electron is accelerated away from the parent ion. Because of the
changing sign of the electric �eld of the linearly polarized driving laser, the electron
reverses direction and is accelerated back (Figure 2.3 b). The third step describes
the recollision of the electron with its parent ion and the resulting emission of an
XUV photon according to
EHH = hν = Ip + Ekin (2.24)
with EHH the photon energy, ν the frequency, IP the ionization energy, and Ekin the
electrons kinetic energy at the time of the recollision (Figure 2.3 c). The probability
for this last step is in the regime of 10−6, signi�cantly limiting the e�ciency of
the overall process of HHG. It can be shown that the maximum obtainable kinetic
energy for the electron is given by
Ekin,max = 3.17 · UP [13] (2.25)
14 2. Theory
1 3 2 1 2 9
Intensi
ty (arb
. units)
H a r m o n i c o r d e r n
Figure 2.4: Schematic high harmonic spectrum. Rapidly decreasing intensity in the
perturbative regime followed by the plateau with constant intensities. The cut-o� is
de�ned by the highest obtainable electron kinetic energies.
A schematic spectrum resulting from high harmonic generation can be seen in Fig-
ure 2.4. The centrosymmetry of the atomic gas leads to the emission of only odd
harmonics. At low harmonic orders the di�erent frequencies couple to each other
and compete for energy. This regime is called the perturbative regime. The intensity
drops rapidly with increasing frequency. The adjacent plateau region is described by
the three step model and the harmonic intensities stay constant. For few cycle laser
pulses with appropriate CEP the highest energy region is the unmodulated cut o�.
It is de�ned by the maximum obtainable photon energy, given by approximately
hνmax = 3.17 · UP + IP (2.26)
with UP the ponderomotive energy and IP the ionization potential.
2.2. Nonlinear optics 15
Electric �eld
electron trajectories
XUV
Figure 2.5: Electric �eld of the driving pulse (red) and electron trajectories (blue)
during high harmonic generation. The free electrons are generated by tunnel ioniza-
tion and get accelerated away from their parent ion. After getting accelerated back
and recolliding they emit an XUV photon (violet). The conditions for HHG are met
every half cycle of the driving laser, generating a train of XUV pulses.
2.2.3 From pulse trains to isolated attosecond pulses
The goal for HHG is often not only the production of radiation with the according
high photon energies but to create single ultra short pulses (τ ≈ 100 as). Figure
2.5 shows the process of high harmonic generation with the according electron tra-
jectories over the course of one driving few cycle IR pulse. It is obvious that the
process described by the three step model can take place every half cycle at high
pulse intensities, leading to a train of XUV pulses, visible as constructive and de-
structive interferences in the spectrum, as depicted in Figure 2.4. As the harmonics
are generated every half cycle, thus with twice the laser frequency, the fringes in the
spectrum have a distance of twice the carrier frequency of the driving laser according
to
F(A(t) + A(t− τ0/2)) = F(A(t)) · (1 + eiωτ0/2) (2.27)
A common technique to obtain isolated single attosecond pulses is spectral �ltering
of the high harmonic cut-o� regime with a high pass �lter [8]. If the highest photon
energies are only generated during one half cycle of the driving pulse, it is possible to
isolate the single resulting XUV pulse by �ltering all lower frequencies. The photon
16 2. Theory
Time
Am
plitu
de
CEP=0
CEP=pi/2envelope
Figure 2.6: Two pulses with identical envelope but di�erent CEP. Blue: CEP = 0,
the electric �eld has a single maximum at the peak of the envelope(red). Green:
CEP = π/2, the electric �eld always has two equivalent peaks with the same �eld
strength.
energies of the individual XUV pulses are determined by the maximum electric
�eld accelerating the electrons according to equations 2.26 and 2.23. To have only
one half cycle generate the highest energies it is important to have control over the
carrier envelope phase (CEP) of few cycle laser pulses. The CEP describes the phase
relation between the maximum of the envelope of the pulse and the relative phase
of the electric �eld. Figure 2.6 shows the electric �eld of two pulses with identical
envelope but di�erent CEPs. Green shows the case of CEP = π/2 containing two
cycles with the same maximum electric �eld, accordingly causing two XUV pulses
with the same energies. Spectral �ltering of the highest energy range in this case
would deliver a double pulse. In the blue case the electric �eld in the middle of the
pulse has a distinct maximum, creating a single XUV pulse with the highest photon
energies. Filtering lower energies isolates this pulse.
2.3. Pulse characterization 17
2.3 Pulse characterization
For all applications in this thesis it is important not only to generate the required
pulses but also to characterize them. Without knowing the properties of the used
probes, all following applications are rough estimates at best. For the temporal
characterization of any property it is necessary for the probe to be in the same regime
or shorter as the observed process. In the regime of few or even sub-femtosecond
pulses this is a challenging requirement, as these are among the fastest controllable
events today. As a consequence several techniques have been established using the
pulse to measure itself.
2.3.1 Autocorrelation
A rather simple tool for the characterization of laser pulses is the intensity auto-
correlation. An identical copy of the pulse is generated e.g. via a beam splitter or
a split mirror. Via a delay stage the two pulses are superpositioned with varying
temporal shift in a nonlinear medium e.g. in a SHG nonlinear crystal. The resulting
delay dependent SHG signal intensity is measured by a photodiode. The recorded
function is a measure for the pulse duration.
A(2)(τ) =
∫ ∞−∞
I(t)I(t− τ)dt (2.28)
While this gives an estimate for the pulse duration, for the deconvolution a pulse
shape has to be assumed. The intensity autocorrelation only allows access to pulse
duration, not to more complex substructures contained in the phase.
2.3.2 FROG
A more advanced technique is "Frequency Resolved Optical Gating"(FROG) [15].
Again the observed pulse is duplicated and superpositioned in a nonlinear optical
medium with an adjustable temporal delay. The resulting signal is resolved spec-
trally. It can be shown that the FROG trace, the measured spectra depending on the
delay, as depicted exemplarily in �gure 2.7, contains the complete information for
18 2. Theory
−50 −25 0 25 500
0.2
0.4
0.6
0.8
1
Delay (fs)
Am
plitu
de (
arb.
uni
ts)
−50 −25 0 25 50
0
1
2
3
4
Delay (fs)
Pha
se (
rad)
Figure 2.7: Exemplary FROG trace and associated retrieved temporal envelope
and phase.
the characterization of the pulse under scrutiny. Di�erent versions of this technique,
depending on the used nonlinear e�ect, have been developed. Commonly used are
for example SHG, THG, self di�raction, or polarization gating. The retrieval of the
pulse's envelope and phase from the trace is done via an iterative numerical algo-
rithm considering the type of nonlinear e�ect used. The evaluation of the trace in
�gure 2.7 is depicted in the bottom two graphs.
2.3. Pulse characterization 19
2.3.3 Streaking
The most direct measurement of the characteristics of a pulse is probably done via
the attosecond streaking technique [16]. Opposite to a FROG measurement, where
the pulse parameters are reconstructed afterwards, streaking allows direct access to
the vector potential of the pulse under scrutiny.
Assuming a free electron can be generated at a certain time t0 in relation to a pulse,
the change in momentum the electron experiences from the rest of the passing pulse
is given by
∆p(t0) = e ·∫ ∞t0
EL(t′)dt′ = e · AL(t0) (2.29)
with EL(t) the laser's electric �eld and AL(t) the laser's vector potential. Measuring
the electron's end velocity depending on the time of generation t0 directly gives the
vector potential of the pulse for all times via
AL(t0) =∆p(t0)
e=me · (v(t0)− v0)
e(2.30)
with v0 the electron velocity from its generation. Via a simple di�erentiation the
electric �eld can directly be retrieved from this data [16].
EL(t) = −∂AL(t)
∂t(2.31)
Attosecond streaking
The generation of free electrons at well de�ned temporal positions in relation to a
few femtosecond pulse is not a trivial task. The most common method is to use
photoionization in a low pressure gas target by an attosecond XUV pulse. The XUV
pulse, generated as previously described in section 2.2.2, has a well de�ned delay
in relation to the generating pulse. The delay between the two and therefore the
time of generation of the electrons, t0, can be adjusted, as described in section 4.1.
Figure 2.8 shows an exemplary streaking spectrogram. Horizontally the delay time is
plotted, the vertical axis represents the energy of the detected electrons. The distinct
20 2. Theory
Figure 2.8: Exemplary streaking spectrogram. Horizontally the delay time between
XUV pulse and the recorded pulse is plotted. The vertical axis represents the energies
of the electrons.
lines visible result from the di�erent energy levels of the used gas, Neon in this case,
and the according starting velocities v0 in equation 2.30.
Chapter 3
Laser FP3
The basis for all investigations of ultra fast phenomena is a laser system generating
high intensity few femtosecond pulses. For the later generation of isolated attosecond
pulses via high harmonic generation, as described in section 2.2.2, a high contrast
between the adjacent maxima of the femtosecond pulses' electric �eld is required.
This implies pulse durations of sub two cycles. The Femtopower 3 (FP3, named after
the oscillator and ampli�cation stage) laser at the Max-Planck-Institute of Quantum
Optics is a system generating sub �ve femtosecond pulses at a central wavelength
of λ = 785 nm with a repetition rate of f = 4 kHz. The > 1 mJ pulses are CEP
stabilized.
I will �rst describe the system in its current state in section 3.1. During the perfor-
mance of this thesis the second CPA stage described in section 3.1.3 was reinstalled,
after the system was previously operated without it. The process of reinstallation is
presented in section 3.2.
3.1 Setup
FP3 is a complex system consisting of several separate stages. The original pulse
generation takes place in the oscillator. The pulses experience a �rst stage of am-
pli�cation in a multipass chirped pulse ampli�er. A second chirped pulse ampli�er,
or booster, increases the pulse energy further. After compression via a transmis-
sion grating the pulses are spectrally broadened in a gas �lled hollow core �ber, to
21
22 3. Laser FP3
achieve the bandwidth necessary for pulse durations of few femtoseconds. The �nal
pulse compression is performed by a chirped mirror compressor. Figure 3.1 shows
a schematic of the complete system. The single components are described in more
detail in the following sections.
cw pump laserλ : 532 nmP : 3W
AOM
PP-MgO:LN (DFG)
dichroic mirror
glass piezo acuated pair of glass wedges stretcher
Q-switchedpump laserλ : 527 nmP : 30 W
Ti:Sapphire oscillatorf : 78 MHz λc : 780 nmτ : 6 fs P : 180 mW
P : 4.7 W
PSD1xy
PSD2xy
transmission
compressor
chirped mirror compressor
PSD3xy
PSD4xy
fL=2000 mm
neon filled hollow core fiber d=380 µm
ROC = -5000 mm
fused silicawedge pair
to experiment
flat mirror
curved mirror
piezo actuated mirror
photodiode
photodetector
feedbackelectronics
transmission
compressed and stretched
output pulse
f-to-2f interferometer
PSDxy
λc : 785 nmτ : <5 fs P : 4.3 W
4 passes - 78 MHz Pockels cell dazzler 5 passes - 4 kHzchirped pulse amplification 1
chirped pulse amplification 23 passes P: 11.5 W
grating
slow CEP stabilization loop
Q-switchedpump lasersλ : 532 nmP : 2 x 55 W
Figure 3.1: Schematic of the FP3 laser system. [17]
3.1.1 Oscillator
The oscillator generating the primary pulses (Ti:Sapphire oscillator in �gure 3.1) is a
commercially available Femtolasers1 Rainbow. The gain medium is a titanium doped
sapphire crystal (Ti:Sa), which is pumped by a Coherent Verdi V6 2, a continuous
1FEMTOLASERS Produktions GmbH, http://www.femtolasers.com/2Coherent Inc., http://www.coherent.com
3.1. Setup 23
wave (cw), frequency doubled Nd:YVO4 laser with a wavelength of 532 nm and a
power of P = 3 W . The oscillator is Kerr lens mode locked (KLM)[3] producing
pulses with a duration of τ = 6 fs, a central wavelength of λc = 780 nm, a pulse
energy of E = 2.3 nJ , and a repetition rate of frep = 78 MHz. A thick glass block
stretches the pulses to a duration of τ = 18 ps, introducing a positive linear chirp, as
preparation for chirped pulse ampli�cation as described in the next section 3.1.2.
3.1.2 Chirped pulse ampli�er
Chirped pulse ampli�cation (CPA) for the optical regime was introduced in 1985
by Strickland and Mourou [18], transferring a technique until then only common in
radar systems. The motivation in both cases is to amplify pulses, radar or laser, into
a regime that would normally be beyond the damage threshold of the ampli�ers.
The basic idea is to �rst stretch the pulses, reducing their intensity. The accessible
ampli�cation factors, while keeping the intensities below the damage threshold, are
now signi�cantly higher. After ampli�cation the pulses are recompressed.
The ampli�er in FP3 (chirped pulse ampli�cation 1 in �gure 3.1) is a commercially
available Femtolasers1 Femtopower compact pro HP/HR. It is a multipass ampli-
�er with a total of nine passes. The necessary stretching of the incoming pulses is
performed dispersively at the output of the oscillator, as described in 3.1.1. The du-
ration of the positively chirped pulses is τ = 18 ps at this stage. The gain medium of
the ampli�er is a Ti:Sa crystal, cooled to a temperature of T ≈ 180 K. It is pumped
optically by a frequency doubled, Q-switched Nd:YLF laser with a wavelength of
λ = 527 nm and a power of P = 30 W at a repetition rate of f = 4 kHz. After the
�rst four passes a Pockels cell acting as a pulse picker reduces the repetition rate to
frep = 4 kHz. To be able to tune the pulses' spectral phase a Dazzler 3 is in place
after the Pockels cell. The Dazzler is an Acousto-Optic Programmable Dispersive
Filter (AOPDF), making it possible to introduce arbitrary dispersion to pulses. The
following �ve more passes through the crystal amplify the laser to a total output
power of P = 4.7 W with an according pulse energy of E = 1.18 mJ .
3Fastlite, http://www.fastlite.com
24 3. Laser FP3
To compensate for daily drifts and alignment changes in the oscillator as well as the
ampli�er, a beam stabilization system is in use. Two position sensitive photodetec-
tors (PSD1,2 in �gure 3.1) record the beam position at di�erent locations along the
beam path. The signals for the PSDs are the re�ection of a thin glass plate in the
beam path (PSD1) and the leakage through a dielectric mirror (PSD2). Necessary
corrections to the beam path are performed by two motorized mirrors (MM1,2 in
�gure 3.1)
3.1.3 Booster stage
To increase the available pulse energy further, a second CPA stage (chirped pulse
ampli�cation 2 in �gure 3.1), that was developed in cooperation with Thales Op-
tronique4 was implemented. Again a Ti:Sa crystal, although with a signi�cantly
higher doping level, is used as a gain medium. To allow for the necessary powers in
this stage, the crystal is cooled to a temperature of T ≈ 70 K by a helium com-
pressor, counteracting thermal e�ects like thermal lensing. Pumping is performed
by two counter propagating Q-switched, frequency doubled Nd:YAG lasers with a
wavelength of λ = 532 nm and a power of P = 55 W each. With a total of three
passes the booster ampli�es the laser to a power of P = 11.5 W .
3.1.4 Spectral broadening
In preparation for the creation of a supercontinuum in a hollow-core-�ber, the pulses
are compressed by a transmission grating compressor, introducing negative second
order dispersion, compensating for the positive chirp remaining from CPA. The
pulses with an original duration of τ ≈ 18 ps are compressed to approximately
τ ≈ 25 fs.
The now nearly fourier-limited pulses are focused into a fused silica hollow-core-�ber
(HCF) with a length of l = 1.5 m. The bore has a diameter of 380 µm and is �lled
with neon gas at a pressure of p = 1.2 bar. Via the nonlinear optical process of
4Thales Optronique, http://www.thalesgroup.com
3.1. Setup 25
500 550 600 650 700 750 800 850 900 950 10000
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
wavelength (nm)
inte
nsity
(ar
b. u
nits
)
Spectrum HCF in
Spectrum HCF out
Figure 3.2: Spectra before and after spectral broadening in a neon �lled hollow-
core-�ber. The spectra are normalized to their respective total energies.
self-phase-modulation (SPM, see 2.2.1), the propagation through the gas causes the
spectral broadening necessary for the later compression to a pulse duration of sub
�ve fs. The required interaction length is achieved by waveguiding in the HCF [5].
Figure 3.2 shows the spectra before and after the spectral broadening in the HCF,
normalized to their respective total energies. The transfer of energy from the initial
spectral range to higher as well as lower frequency areas is clearly visible. The origin
of the pronounced peak at λ ≈ 750 nm is not completely clear. A comparison of
the spectra with and without booster is depicted in �gure 3.3. Though also present
before the installation, the peak increases signi�cantly. Due to this indicated power
dependence a nonlinear e�ect like self-focusing in the �ber is probably responsible.
Continued testing with di�erent �ber diameters and focusing should be performed
in the near future. Di�erential pumping of the �ber, generating a pressure gradient
within, has been proposed [19] and experimentally realized [20] to minimize the
mentioned nonlinear e�ect and is another possibility for improvements. The total
power after the HCF is P = 5.7 W .
26 3. Laser FP3
5 0 0 5 5 0 6 0 0 6 5 0 7 0 0 7 5 0 8 0 0 8 5 0 9 0 0 9 5 0 1 0 0 00 . 0 0 0
0 . 0 0 2
0 . 0 0 4
0 . 0 0 6
0 . 0 0 8
0 . 0 1 0
0 . 0 1 2
Int
ensity
(arb.
units)
W a v e l e n g t h ( n m )
w / o b o o s t e r w / b o o s t e r
Figure 3.3: Comparison of the spectra with and without the booster after the
spectral broadening via SPM in a neon �lled hollow-core �ber. The spectra are
normalized to their respective total energy. Though also present before, the peak at
λ ≈ 750 nm increased signi�cantly with the installation of the booster.
3.1.5 Compression
Besides the spectral broadening the HCF also introduces positive second order dis-
persion, making it possible to compress the pulses further. For the tuning of the
amount of second order dispersion the beam passes a pair of glass wedges under
Brewster-angle. After compression by a set of eight negatively chirped mirrors, the
pulses have a duration of τ ≈ 5 fs. With the repetition rate of frep = 4 kHz and a
total power of P = 4.3 W the pulse energy resembles E = 1.1 mJ .
3.1.6 Carrier envelope phase stabilization
For the intended applications of the laser system described so far, a further property
is important. The generation of isolated attosecond pulses by high harmonic genera-
3.2. Installation of the booster stage 27
tion (section 2.2.2) as well as the streaking measurements (section 2.3.3) performed
in AS2 (chapter 4) require control over the carrier envelope phase. The system is
equipped with an active stabilization consisting of two independent parts. The �rst,
the fast loop, is operating at MHz repetition rates in the oscillator (section 3.1.1).
The phase is measured via the f-to-0 scheme [21], with a PP-MgO:LN crystal as
a nonlinear medium. The stabilization is performed by a PI-controller varying the
pump power of the Verdi via an acousto-optic-modulator(AOM). A second stabi-
lization system, the slow loop, detects the phase after the HCF, therefore being able
to compensate for slow drifts throughout the complete system, e.g. by air currents.
The measurement is performed with the f-to-2f scheme [22] (f-to-2f interferometer in
�gure 3.1). The system is feeding back to a pair of glass wedges (piezo actuated pair
of glass wedges in �gure 3.1) after the glass stretcher in front of the ampli�er. The
later system allows setting the CEP of the generated pulses to arbitrary values.
3.2 Installation of the booster stage
FP3 can be set up to work only with the �rst stage of chirped pulse ampli�cation.
In that case the pulses are recompressed and spectrally broadened directly after the
CPA stage. The output power in that con�guration is P = 1.8 W which resembles a
pulse energy of E = 0.45 mJ . For the UV generation necessary for the experiments
investigating ozone (see chapter 5) this was not su�cient. For that reason the avail-
able but at that time not operational booster stage was reinstalled. The addition of
a complex component like the booster has some severe implications that have to be
considered in the later parts of the laser. For a short pulse duration as required here,
the subsequent components need to be adapted to the changed input parameters.
The obviously desired increase in pulse energy leads to the appearance of nonlinear
e�ects in previously linear regions. The passing of more distance through air as well
as the added optical elements leads to a change in the pulses spectral phase.
28 3. Laser FP3
740 760 780 800 820 840 860
0
0.2
0.4
0.6
0.8
1
Wavelength λ (nm)
Inte
nsity
(arb
. uni
ts)
Spectrum booster inSpectrum booster out
740 760 780 800 820 840 860
0
0.2
0.4
0.6
0.8
1
Wavelength λ (nm)
Inte
nsity
(arb
. uni
ts)
Spectrum booster outSpectrum compressor out
740 760 780 800 820 840 860
0
0.2
0.4
0.6
0.8
1
Wavelength λ (nm)
Inte
nsity
(arb
. uni
ts)
Spectrum compressor outSpectrum fiber in
a) b)
c)
Figure 3.4: Comparison of spectra at di�erent positions. The spectra are normalized
to their total energy. a) shows the spectra before and after the booster stage. As
desired, no change is visible. b) shows the spectra after the booster and after the
grating compressor. Slight losses at the red �ank are probably caused by mirrors in
the beam's path. In c) the output of the compressor is compared with the input of the
hollow core �ber. The temporal compression to τ ≈ 25 fs and spatial focusing cause
intensities high enough for self-phase modulation in air to set in. The generation of
new spectral components in outer areas is clearly visible.
3.2.1 Spectra at di�erent locations
A convenient indicator to monitor the impact of the di�erent parts of the setup, and
to locate possible problems is the laser's spectrum at di�erent locations. Though
some changes are to be expected, most components are selected to fully support
the spectral pro�le of the pulses. Signi�cant changes usually indicate errors or fail-
ures. Figure 3.4 a) shows the laser's spectrum before and after the booster stage.
3.2. Installation of the booster stage 29
No signi�cant di�erence is noticeable. This is desired and to be expected. The pulse
generated and ampli�ed in Ti:Sa crystals is further ampli�ed in yet another such
medium, having the same spectral ampli�cation pro�le. Though gain narrowing is
expected in principle, the ampli�cation factor is too low for this to have a consider-
able impact. The recompression of the pulses in the grating compressor (transmission
grating compressor in �gure 3.1) also does not change the spectrum of the pulses
signi�cantly as shown in �gure 3.4 b). Slight losses at the red �ank are probably
caused by the propagation over several mirrors. These are necessary for the recol-
limation after the booster stage as well as beam guiding because of the geometric
setup of the separate parts of FP3.
The temporal recompression to τ ≈ 25 fs and spatial focusing into the hollow-core
�ber (HCF) with d = 380 µm leads to intensities causing self-phase modulation
in air already before the HCF. Figure 3.4 c) depicts the spectrum at the output
of the compressor in comparison with the spectrum at the beginning of the �ber.
The di�erence is clearly visible. Below the wavelength of λ = 750 nm and above
λ = 830 nm new spectral components are generated. Although not desired, the
change in the spectrum is still in an acceptable regime.
3.2.2 Pulse optimization for compression via feedback
All previous considerations excluded the phase of the pulses. This is due to the fact
that the spectral amplitude is far easier accessible than the phase. None the less,
especially for the compression of pulses, the phase is a deciding parameter. In the
intended regime of fs pulses a �at phase is crucial for a minimal pulse duration. All
conventional shaping techniques can only compensate for certain types of phase. The
grating compressor used here, compensates second order dispersion. The majority of
the pulse's second order spectral phase was introduced intentionally for the process
of chirped pulse ampli�cation and the compressor is designed to compensate that.
The amount unintentionally generated by optical elements is far less and can easily
be handled by the compressor. A challenge represent the higher orders of dispersion.
In the present setup there is no possibility to compensate these orders at the position
of the compressor.
30 3. Laser FP3
The Dazzler, present in the CPA stage, allows the con�guration of arbitrary dis-
persion. To achieve the goal of a �at phase after the compressor, it proved to be
a successful method to use an iterative feedback procedure to the Dazzler. Figure
740 760 780 800 820 840
0
0.2
0.4
0.6
0.8
1
Wavelength λ (nm)
Inte
nsity
(ar
b. u
nits
)
Spectrum
740 760 780 800 820 840
4
6
8
10
12
14
16
Wavelength λ (nm)
Pha
se (
rad)
Start phaseAfter first feedbackFinal phase
Figure 3.5: Spectrum and phases after the grating compressor retrieved via FROG
measurement. In the regime of signi�cant spectral intensities a �at phase could be
achieved via the feedback technique. a) Spectrum of the pulses. b) Spectral phases.
Blue: Starting phase without feedback to the Dazzler. Green: First iteration. Spectral
phase after feedback of the blue phase. Red: Second iteration. Spectral phase after
feedback of the green phase.
3.5 shows the spectrum as well as the phases at di�erent steps of the procedure.
In every step the pulses were characterized via a FROG measurement (see section
2.3.2). The retrieved spectral phase was then subtracted from the current Dazzler
phase settings. In the starting phase, blue, a signi�cant change of the phase with
the wavelength is still present. This already decreased considerably after one itera-
tion as can be seen in the green plot. The red plot depicts the �nal phase after two
3.2. Installation of the booster stage 31
iterations. For the regime of signi�cant spectral intensities between λ = 750 nm and
λ = 830 nm a nearly �at phase could be achieved.
3.2.3 Spectral broadening
The change in pulse energy and duration requires an adaptation of the parameters
for the hollow-core �ber. The setup is determined by the gas pressure, the bore di-
ameter of the �ber, the �ber length, and the focal length used for the incoupling.
The parameters used for the evaluation of the con�guration are the spectral broad-
ening, the energy throughput as well as the resulting mode after the �ber. We tried
several di�erent con�gurations. The best trade-o� between the target parameters
was achieved with a focal length of l = 2 m, a bore diameter of d = 380 µm, a
�ber length of l = 1.5 m, and a neon pressure of p = 1.2 bar. In comparison to
the prior setup without the booster stage, this means an increase in focal length,
l = 1.5 m previously, an increase in the �bers bore diameter, d = 250 µm previously,
an increase in the �ber length, l = 1 m previously, and a decrease in gas pressure,
p = 1.6 bar previously. All changes are in the expected direction as the process of
SPM is sensitive to the involved intensities. To keep the intensities approximately
constant, the increased pulse energy requires an increase in the diameter of the fo-
cus and therefore an increase in the focal length. The larger focus requires a larger
�ber bore diameter. With these two parameters �xed, the gas pressure is a trade-
o� between transmitted energy and spectral broadening. A pressure of p = 1.2 bar
proofed to be the optimum in our case.
3.2.4 Final compression
The �nal compression is performed via a chirped mirror compressor (CMC)[23]. For
best results the chirped mirrors need to be designed for the spectral phase of the
incoming pulse, compensating not only second but also higher orders of dispersion.
The mirrors in use at the present time are matched to the setup without the booster
and are accordingly not optimal. The feedback mechanism described in 3.2.2 is not
applicable as changing the Dazzler settings would also change the compression in
front of the �ber. The only available degree of freedom for the tuning of the phase
32 3. Laser FP3
is by a pair of glass wedges, mainly changing second order dispersion. Figure 3.6
a)
b)
0 5 10 15 20 25−2
−1
0
1
2x 109
delay [fs]
elec
tric
fiel
d
0 5 10 15 20 250
0.2
0.4
0.6
0.8
1FWHM:5.4008
delay [fs]
inte
nsity
env
elop
e
Figure 3.6: Streaking spectrogram resembling the vector potential of the electric
�eld a), and evaluated electric �eld and envelope b) of the pulses after the chirped
mirror compressor
depicts the streaking measurement resembling the vector potential of the electric
�eld a), and the evaluated electric �eld and envelope b) of the pulses after the
CMC with the not optimal mirrors. Calculations from the available bandwidth show
that further compression is possible. The measured spectrum allows for a Fourier-
limited pulse duration of τ = 3.4 fs. To obtain optimal compression, the pulse was
characterized after the �ber by means of an XFROG[24] and new chirped mirrors
are under development accordingly. The goal of sub �ve femtosecond pulse durations
should be reached with the exchange of the compressor.
Chapter 4
Beamline AS2
Besides the laser system, generating pulses with few femtosecond duration, a com-
plex optical installation is necessary to perform experiments investigating ultra fast
dynamics. The intensities involved and the wavelength regime of the to be gen-
erated XUV attosecond pulses require vacuum to prevent nonlinear e�ects during
pulse propagation, and reabsorption respectively.
All experiments described in this thesis were performed at the AS2 beamline at the
Laboratory for Attosecond Physics of the Max-Planck Institute of Quantum optics
in Garching.[25] Though it is possible to work with di�erent laser systems in the
beamline, during the performance of this thesis only the described laser system FP3
(Chapter 3) with a repetition rate of 4 kHz and a central wavelength of 785 nm was
in use.
4.1 Beamline setup
The beamline is separated into a set of vacuum chambers, which are designed to
ful�ll di�erent tasks. In the high harmonic chamber 4.1.1 XUV pulses are generated
from the incoming VIS/IR beam.
The interferometric chamber 4.1.2 is constructed as a Mach-Zehnder interferometer.
The VIS/IR beam and the previously generated XUV beam are spatially separated
into di�erent arms of the interferometer, making it possible to individually adjust
both beams to the experimental requirements and introduce a temporal delay. At
33
34 4. Beamline AS2
the end of the chamber both arms are spatially recombined. The interferometric
chamber also contains the diagnostic devices for the high harmonic generation.
The experimental chamber 4.1.3 contains instruments to investigate gases and solids
with di�erent experimental techniques including streaking and transient absorption.
Furthermore it contains additional diagnostics.
4.1.1 High harmonic generation
MM1
MM2
FM
MA
HHT
E = 0.9 mJf = 4 kHzλ = 785 nmτ < 5 fs
c
Figure 4.1: High harmonic chamber with the motorized aperture (MA), the focus-
ing mirror (FM), the two motorized folding mirrors (MM1,2), and the gas target
for high harmonic generation. The VIS/IR laser enters the chamber at the top and
leaves it collinear with the generated XUV pulses on the left side
The high harmonic chamber is the �rst chamber of the AS2 beamline system. It
is depicted in �gure 4.1. For the generation of high harmonic XUV pulses (HHG),
the incoming laser pulses are focused into a gas target, consisting of a small tube
of stainless steel with a drilled hole of approximately 300 µm diameter (HHT in
4.1). Neon with a pressure ranging from 100 to 250 mbar, depending on the desired
4.1. Beamline setup 35
photon energies of the XUV pulses, adjusted to optimize phase matching for the
HHG process, is the medium for the process. For optimal positioning of the target
in relation to the focus, it is mounted on a motorized translational stage, which
can be moved in the propagation direction. Height as well as perpendicular position
are adjustable by picomotors1. The layout of the chamber allows for a variation
of the focusing length onto the target and thus the focus size. Therefore di�erent
peak intensities in the target can be achieved. Typically a spherical mirror with a
radius of curvature (ROC) of −1600mm (FM in 4.1) is used. To allow the steering
of the laser under vacuum conditions, two of the folding mirrors in the chamber are
motorized (MM1,2 in 4.1). For the adjustment of the intensity of the incoming beam
a motorized aperture (MA in 4.1) is in place. As the use of a gas target in vacuum
signi�es a great impact on the quality of the vacuum, the chamber is equipped with
two turbo molecular pumps, optimized for high gas load, to keep the degradation to
a minimum and avoid reabsorption of the generated XUV radiation.
4.1.2 Interferometer
The second chamber of the AS2 beamline contains a Mach-Zehnder Interferometer
depicted in �gure 4.2. The incoming collinear VIS/IR and XUV pulses are spatially
separated by the use of a perforated silver mirror (PM1 in 4.2) with variable hole
diameters ranging from 1 mm to 2.5 mm. Because of the lower divergence of the
XUV beam, its mode is signi�cantly smaller than the VIS/IR mode. Centered in
the middle of the VIS/IR beam, it can pass through the hole in the mirror, while
the major part of the VIS/IR mode is re�ected. A rough estimate of the ratio of the
divergences can be made by assuming equal beam waist size in the HHG target and
using
θ =λ
πw(4.1)
with θ the radial beam divergence, λ the laser wavelength, and w the beam waist
[26]. With a central IR wavelength of 785nm and a XUV wavelength of 10nm this
1Newport Corporation, http://http://www.newport.com
36 4. Beamline AS2
Z1Z2PM1
HHG
TS
FW1
FW2PM2
T
G
XUV CCD
F
MLM
Exp
MADS
Figure 4.2: Interferometer chamber. VIS/IR and XUV beam enter the chamber
collinearly at the top (HHG). They get separated at the �rst perforated mirror
(PM1). The XUV passes through metal �lters in a motorized �lter wheel (FW1)
and gets redirected by a multi layer mirror (MLM) towards a second perforated
mirror (PM2). Via a translational stage (TS), mirrors can be moved into the XUV
beam path to divert it onto a XUV sensitive camera (XUV CCD). The beam either
passes a second mirror to monitor the beam pro�le or a re�ective grating (G) to
resolve the spectrum. On both ways a second �lter wheel (FW2) with metal �lters
is passed. The VIS/IR coming from PM1 passes a focusing telescope (Z1) with
a piezo actuated delay stage (DS). After a focus (F) a second Z (Z2) is passed.
Via a motorized aperture (MA) the VIS/IR intensity can be adjusted. VIS/IR and
XUV are recombined at PM2 and focused via a toroidal mirror (T). They leave the
chamber towards the experimental chamber (Exp).
4.1. Beamline setup 37
results in
θXUV ≈1
79θIR (4.2)
After passing their respective arms, the XUV and the VIS/IR beams are recombined
at a second perforated mirror (PM2 in 4.2).
XUV arm and diagnostics
The XUV arm has three di�erent settings. For the transmission through the inter-
ferometer (transmission setting) used for experiments, the beam is re�ected by a 45◦
XUV multilayer mirror (MLM in 4.2) and is sent to the second perforated mirror
(PM2 in 4.2) for spatial recombination with the VIS/IR. To �lter out remaining
VIS/IR components, thin metal �lters2 are placed in a motorized �lter holder (FW1
in 4.2), which allows for the selection of up to six di�erent �lter sets under vacuum
conditions. The combination of the �lters and the characteristics of the multilayer
mirror de�ne the spectral transmission characteristics for the XUV pulses and are
used to �lter for single XUV pulses, as described in 4.1.1. They are adjusted for the
individual experimental requirements.
The second setting is designed for the characterization of the XUV beam pro�le
(beam pro�le setting). It is also used for the adjustment of the HHG parameters to
maximize XUV photon �ux. Depending on the XUV photon energies, an 80% Au,
20% Pd or a Ni mirror is moved into the beam via a motorized translational stage
(TS in 4.2). To obtain high re�ectivity, an angle of incidence of > 80◦ is chosen.
Figure 4.3 shows the energy dependent re�ectivity of the used mirrors for an angle
of incidence of 80◦. Regarding re�ectivity, the Ni mirror obviously is only bene�cial
if the expected photon energies exceed approximately 175 eV. Nevertheless, for the
recording of the spectrum the �atter characteristics is an advantage for lower energies
as well.
2Lebow Co., http://www.lebowcompany.com/
38 4. Beamline AS2
4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 1 6 0 1 8 0 2 0 0 2 2 00 . 10 . 20 . 30 . 40 . 50 . 60 . 70 . 8
Refle
civity
P h o t o n E n e r g y ( e V )
8 0 % A u 2 0 % P d N i
Figure 4.3: Re�ectivity of 80% Au, 20% Pd and Ni mirrors with an angle of inci-
dence of 80◦, depending on the photon energy. For the re�ectivity the use of the Ni
mirror is only bene�cial if the expected energies exceed approximately 175 eV. The
�atter characteristics is an advantage for the recording of spectra for lower energies
as well.
Source: Lawrence Berkeley National Laboratory, The Center for X-Ray Optics,
http://henke.lbl.gov/optical_constants/
The beam is detected on a XUV sensitive CCD camera3(XUV CCD in 4.2). To pro-
tect the camera from exposure to too high intensities and to remove stray light, a
thin metal �lter, mostly 500 nm Zr2, is placed in front of it. To adjust the intensity
of the incoming beam and to �lter for selected photon energies, a second motorized
�lter holder (FW2 in 4.2) is used in this beam path. Importantly, the metal �lters
also remove the fundamental VIS/IR beam.
For the third setting (spectrometer setting), the beam is picked by the same type of
mirror as for setting two. The XUV passes through the same motorized �lter holder
as for the beam pro�le setting, to select speci�c spectral properties. Instead of a
3PIXIS XO, Roper scienti�c, http://www.roperscienti�c.com/
4.1. Beamline setup 39
second metal mirror it is guided onto a re�ection grating 4 (G in 4.2) and further
through the same static metal �lter to the PIXIS XUV camera. A home made
program [14] reads the intensities on the camera chip with full vertical binning and
translates them into spectral intensities. The spectrometer is calibrated by placing
metal �lters with well de�ned absorption edges into the beam via the motorized �lter
holder, typically 500 eV Aluminum and 150 eV Silicon. The transmission functions
are depicted in Figure 4.4.
6 0 8 0 1 0 0 1 2 0 1 4 0 1 6 00 . 00 . 10 . 20 . 30 . 40 . 50 . 60 . 70 . 8
Refle
civity
P h o t o n E n e r g y
5 0 0 n m A l 1 5 0 n m S i
Figure 4.4: Transmission functions of 500 nm Aluminum and 150 nm Silicon, de-
pending on the photon energy. The absorption edges at 73 nm and 100 nm are used
to calibrate the XUV spectrometer.
Source: Lawrence Berkeley National Laboratory, The Center for X-Ray Optics,
http://henke.lbl.gov/optical_constants/
IR arm
The part of the VIS/IR beam that is re�ected from the �rst perforated mirror passes
through a telescope in Z-con�guration (Z1 in 4.2)generating an intermediate focus.
4Hitachi (001-0266), http://www.hitachi-hitec.com/
40 4. Beamline AS2
The �rst telescope mirror is mounted on a piezo actuated delay stage (DS in 4.2).
The piezo system �ne tunes the path length for the streaking measurements de-
scribed in 2.3.3 delaying the VIS/IR in respect to the XUV pulse. After the focus,
the beam passes a second Z (Z2 in 4.2) with one �at and one concave mirror to
adapt the divergence of the VIS/IR beam to the one of the XUV. The �at mirror
is motorized with a picomotor. The motorization is used to adjust the optical path
length of the VIS/IR pulses to that of the XUV. The beam then is recombined with
the XUV at the second perforated mirror (PM2 in 4.2).
The two Zs with the in between lying focus are the big advantage of the design of
AS2 over collinear con�gurations. The layout makes the setup adaptable to various
experimental applications, like e.g. the generation of third harmonic UV pulses, as
described in the next section, or to perform transmission experiments, while still
keeping the temporal synchronization with the XUV pulses.
Third harmonic UV generation
One application of the versatility of AS2's interferometric chamber is the generation
of third harmonic UV radiation from the driving VIS/IR pulses at the intermediate
focus position (F in �gure 4.2).
The generation of ultra short UV pulses in the regime of λ ≈ 260 nm and τ ≈ 3 fs
is not a trivial task. The pulse durations involved imply a spectral bandwidth ex-
cluding crystals as nonlinear medium due to phase matching considerations as well
as their dispersive behavior stretching the pulses. The use of an atomic gas as a
medium, Neon in this case, has the disadvantage of low e�ciency in the regime of
10−3 and lower. Experiments using a simple gas-target design similar to the high-
harmonic-generation targets, a steel tube with a predrilled hole, requires very high
gas pressure. Figure 4.5 depicts the pressure dependence of the conversion e�ciency
in such a target at driving pulse energies of E ≈ 220 µJ and pulse durations of
τ ≈ 5 fs. Already technically challenging pressure regimes are necessary before the
yield reaches saturation. The high gas pressure has severe disadvantages. The ex-
hausting gas signi�cantly degradates the quality of the vacuum, profoundly increas-
ing reabsorption of the XUV pulses. First attempts at the application of di�erential
4.1. Beamline setup 41
3 4 5 6 7 83 . 0 x 1 0 - 4
4 . 0 x 1 0 - 4
5 . 0 x 1 0 - 4
6 . 0 x 1 0 - 4
7 . 0 x 1 0 - 4
Effic
iency
P r e s s u r e a b s . ( b a r )
Figure 4.5: Pressure dependence of the third harmonic conversion e�ciency in a
simple gas target. Challenging pressure regimes are reached before the e�ciency
reaches saturation
pumping could not prevent a noticeable decrease in XUV pulse energy. Di�erent
attempts were made to improve the e�ciency of the di�erential pumping and de-
crease the amount of escaping gas. A signi�cant improvement was the introduction
of a second pumping stage as depicted in �gure 4.6. The gas coming from the target
T gets dammed by the bottlenecks B1,2 and pumped at the respective stages. A
comparison of the pressure in the interferometric chamber with and without the
second pumping stage showed a decrease of the pressure from p = 3.0 · 10−2 mbar
to p = 7.7 · 10−3 mbar. Though an improvement, the vacuum quality in the inter-
ferometric chamber is still impaired. The pressure without the Neon from the third
harmonic target is at p = 1.4 · 10−3 mbar. Experiments to close o� the pumping
stage with thin glass plates failed because they disrupted the pulses dispersively. Us-
ing pellicles for that purpose is a promising approach although they are frequently
42 4. Beamline AS2
Ne
to pumps
B1 B1T
B2B2
Figure 4.6: Schematic of the di�erential pumping stage to prevent vacuum degra-
dation by the escaping gas from third harmonic generation. The gas originates from
the target T in the middle. It gets pumped at two separate stages while getting
dammed at the inner and outer bottlenecks B1,2.
damaged by the laser.
A di�erent approach is a change of the target design. To increase the interaction
length and therefore decrease the necessary gas pressure, two designs were tested.
In both cases the laser would pass through two pieces of a fused silica hollow-core
�ber with a bore diameter of d = 400 µm and a length of l ≈ 2 mm each. The gas
is injected in between the two pieces. The challenge in both cases is the alignment
of the �bers. Figure 4.7 shows the di�erent designs: a) the simple steel tube used
before, b) "hydrant" design with two pieces of hollow-core �ber in a guiding steel
tube, c)"tra�c light" design with the �ber pieces in drilling holes in a solid aluminum
block. The �rst attempt, the "hydrant" design proved to be too unstable, due to
the �exibility of the used steel tube. It was not possible to align the �ber pieces in
a straight line. The second attempt, the "tra�c light" design, generated promising
results. Opposite to the behavior in �gure 4.5 saturation in e�ciency was reached
at p ≈ 5 bar while additionally the amount of escaping gas was decreased. The
4.1. Beamline setup 43
Steel tube Hydrant Tra�c lighta) b) c)
Predrilled hole
HCF
HCF
Figure 4.7: Di�erent gas target designs for third harmonic generation. a) Simple
steel tube with predrilled hole. Requires high gas pressure causing a large amount
of leaking gas. b) "Hydrant" design: Steel tube with two embedded pieces of hollow-
core �ber (HCF). Due to the instability of the steel tube the alignment of the HCF
parts is very challenging. c) "Tra�c light" design: The HCF pieces are embedded in
drilling holes in a solid aluminum block. Alignment works �ne, reduces the required
gas pressure and amount of leaking gas signi�cantly.
e�ciency of UV generation with this target at p = 3 bar was equivalent to the case
of p = 8 bar with the steel tube.
Recombination and focusing
After passing their respective arms of the interferometer, the XUV and VIS/IR
beams are recombined via a second perforated mirror. Again the XUV mode passes
through the hole in the center of the mirror, while the VIS/IR part is re�ected in
direction of the XUV. While the direction of the XUV is de�ned by the earlier beam
path and the position of the hole, it is possible to adjust the subsequent direction
of the VIS/IR by the orientation of the mirror.
The again collinear beams of VIS/IR and XUV are both focused via a Nickel coated,
44 4. Beamline AS2
toroidal mirror (T in 4.2)5 with an angle of incidence of 86◦ from the normal axis
onto the same spot.
4.1.3 Experiment chamber
The layout of the experimental chamber is depicted in 4.8. Possible experiments
range from transient absorption measurements to photo electron streaking. For the
later the chamber is equipped with a time of �ight electron spectrometer 6 (TOF in
4.8). It is directed at a gas nozzle (N in 4.8), that is motorized in all three axes, to
simplify the alignment in the focus. Furthermore the chamber contains several tools
for the alignment and diagnostic of the incoming beams.
Diagnostics
Beam pro�le and spectrum A second XUV sensitive CCD camera3 (XUV CCD
in 4.8) is in use in the experimental chamber. It ful�lls the same beam pro�le and
spectrum measurements as the earlier mentioned one in the interferometer. An 80%
Au 20% Pd mirror de�ects the XUV beam onto the camera, to monitor the beam
pro�le as well as its pointing through the second perforated mirror in the inter-
ferometer. For transient absorption measurements, a second grating (G in 4.8) in
combination with a 200 µm broad slit is available to measure the spectrum of the
HH pulses. Both, plane mirror and grating, are moved into the beam path by a
linear translational stage (TS3 in 4.8).
Imaging An imaging system is used to spatially align the XUV and VIS/IR foci
relative to each other. It is also used to adjust the optical path length of the VIS/IR
beam through the interferometer to the path length of the XUV pulses.
The system consists of a plane silver mirror, moved into the beam path by a linear
translational stage (TS3 in 4.8), a focusing lens (L in 4.8), and a CCD camera (IR
CCD in 4.8), sensitive in the visible and IR spectral range.
To detect the position of the XUV beam, all metal �lters in its path are removed, so
5Société Européenne de Systémes Optiques, http://www.seso.com/6Stefan Kaesdorf, http://http://www.kaesdorf.de
4.1. Beamline setup 45
IR CCD
P, UVD
XUV CCD
TS1,2
TS 3TOF
NG
L
Figure 4.8: Experimental chamber. XUV and VIS/IR enter the chamber from
the left. They are focused in front of a gas nozzle (N) and under a time of �ight
electron spectrometer (TOF). The nozzle is moveable in all three directions via two
translational stages (TS 1,2) and a pico motor. Via mirrors on a third translational
stage (TS 3) the beams can be steered towards di�erent targets. Without diversion
they leave the chamber towards a power meter or diagnostics for UV pulses (P,
UVD). To permit the transmission of UV the window at this �ange consists of
quartz. The XUV can be sent to a XUV sensitive camera (XUV CCD) either via a
mirror for a beam pro�le or via a grating (G) to read the spectrum. An IR imaging
system with a lens (L) and an IR sensitive camera (IR CCD) is also available.
the otherwise blocked VIS/IR, remaining from the HHG, passes through the system.
Though larger in size, the position of the VIS/IR passing through the XUV arm of
the spectrometer is a good indicator for the position of the XUV.
To con�gure the di�erence in optical path length and therefore adjust the tempo-
ral overlap of the VIS/IR and XUV pulses, again the VIS/IR components passing
46 4. Beamline AS2
through the XUV path are used. For small, day to day, readjustments, the CCD
camera is su�cient for detection. In the temporal overlap, while moving the piezo
positioning system in the VIS/IR arm of the interferometer, modulations in inten-
sity, caused by interference of the two pulses, are clearly visible. These modulations
are precise enough for a rough adjustment.
For larger adjustments, e.g after a reconstruction of prior parts, the CCD camera
can be replaced by a spectrometer. Considering the spectrum of the superposition
of a pulse with a time delayed but otherwise identical copy of itself,
F(A(t) + A(t− t0)) = F(A(t)) · (1 + eiωt0) (4.3)
with A(t) an arbitrary pulse, F the Fourier transform, t0 the time delay between
the two pulses, it can be seen that the magnitude of the time delay is indicated
by the period of modulations in the recorded spectrum. Maximizing the period, by
adjusting the path lengths in the interferometer, minimizes the temporal delay.
Photo electron spectroscopy and streaking
The most common measurement technique in AS2 is the detection of photoelectron
spectra, streaked in the VIS/IR �eld (streaking 2.3.3). A gas nozzle, fabricated out
of a thin glass tube (N in 4.8), is positioned in the XUV beam's focus. To prevent
electrostatic charging, the glass is coated with graphite. The nozzle's position is
adjustable in all three axes. In beam direction and horizontally perpendicular the
con�guration is performed by translational stages (TS1,2 in 4.8), the height can
be adjusted by a pico motor. This motorization is necessary to align the nozzle in
relation to the �xed beam path. The photoelectrons coming from the gas, ionized
by the XUV pulses, are detected and spectrally resolved by a time of �ight electron
spectrometer6(TOF in 4.8). The signal from the TOF is read by a Fast ComTec
time digitizer7, that is synchronized with the repetition rate of the laser pulses via
an electronic trigger. The evaluation of the measured data is conducted by a home
made LabView program. [14]
For streaking measurements, the VIS/IR is focused at the same position as the
7FAST Comtec GmbH, p7887, http://www.fastcomtec.com
4.2. New software control system for the beamline 47
XUV. The temporal delay between the VIS/IR and XUV pulses is varied by the
piezo positioning system in the interferometer.
4.2 New software control system for the beamline
The necessity of experiments in vacuum makes the setup and control more challeng-
ing, because moveable parts have to be motorized. As installations quickly become
very complex, it is advantageous to combine the control of the motorization in a
centralized program. In this section the setup and development of a new such con-
trol system is described.
Due to malfunctions of the previously employed positioning systems, they were re-
placed by technically more precise and reliable devices by PI miCos 8. The stages
are VT-80 translational stages with a nominal range of 150mm. The �lter wheels
were replaced with the model AFW-65 which can hold up to six �lters. All devices
are controlled by SMC pollux controllers which are daisy-chained and can all be
addressed over a single serial port. As the positioning devices in the experimental
chamber are from the same manufacturer and controlled by SMC pollux controllers
as well, integration into a single system was possible without problems.
An addition to the existing control possibilities poses the introduction of computer
control for motorized �ipper mounts like the New Focus9 8892-K. For this the TTL
connections on the handpads are addressed by a NI10 USB-6009 analog-digital in-
terface. By using the digital channels on the USB-6009 it is possible to control up
to twelve �ips with a relative low cost device.
4.2.1 Programmatic structure of the control system
The program is developed in a client-server architecture, to make it possible to con-
trol the beamline from di�erent computers. That way, the previous restriction to
the computer with the physical hardware connection to the pollux controllers, to
move stages or �lter wheels, could be removed. Apart from the possibility of using
8PI miCos GmbH, http://www.pimicos.com/9Newport Spectra-Physics GmbH, http://www.newport.com/10National Instruments Corporation, http://www.ni.com/
48 4. Beamline AS2
the control interface on di�erent computers around the beamline, all low level func-
tions, for integration in other programs, are available over the network. This makes
it possible to combine e.g. the movement of a stage and the streaking measurement
in one program, allowing for further automation than possible until now. All men-
tioned programs are developed in LabView. For further descriptions it is assumed
that the reader is familiar with the basic concepts of LabView programing.
For the realization of the project the LabView vi server is used. It o�ers the possibil-
ity to call and transfer parameters to vis on a di�erent computer over the network.
Though the call comes from a remote computer, the execution of the vis is local,
they run on the server. For the transfer of data, like the current positions of the
stages or the �lter wheels, the LabView shared variable engine is used. It allows the
remote access of variables over the network.
As with all network based applications, security is a relevant topic here as well.
The use of static IP addresses in the MPQ network makes it possible to use the vi
servers and shared variable engines own white list based security system for this.
Only computers that are manually registered on the server are able to access the
system.
4.2.2 Access control
With the possibility to control the system from di�erent computers, the problem
arises, that di�erent computers might send con�icting commands to the server at
the same time. To prevent this, an access control system is implemented. The server
will only grant access to the system to one program at a time. Every program that
wants to take control, has to request an access code which is generated randomly
and stored on the server. If the server detects a valid stored code, the request for
a new one will be denied. All low level functions, like the relative movement of a
stage, require the calling client to send the access code which will be compared to the
stored one before the command is executed. Should the submitted code be invalid,
an error is returned. Before any other program can take control, the program in
command has to release control which will delete the access code on the server and
make control available to other clients.
4.2. New software control system for the beamline 49
4.2.3 Server program
The server program has to be running on the computer that is physically connected
to the pollux controllers, for all remote functions to work. At start up, the server
initializes the communication with the controllers. It further creates the semaphore
"micossem", which is used by all server sided functions, to ensure that requests to
the controllers don't con�ict with each other. During run time the program contin-
uously updates the positions of the connected devices to the shared variable array
"positions", running on the server computer. For the case of a program not releasing
control, the access control system can be reset in the server program.
4.2.4 Low level functions available for further development
To simplify the use of the distributed system, envelope vis, calling the actual vis on
the server, were developed. They all have the nomenclature "rc_actual_name", "rc"
standing for "remote call". These vis need two �les in the same folder, "server_ip.txt"
and "server_path.txt", containing the server IP and the server path respectively.
While these values usually stay the same, two vis to change them, "set_server_ip"
and "set_server_path", were developed. The available functions are listed in table
4.1. All Functions have error in- and outputs and, apart from "request control",
require the access code.
Function "request control"
The function "request control" submits the ip of the requesting computer to the
server and checks if the control is available. It has three return values: "access
granted" (boolean), "access code" (-1 if control not available), and "ip in control"
(the IP of the computer that currently has the control over the system). If the
function returns access granted = true and access code 6= −1, the program is now
in control and has to release control, before any other program can gain access.
50 4. Beamline AS2
Function name Description Inputs Outputs
calibrate �lter Executes calibrationrun of a �lter wheel.Stops at zero position.
Controller number
calibrate stage Executes calibrationrun of a stage. Stopsat zero position.
Controller number
check code Checks if the accesscode is still valid.
boolean: code valid
move �lter absolute Moves �lter wheel toan absolute position[◦]
Controller number,position
move �lter relative Moves �lter wheel thespeci�ed distance [◦]
Controller number,distance
move stage absolute Moves stage to an ab-solute position [mm]
Controller number,position
move stage relative Moves stage the spec-i�ed distance [mm]
Controller number,distance
release control Releases control of thesystem. Invalidatesthe access code.
request control See separate section in4.2.4.
Requesting IP Access grantedAccess codeIp in control
set stage zero Sets the current posi-tion of a stage as zeromark.
stop movement Stops all movement.
Table 4.1: Functions remotely available for the control of the beamline.
4.2.5 Programs developed or extended
The advantage of the new system is the possibility to combine automated movement
with data acquisition. Because of the implementation as a network based application,
this is possible across di�erent computers and can easily be included in existing
measurement systems.
4.2. New software control system for the beamline 51
Beam pro�le measurements
For a great variety of experiments it is necessary to know the spatial beam pro�le
in detail. An easy way to evaluate this parameter is to record it with a CCD camera
at di�erent positions of the beam. An automated combination of image acquisition
and the movement of a translational stage makes it possible to measure this in great
detail in an acceptable amount of time. This method was implemented for three
di�erent camera systems, WinCamD from DataRay11, FireFly from PointGrey12,
and CinCam from Cinogy13
For the evaluation of the pro�le of an XUV beam, direct measurements, like de-
scribed earlier, are not possible because of the size of the cameras necessary in that
wavelength regime. A possible method is the so called "knife edge" measurement.
A razor blade is stepwise inserted into the beam perpendicularly, blocking o� parts
of it. Measuring the transmitted power and knowing the precise step size, it is now
possible to reconstruct the beams pro�le in the direction of the moving blade, by
�tting an error function to the acquired data. As the results of this method are
only signi�cant if the step size of the moving blade is small enough, it requires a
great amount of single measurements. This makes it a time consuming procedure if
performed manually. The implemented combination of automated camera readout
and stage movement makes the method suitable for regular use.
Streaking series measurements
For the measurement of the intensity dependent e�ect of the transition of VIS/IR
pulses through SiO2 (see chapter 6), a series function was developed. The pulse af-
ter the transition was characterized by streaking measurements. The intensity of the
pulses was adjusted by the position of the SiO2 target in relation to the VIS/IR focus
in the interferometer. The power of the VIS/IR beam was recorded in front of the
SiO2 target (incoming power) and behind the streaking target (transmitted power).
The developed series function allows to enter an arbitrary amount of positions for
the SiO2 target, to then be measured automatically.
11DataRay Inc., http://www.dataray.com12PointGrey, http://www.ptgrey.com13CINOGY Technologies GmbH, http://www.cinogy.com
52 4. Beamline AS2
For each positional step, the program moves a thick glass wedge into the beam by a
motorized �ip mount, diverting it to a power meter. That way, at the same time, the
beam is blocked from its regular path, making it safe to move the SiO2 target. With
the target in position and the incoming power recorded, the wedge is removed. The
transmitted power is recorded by a second power meter behind the experimental
chamber and the streaking measurement is initialized. After the streaking, again,
the transmitted power is recorded.
The automation of the procedure not only increases the speed of the measurements
but equally important eliminates human in�uences and errors, increasing compara-
bility of the collected data.
Chapter 5
Ozone
Ozone is a vital component of the earth's atmosphere. The so called ozone layer in
an altitude of h ≈ 10− 50 km is an important part of the atmosphere's protection
against harmful UV radiation damaging the DNA of plants and animals alike. Es-
pecially radiation in the UV-B, λ = 280 − 315 nm, and UV-C, λ = 100 − 280 nm,
band is hazardous to living beings. The cycle between molecular oxygen and ozone
absorbs in this regime, reducing the intensity of the radiation by many magnitudes.
The importance of ozone for the absorption of UV radiation is long known and �rst
studies date back to the 19th century [27][28]. Investigations of the process of photo-
chemical generation of ozone started approximately 20 years later, at the start of
the 20th century[29].
Ozone in the stratosphere is mainly generated by the absorption of UV radiation in
the regime below λ < 240 nm by molecular oxygen, O2.
O2UV−C−−−−→ 2 O (5.1)
O +O2 +M → O3 +M (5.2)
M in this equation is an additional molecule absorbing excess energy from the pro-
cess. The decomposition of ozone again absorbs UV. With lower absorbed photon
energies, this reaction extends the absorbing regime of the atmosphere through the
53
54 5. Ozone
lower energetic part of the UV-C and the whole UV-B band.
O3UV−B−−−−→ O2 +O (5.3)
This process is mostly immediately followed by the repeated generation of ozone as
described by equation 5.2, keeping the ozone concentration nearly constant.
Though ozone is long studied and well known, time resolved measurements of the
�rst few femtoseconds of the process of UV absorption and ozone dissociation have
not been performed yet. This chapter describes such measurements.
5.1 Basic measurement principle
Though the absorption spectrum of ozone is well known, the attosecond dynamics
of photoexcitation and subsequent dissociation are still under investigation. Figure
5.1 shows the di�erent absorption bands of ozone with their cross sections. The
strongest absorption is observed in the Hartley band [27] in the UV, followed by
the energetically adjacent Huggins band [31] and the Chappuis band [28] in the vis-
ible spectral regime. Due to the high cross section, absorption in the Hartley band
was selected for �rst experimental investigations. Theoretical calculations show that
with a 3 fs UV pump pulse with peak intensities of I = 1013 W/cm2 and a central
wavelength of λ = 260 nm considerable populations of up to 40% in the Hartley
band should be obtainable [32]. The temporal evolution of the excited state should
then be detectable via photoelectron spectroscopy with attosecond probe pulses.
The experiments are performed in the experimental chamber of the AS2 beamline
(chapter 4) with the FP3 laser (chapter 3). The necessary UV pump pulses for the
experiment are generated via third harmonic generation as described in section 4.1.2,
the attosecond probe pulses via HHG as described in section 4.1.1. The ozone is in-
troduced to the experimental chamber via a gas nozzle generating a constant �ow
of ozone in the common focus of the pump and the probe beam. At the timescale
5.2. Production of ozone 55
Figure 5.1: Ozone absorption cross sections. [30]
considered, the movement caused by this �ow can be neglected. Photoelectron detec-
tion is performed by a time of �ight electron spectrometer. To resolve the temporal
evolution of the excited state the delay between pump and probe is varied.
5.2 Production of ozone
Due to the instability of ozone, it is not commercially available bottled like e.g.
oxygen. For the same reason a long time storage is not possible. The purity of con-
centrated ozone degradates on a timescale of few hours and a high level of purity,
> 80%, is a necessity for the experiments. Furthermore, concentrated ozone always
poses a risk, due to toxicity as well as explosivity. Therefore it is only produced for
experiments and remaining amounts are disposed afterwards.
56 5. Ozone
to experiment
to pump
U
ozonizer O + 2
2
O2 O3
T
LN
silica gelheat bu�er
reservoir
Figure 5.2: Schematic of the setup for ozone production and puri�cation.
Figure 5.2 depicts the setup for the generation and distillation of ozone schemati-
cally. The in-time production of ozone in the lab is performed by a commercially
available ozonizer using an electric discharge for the generation. Fed with molecular
oxygen, the ozonizer produces a mixture of ozone and oxygen with a concentration
of approximately 2%. The mixture is routed through and the contained ozone ad-
sorbed at silica-gel cooled to a temperature of T = −90 ◦C. For the generation of the
required temperatures the silica gel �lled glass tube is embedded in a container �lled
with small steel plates acting as a heat exchange and temperature bu�er. The con-
tainer is submerged in liquid nitrogen. As a side e�ect the steel plates also prevent
sudden heating to critical temperatures should the nitrogen evaporate unnoticed.
Since the temperature of liquid nitrogen, T = −196 ◦C, is too low the glass tube is
heated electrically. The temperature is regulated by a Jumo cTron 041 temperature
controller, switching the heating accordingly. The method of storing ozone with sil-
ica gel is long known and serves di�erent purposes [33]. The adsorption in the gel
greatly decreases the risk of explosions. A comprehensive study of the adsorption
of ozone on silica gel by Cook in 1959 showed that under the mentioned conditions
even the intentional generation of an electric spark near silica gel, loaded with up
1JUMO GmbH & Co. KG, http://www.jumo.de
5.3. Pressure control 57
to 10% of its weight, did not cause an explosion [34]. Besides safe storage, the silica
gel is used for the puri�cation process. At the used conditions during production,
p = 550 mbar, T = −90 ◦C, mainly ozone is adsorbed while the majority of oxygen
passes the gel unchanged. After the production period of t ≈ 20 min, depending on
the required amount, the �ow of gas is stopped and the container evacuated. Still at
the temperature of T = −90 ◦C this causes the adsorbed oxygen to evaporate, while
the ozone stays bound. At a �nal system pressure of p ≈ 3 mbar the bound ozone
has a concentration of approximately 95%. To release the ozone for experiments, the
silica gel is heated to temperatures of approximately T ≈ −20 ◦C. As a bu�er for
the system pressure a reservoir volume is attached. Control measurements to verify
the ozone's purity show that after a time of t = 2 h the purity has degraded to ap-
proximately 80%. The measurements were performed by exploding a small amount
of the oxygen-ozone mixture in a well de�ned volume. When the temperature after
the reaction has readjusted to the value before, the purity can be calculated from
the pressures by
x = 2 · pe − psps − p0
(5.4)
with x the purity, pe the end pressure, ps the start pressure, p0 the residual pres-
sure.
5.3 Pressure control
For the described experiments a constant �ow of ozone into the experimental cham-
ber and therefore a constant pressure in the ozone storage is required. To control the
emerging pressure the temperature of the silica gel has to be varied. The relation
between the system pressure and the applied temperature is not constant over time
and the temperature has to be adjusted constantly.
58 5. Ozone
5.3.1 Automation
To keep the system pressure at a constant value an automation system was devel-
oped. The systems pressure as well as the temperature are constantly monitored by
a LabView program that adjusts the set temperature of the Jumo cTron controller
as needed. Starting from a stable pressure, after manually heating to approximately
T = −20 ◦C, it proofed to be su�cient to use a simple algorithm. The program
in-/decreases the temperature by 1 ◦C, should the pressure di�er from a set value
by more than a con�gurable threshold, typically 2 mbar. Due to the dangerous na-
ture of ozone, safety thresholds were implemented. The system will be reset to a
system temperature of T = −90 ◦C should a con�gurable temperature or a con�g-
urable di�erence from the set pressure be exceeded.
5.4 Experimental results and outlook
First experiments trying to detect the excitation in the Hartley band were performed
using the setup of FP3 without the booster stage, limiting the available IR pulse
energy for the UV generation to E = 0.2 mJ . The resulting peak UV intensities
are in the regime of I = 1012 W/cm2 at a pulse duration of τ ≈ 3 fs. Theoretical
calculations predict populations in the regime of 1% for these parameters. Detection
of the excitation under these conditions was expected to be challenging and was not
successful in the end. No di�erence from the ground state photoelectron spectrum
could be determined.
The required increase in pump intensity led to the decision to upgrade the lasers
power by the reinstallation of the booster stage (see section 3.2). To further increase
the UV intensities, a change in the focusing is planned. Instead of the toroidal mirror
(T in �gure 4.2) an o� axis parabolic mirror directly in the experimental chamber
will be used. The thus reduced focal length decreases the focus size and therefore
increases the intensity. With the increased power and changed focusing intensities of
up to I = 1013 W/cm2 are expected. The calculated resulting populations of up to
40% should easily be detectable[32]. The con�rmation of the calculated time scale
5.4. Experimental results and outlook 59
by time resolved measurements would be a great progress in the investigation of
ozone's dynamic behavior.
Chapter 6
Silicon dioxide
Most of modern information technology is based on manipulating and controlling the
properties of matter with electric �elds. Hitherto existing applications work in the
regime of microwaves. Extending this to optical frequencies requires high intensity
electric �elds and the application of larger band gap materials which are dielectrics.
Since the changes in the electronic properties induced by external �elds are coupled
to the optical response of a material, the investigation of the nonlinear polariza-
tion permits a deeper understanding of the underlying processes.[35] Following the
experiments by Schultze et al. the intensity dependent e�ects of the transition of
ultrashort pulses through SiO2 are investigated directly. The pulses pass through
thin plates of SiO2 and are subsequently characterized by streaking. Depending on
the peak intensity at the target, the resulting waveforms are analyzed.
6.1 Experimental setup
The experiment is performed in the AS2 beamline (see chapter 4). The target, a thin
plate of silicon dioxide between 12 and 100 µm, is mounted on a motorized trans-
lational stage at the intermediate focus of the interferometric chamber, F in �gure
4.2. To minimize re�ective losses the target is mounted in Brewster con�guration.
For the adjustment of the on target intensity the longitudinal position in relation
to the focus is varied, changing the beam diameter on the target and therefore the
intensity. To quantitatively determine the intensity, depending on the position, the
61
62 6. Silicon dioxide
total power of the incoming laser was measured with a powermeter and the beam
pro�le characterized as described in section 4.2.5. The pulse duration is measured
by streaking without target. For the evaluation of resulting e�ects the pulse after
the target is characterized by streaking in the experimental chamber. To monitor
potential temporal �uctuations in the laser's power or other intensity changes during
the measurement, the transmitted power is recorded before and after the streaking.
In order to thoroughly detect intensity dependent changes in the polarization of
the fused silica, altering the phase of the transmitted laser beam, it is necessary
to compare measurements at di�erent intensities. Therefore the sample is alternat-
ingly exposed to high (up to 1013 W/cm2) and low (below 1011 W/cm2) intensities.
At the applied low intensities no nonlinear response is to be expected. Only if the
measurements at low intensities are reproducible, systematic errors can be excluded.
Due to the necessary number of single measurements and the number of single steps
for each measurement, an automation was developed as described in section 4.2.5.
Using an automated procedure not only increases the speed of the experiment and
therefore the amount of recordable data but also increases comparability as possible
human in�uences are minimized.
6.2 Results
Figure 6.1 depicts an exemplary measurement of one high intensity transition with
a peak intensity of Imax = 1.82 · 1013 W/cm2 (red graph) in comparison with two
references, Imax ≈ 1012 W/cm2 (black and blue), at a 12.6 µm thick fused silica tar-
get. The upper graph shows the electric �elds evaluated from the vector potentials
recorded by streaking, the lower the �elds' envelopes. A high degree of accordance
between the two references is clearly visible. The reproducibility of the measure-
ments at low intensities show that the system runs with satisfying stability. In the
high intensity regime two e�ects mainly caused by the nonlinear refractive index
(Kerr e�ect) are visible. A phase shift to higher delay values representing self-phase
modulation can be seen at the highest intensities. Examining the envelopes, a shift
of the maximum to larger delay values is visible. This intensity dependent change
6.2. Results 63
0 5 10 15 20 25
−2
0
2
x 109
Delay (fs)
Ele
ctric
fiel
d (V
/m)
0 5 10 15 20 250
1
2
3x 109
Delay (fs)Ele
ctric
fiel
d en
velo
pe (
V/m
)
Imax
=0.09⋅ 1013 W/cm2
Imax
=1.82⋅ 1013 W/cm2
Imax
=0.10⋅ 1013 W/cm2
Figure 6.1: Comparison of the resulting electric �elds and �eld envelopes of pulses
after the transition through a 12.6 µm thick plate of amorph SiO2. Black and blue:
Reference measurements with maximum intensities at the target in the regime of
I ≈ 1012 W/cm2. Red: Measurement with a maximum intensity at the target of
I = 1.82 · 1013 W/cm2. While no di�erence is visible in the temporal regime of
low intensities for delays of t < 5 fs and t > 15 fs, two e�ects, caused by the
intensity dependent refractive index, can clearly be seen during the pulse. i) Phase-
shift delaying the electric �eld. ii) Self-steepening delaying the peak of the envelope.
in the group velocity of the pulse causes a decrease of the slope at the leading edge
of the pulse and an increase of the slope at the trailing edge. The e�ect is called
self-steepening [36]. As there is no variation detectable after the pulse, the e�ects
are reversible on a timescale of femtoseconds.
The experiments were performed with di�erent target thicknesses as well as for
di�erent pulse intensities. A comprehensive evaluation of the recorded data and
a theoretical modeling of the underlying processes is still in progress and a �nal
64 6. Silicon dioxide
conclusion has not been reached yet. Possible higher-order nonlinear e�ects than
described here are still under investigation by M. Stockmann, K. Yabana, and V.
Yakovlev.
Chapter 7
Conclusion
During the performance of this thesis major changes in the infrastructure of an
attosecond beamline were made. Driven by technical necessity, the exchange of the
motorized parts in the beamline was used as an opportunity to redesign and improve
the associated control system. New possibilities in automation were developed and
proved to be of great help in the performance of complex experiments.
The investigation of the nonlinear response of silicon dioxide already bene�ted from
the new control system. The increase in speed made previously time consuming
procedures, like e.g. the detailed characterization of the beampro�le, feasible for
frequent use. Though the evaluation of the recorded data and the development of a
theoretical model is not concluded yet, the experiments were successful. The changes
in the waveform are clearly detectable and the reversibility of the observed e�ects
on a femtosecond timescale could undoubtedly be shown.
The requirement of higher pulse energies for the generation of UV in the ozone ex-
periment led to the installation of the additional ampli�er in the laser. Though a
time consuming undertaking and not completely �nished as of the writing of this
thesis, the resulting improvement in power is promising. Already without optimized
pulse compression new IR intensity regimes are accessible. With the upcoming ex-
change of the chirped mirror compressor again sub 5 fs pulse durations are to be
expected, making the FP3 laser system a valuable tool for future experiments.
With the upgraded laser system time resolved detection of the UV excitation of
ozone in the Hartley band should be possible and, looking further into the future,
65
66 7. Conclusion
even investigations of IR excitation in the Chappuis band could be successful.
With the improvements made the combination of FP 3 laser system and AS2 beam-
line is a prime tool for the investigation of ultra fast physics.
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B Acknowledgments
Without the help and support of many people the performance of this thesis would
not have been possible. I would like to thank especially:
Prof. Dr. Reinhard Kienberger for giving me the opportunity to perform this
thesis in his group
Michael Jobst as my supervisor for the patient introduction into the complex
techniques necessary for the performance of this thesis
Tobias Latka, Annkatrin Sommer, Elisabeth Bothschafter, Wolfgang Schwein-
berger for all the help, the willingness to answer my questions, and in general for
making the lab a pleasant environment to work in
Hanieh Fattahi, Konrad Hütten for the entertainment
Dr. Hristo Iglev for awakening my interest in the �eld of nonlinear optics
Dr. Karl Dressler
v
