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L. Klar
Quantum vacuum nonlinearities in the all-optical regime
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2022)

Abstract: In this work, we demonstrate how new theoretical concepts enable measurements of the signature of the QED vacuum nonlinearity beyond the background in collision experiments of all-optical high-intensity laser pulses. Using the vacuum emission picture, we develop the method of channel analysis of the signal. Based on these findings, we study two different experimental scenarios and identify discernible signals. In the first case, we consider the collision of two high-intensity laser pulses that differ only in their focus waist sizes. We present a numerical method to identify the regions where the signal dominates the background. Furthermore, we use this to investigate the behavior of the discernible signal, particularly with respect to the effects of the waist size of the probe beam. Of particular note, maximization of the measurable signal photons is not achieved by minimal focusing. This can be explained by the interplay of intensity in the interaction volume and decay behavior of the background in the far field. With the help of an elliptical cross section of the probe pulse, the signal can be further enhanced. Moreover, we show that a discernible signature of vacuum birefringence is achievable in the all-optical regime. In a second setup, elastic and inelastic photon-photon scattering mediated by the nonlinearity of the quantum vacuum is investigated. Based on a collision of four laser pulses of different oscillation frequencies, we observe signals in regions beyond the forward direction of the driving lasers as well as with frequencies beyond the laser frequencies. These features allow us to measure the signal beyond the background. The preceding channel analysis not only helps in the interpretation of the results, but it also allows effective amplification of the signal while maintaining experimental constraints.

V. Kosheleva
QED corrections to the hyperfine splitting and g factor of few-electron ions
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2022)

Abstract: Quantum electrodynamics (QED) is the first quantum field theory that describes all phenomena associated with electrically charged particles. Despite its mathematical complexity, it is quite effective in describing and predicting experimental results. With the introduction of lasers, atomic spectroscopy is constantly evolving, contributing to QED testing and continuous improvements in the precision of physical constants determination. Atomic systems offer many opportunities for high-precision QED tests. In the present dissertation, we focus on the magnetic sector of QED: the hyperfine structure and the Zeeman effect in few-electron ions.
We present the systematic QED treatment of the electron correlation effects in the ground-state hyperfine structure in lithiumlike ions for the wide range of nuclear charge numbers Z = 7 - 82. The one- and two-photon exchange corrections are evaluated rigorously within the QED formalism. The electron-correlation contributions due to the exchange by three and more photons are accounted for within the Breit approximation employing the recursive perturbation theory. The calculations are performed in the framework of the extended Furry picture, i.e., with the inclusion of the effective local screening potential in the zeroth-order approximation.
In comparison to previous theoretical computations, we improve the accuracy of the interelectronic-interaction correction to ground-state hyperfine structure in lithiumlike ions. The g factor of a bound electron is a rigorous tool for verifying the Standard Model and searching for new physics. Recently, a measurement of the g factor for lithiumlike silicon was reported and it disagrees by 1.7! with theoretical prediction [D. A. Glazov et al., Phys. Rev. Lett. 123, 173001 (2019)]. Attempting to resolve this deviation another theoretical value for silicon has been delivered. It results in a disagreement with experimental value [V. A. Yerokhin et al., Phys. Rev. A 102, 022815 (2020)]. We perform large-scale high-precision computations of the interelectronic-interaction and many-electron QED corrections to determine the cause of this disagreement. Similar to the case of hyperfine splitting, we carry out the calculations within the extended Furry picture of QED. And we carefully analyze the final values’ dependence on the binding potential. As a result, the agreement between theory and experiment for the g factor of lithiumlike silicon improves significantly. We also present the most accurate theoretical prediction for lithiumlike calcium too, which perfectly agrees with the experimental value.

M. Schwab
Relativistic electron-cyclotron resonances in laser Wakefield acceleration
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2022)

Abstract: Laser plasma accelerators (LPAs) have the potential to revolutionize research fields that rely on relativistic particle beams and secondary radiation sources thanks to their 10-100 GV/m accelerating fields. In the Laser Wakefield Acceleration (LWFA) scheme, a relativistically intense pump or driver laser is focused into a low-Z gas target, ionizing the gas and driving a relativistic, electron plasma wave. Under the proper conditions, such a plasma wave can be used to accelerate electrons to GeV kinetic energies in only centimeters of plasma propagation. As LPAs continue to be tested and refined, nondestructive measurement techniques must be developed to further investigate and understand the dynamic laser-plasma interaction as well as to help ensure reliable operation and measurement of future accelerator facilities based on plasma technology.
In this thesis, experiment, theory and simulation are combined to investigate the magnetized, relativistic plasma coinciding with the pump laser at the front of the plasma wave. Experimentally, the Jeti 40 TW laser system was used at the Institute of Optics and Quantum Electronics in Jena, Germany to drive a LWFA in tenuous plasma. The plasma wave was then shadowgraphically imaged using a transverse, few-cycle probe pulse in the visible to near-infrared spectrum and an achromatic microscope using various polarizers and spectral interference filters. The resulting shadowgrams were sorted depending on the properties of the LWFA’s accelerated electron bunches, and subsequently stitched together based on the timing delay between the pump and probe beams. This allowed for the detailed investigation of the laser-plasma interaction’s propagation and evolution as imaged in different polarizations and spectral bands.
The resulting data showed two primary signatures unique to the relativistic, magnetized plasma near the pump pulse. Firstly, a significant change in the brightness modulation of the shadowgrams, coinciding with the location of the pump pulse, is seen to have a strong dependence on the pump’s propagation length and the probe’s spectrum. Secondly, after ~1.5 mm of propagation through the plasma, diffraction rings, whose appearance is polarization dependent, appear in front of the plasma wave. A mathematical model using relativistic corrections to the Appleton-Hartree equation was developed to explain these signals. By combining the model with data from 2D PIC simulations using the VSim code, the plasma’s birefringent refractive index distribution was investigated. Furthermore, simulated shadowgrams of a 3D PIC simulation using the EPOCH code were analyzed with respect to the aforementioned signals from magnetized, relativistic plasma near the pump pulse.
The results of the study present a compelling description of the pump-plasma interaction. The previously unknown signals arise from relativistic, electron-cyclotron motion originating in the 10s of kilotesla strong magnetic fields of the pump pulse. Advantageously, a VIS-NIR probe is resonant with the cyclotron frequencies at the peak of the pump. With further refinement, the measurement of this phenomenon could allow for the non-invasive experimental visualization of the pump laser’s spatiotemporal energy distribution and evolution during propagation through the plasma.

M. Vockert
Die radiative Elektroneneinfang als Quelle stark linear polarisierter Röntgenstrahlung
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2022)

Abstract: A good way to test the common theories in atomic physics and astronomy is to determine the degree of polarization of the emitted radiation. The short wavelengths in the X-ray range make a direct determination of the polarization impossible and the use of known interaction mechanisms necessary. A simple mechanism with significant anisotropy with respect to the polarization of the incident photons and a high effective cross section in the low to medium keV energy range is Compton scattering. Taking advantage of position- and energy-sensitive semiconductor detectors, this anisotropy provides the basis for Compton polarimetry.
Double sided segmented semiconductor strip detectors have therefore been used for polarization determination for several years. Within the SPARC collaboration of FAIR, the design of a Si(Li) polarimeter has now been further developed. This novel Compton polarimeter with a cooled first preamplifier stage is characterized in detail in this work. Compared to the previous models, it allows for a better energy resolution and a more precise polarization determination, as well as for the first time a precise determination of the degree of polarization and the orientation of the polarization vector at photon energies well below 100 keV. This makes the emission properties of radiative transitions of heavy atoms accessible for polarization spectroscopy for the first time.
Until now, the precision of the determination of the degree of polarization was largely limited by the statistics of the investigated data set. Studies based on simulations, which are presented in this thesis, show that for the sizes of experimental data sets available here, statistical uncertainty continues to dominate systematic sources of error. In particular, the improved detector setup allowed for the first time the determination of the degree of polarization for radiative electron capture into the K-shell (KREC) of ions for the previously inaccessible range of photon energies below 70 keV. Close to complete polarization has been demonstrated for this important electron capture process, which is very prominent in collisions of heavy ions and light targets. This demonstration was achieved for the interaction of a Xe54+ ion beam with an H2 gas target and a K-REC photon energy of 56 keV. In the present work, it has thus been shown that radiative electron capture (REC), in particular into the K-shell, is one of the most significant mechanisms of the production of strongly linearly polarized X-rays. In particular, with variation of projectile ion and energy and observation angle, it provides a well-defined source of polarized X-rays with tunable energy and, at the same time, variable polarization properties.

M. Gebhardt
Power scaling of few-cycle short-wavelength infrared laser sources for nonlinear frequency conversion
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2022)

Abstract: To be added


S. Hell
Space- and Polarization-Resolved Investigations of Rear Side Optical Radiation from High-Intensity Laser-Solid Interaction
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2021)

Abstract: Thin aluminum foils (0.4-8µm) have been irradiated by laser pulses at relativistic intensities.
Hot electrons, which are periodically accelerated in the laser field at the foil front side,
emit coherent optical radiation (COR) at the foil rear side. COR has been investigated spaceand polarization-resolved to study hot electron transport through dense matter. This is important for further progress in laser-driven ion acceleration and fast ignition inertial confinement fusion. The COR source size increased from 1.2 µm to 2.3 µm with foil thickness. This is significantly smaller than the laser focal width of 4 µm and therefore indicates that pinching
or filamentation influenced the propagation of the diverging hot electron current. The
strong increase of the COR energy at the laser wavelength λ = 1030nm and λ/2 with laser
intensity I_L has been explained by considering an intensity dependent hot electron number
N and temperature T_h in a coherent transition radiation (CTR) model. Fitting this CTR
model to the experimental data allowed to determine Th which increases with I_L but slower
than expected. The CTR model fits also showed that about 40% of the hot electrons have been
accelerated at the laser frequency 60% at SHG, without significant changes with I_L.
Hence, hole boring must have deformed the plasma surface. The COR polarization, measured
at SHG, shows strong spatial changes along the COR emission region and varies with
I_L, foil thickness and the COR source size at the foil rear surface.

D. Hollatz
Detection of positrons from Breit-Wheeler pair formation
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2021)

Abstract: This work explores the experimental observation of the Breit-Wheeler process, first described by Gregory Breit and John A. Wheeler in 1934 [1], where two photons collide to form an electron positron pair from the quantum vacuum. The specific challenge thereby is the low cross section of a few 10e 29 m2 or 0.1 b combined with the requirement of photon energies in the range of mega electronvolt. Such beams can be provided by particle accelerators, for instance LCLS at SLAC or the European XFEL at DESY. Experiments exploring photon photon collisions with conventional accelerators were done in the past, for example E144 at SLAC in 1997 [2], however the two photon process described by Breit and Wheeler has not yet been observed. Over the last few decades, novel laser driven plasma based particle accelerators (LWFA) made significant progress [3, 4, 5, 6], allowing the production of the required photon beams to study the Breit-Wheeler process at pure laser facilities [7, 8, 9]. The work in hand explores the challenges related to such an experiment specifically at high power laser facilities using the example of Astra Gemini, a multi 100TW dual beam system at the CLF in England. In an experiment, multi 100MeV γ-rays from LWFA electron bremsstrahlung and 1-2 keV x-rays from Germanium M-L shell transition radiation are collided to produce pairs through the Breit-Wheeler process. A detection system to measure those pairs composed of a permanent magnet beam line and shielded single particle detectors is developed and tested within this thesis. The acquired data allows an estimate of the requirements for future experiments to measure the two-photon Breit-Wheeler process.

R. Klas
Efficiency scaling of high harmonic generation using ultrashort fiber lasers
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2021)

Abstract: -

S. Tietze
Compact XUV and X-Ray sources from laser-plasma interactions: theoretical and numerical study
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2021)

Abstract: In this thesis the generation of high order harmonics of ultrashort and high intensity laser pulses from solid density plasmas, so called surface high harmonic generation (SHHG), is studied. With SHHG, a compact source of coherent XUV and X-Ray radiation becomes possible. The results are obtained numerically using 1D and 2D Particle-In-Cell (PIC) computer simulations, which are supported by analytical models. This work focusses on two main issues of SHHG to date, pulse isolation and generation efficiency. It is shown that a single attosecond pulse (AP) can be obtained from a few-cycle incident laser pulse by choosing a suitable carrier-envelope phase (CEP), depending on the density and shape of density gradient of the target. An analytical model providing an interpretation of the results obtained from PIC simulations is presented. Spatial isolation of APs can be achieved using the attosecond lighthouse effect, but surface denting is detrimental to the separation of APs. PIC simulations are used to explain an experimental result, where a separation of pulses was not possible due to surface denting. Furthermore it is shown that the angular spectral chirp corresponds to the depth of the surface denting. The efficiency of SHHG can be enhanced greatly by reflecting the beam coming from a first target off a second target. Of major importance for the efficiency is the relative phase between harmonics on the surface of the second target. The relative phase changes even when propagating in free space due to the Gouy phase. To maximize the efficiency gain, a parametric study using PIC simulations has been performed to find the optimal distance between two targets.

N. Stallkamp
Confined ensembles of highly charged ions for studies of light-matter interaction at high intensities: the HILITE Penning trap setup
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2021)

Abstract: The investigation of light-matter interactions is based on the description of the `photoelectric effect' in the early 20th century. The development of the first laser systems, especially of systems with high intensities and/or high photon energies, allowed to study previously unknown, non-linear effects like multiphoton or tunnel ionisation processes, which became subject of theoretical descriptions and experimental studies. Independently, the storage techniques for charged particles (electrons and ions) developed in parallel and different kind of devices, like Paul and Penning traps, had been invented in the 1950s and 1960s to study fundamental parameters of matter (for instance g-factor, mass etc.) with previously unknown accuracy. The HILITE experiment, presented within this thesis, is designed to combine and use for the first time the advantageous properties of target preparation a Penning trap can provide, like ensemble temperature, purity and localizability, in order to investigate laser-ion interactions at high intensities. Particular attention was paid to the compactness of the setup in order to be capable to transport the experiment to different laser facilities and perform experiments on site. In the frame of this thesis, the experimental setup was built and put into operation in terms of its dedicated ion source, ion selection, beam transport, deceleration and capture inside the Penning trap at the GSI Helmholtzzentrum für Schwerionenforschung GmbH. During commissioning, the storage and non-destructive detection of pure ion ensembles within the trap was demonstrated. The individual components have been characterised and their operation was checked. Additionally, a proposal was handed in for the first beamtime at an external laser facility (FLASH at DESY), which was granted and carried out. The interaction between the laser and low charged ions could be verified.

A. T. Schmitt
Kombination von hochpräziser Polarimetrie mit Spektroskopie im Röntgenbereich
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2021)

Abstract: Magnetism, superconductivity, and other macroscopic quantum effects are based on symmetry breaking in solids. Their atomic and molecular structure can be studied using linearly polarized X-rays, where a change of the polarization state of the transmitted beam enables conclusions about electronic anisotropies in the material. Responsible for a change of the polarization state are the optical effects dichroism and birefringence. While X-ray absorption spectroscopy is a well-established method for the detection of dichroism, the effect of birefringence in the vicinity of an X-ray absorption edge is little studied. This work presents the first comprehensive experimental and theoretical investigation of X-ray birefringence and dichroism at the Cu K-absorption edge for two different model substances, CuO and La2CuO4. For this purpose, high-precision X-ray polarimetry, which detects changes of the polarization state with utmost sensitivity, was further developed into a spectroscopic method.

J. Hornung
Study of preplasma properties using time-resolved reflection spectroscopy
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2021)

Abstract: The aim of this work was to develop a new diagnostic method to probe preplasma properties in laser-plasma interaction experiments, using the time-resolved measurement of the laser pulse reflected by the plasma. Its spectral change over time can be attributed to the motion of the critical-density position of the plasma, which can be correlated with the preplasma properties that are present at the beginning of the interaction. 2-D particle-in-cell (PIC) simulations showed a correlation between the blue shift of the spectrum at the temporal beginning of the laser pulse and the expansion velocity of the preplasma, which can be used to derive the corresponding electron temperature. In addition, a correlation between the acceleration of the reflection point into the plasma and the density scale length has been observed. This has also been confirmed by an analytical description of the holeboring velocity and acceleration, which has been developed to include the effect of the preplasma scale length. To verify this method, two experimental campaigns were performed at the PHELIX laser system, while employing different temporal contrasts using so-called plasma mirrors. The experimental observations matched the predictions made by the numerical simulations. By comparing the maximum red shift of the spectrum with the results of the analytical description, the scale length of the preplasma was determined to be (0.18+-0.11) m and (0.83+-0.39) m with and without plasma mirror, respectively. At last, two further experimental campaigns to improve laser-ion acceleration at PHELIX were carried out. First, by increasing the laser absorption during the interaction using a p-polarized laser pulse and second, by increasing the laser intensity. The latter led to the generation of protons with a maximum energy of up to 93 MeV, for a laser intensity in the range of 8e20 W/cm^2, resulting in a new record for the laser system PHELIX.

S. Panahiyan
Toward quantum control in discrete-time quantum walks
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2021)

Abstract: Discrete-time quantum walks are among the branches of quantum information and computation. They are platforms for developing quantum algorithms for quantum computers. In addition, due to their universal primitive nature, discrete-time quantum walks have been used to simulate other quantum systems and phenomena that are observed in physics and chemistry. To fully utilize the potentials that the discrete-time quantum walks hold in their applications, control over the discrete-time quantum walks and their properties becomes essential. In this dissertation, we propose two models for attaining a high level of control over the discrete-time quantum walks. In the first one, we incorporate a dynamical nature for the unitary operator performing the quantum walks. This enables us to readily control the properties of the walker and produce diverse behaviors for it. We show that with our proposal, the important properties of the discrete-time quantum walks such as variance would indeed improve. To explore the potential of this proposal, we apply it in the simulations of topological phases in condensed matter physics. With our proposal, we can control the simulations and determine the type of topological phenomena that should be simulated. In addition, we confirm simulations of topological phases and boundary states that can be observed in one-, two- and three-dimensional systems. Finally, we report the emergence of exotic phase structures in form of cell-like structures that contain all types of topological phases and boundary states of certain classes. In our second proposal, we take advantage of resources available in quantum mechanics, namely quantum entanglement and entangled qubits. In this proposal, we use entangled qubits in the structure of a quantum walk and show that by tuning the initial entanglement between these qubits and how these qubits are modified through the walk, one is able to produce diverse behaviors for the quantum walk and control its behavior.


H. T. Olgun
Efficient high energy laser-driven multicycle terahertz generation for accelerators
Universität Hamburg, Fakultät für Mathematik, Informatik und Naturwissenschaften (2020)

Abstract: Optically generated, narrowband multi-cycle terahertz (MC-THz) radiation
has the potential to revolutionize electron acceleration, X-ray free-electron
lasers, advanced electron beam diagnostics and related research areas. However, the currently demonstrated THz generation efficiencies are too low to
reach the requirements for many of these applications.
In this project, a MC-THz generation approach via difference frequency
generation (DFG) driven by a laser with a multi-line optical spectrum was
investigated with the aim of increasing the conversion efficiency. For this purpose, a home-built, Yb-based laser source with a multi-line optical spectrum
was developed. This laser source was amplified to tens-of-millijoule using a
regenerative and a four-pass amplifier; it was used to generate MC-THz in
magnesiumoxid-doped periodically poled lithium niobate (MgO:PPLN) and
rubidium-doped periodically poled potassium titanyl phosphate (Rb:PPKTP).
With this laser system, the highest optical-to-THz conversion efficiencies (CE)
of 0.49% with a pulse energy of 30 mJ at 0.29 THz, and 0.89% with a pulse
energy of 45 mJ at 0.53 THz in MgO:PPLN were achieved. These results compare well with 2-dimensional numerical simulations. In addition, Rb:PPKTP,
which has a promising figure-of-merit compared to MgO:PPLN, achieved
a CE of 0.16% with a pulse energy of 3 mJ at 0.5 THz. Next, to scale this
laser system to tens of millijoule MC-THz output, large aperture crystals
for both MgO:PPLN and Rb:PPKTP were investigated using a commercial
laser, producing 200 mJ with a pulse duration of 500 fs at 1030 nm; although
in this case an older method of optical rectification (OR) was used, achieving
less efficiency than the multi-line source. With MgO:PPLN crystals of
aperture size 10x15mm2, a CE of 0.29% at 0.35 THz was achieved with a
pulse energy of 260 mJ. This is the highest known CE value using OR. In addition, wafer-stacks with alternating crystal-axis orientation of aperture size
of 1” for LN and 10x10mm2 for KTP were successfully tested. Two novel
experiments were performed with LN wafers: multi-stage wafer-stacks in
a serial configuration with multi-output THz radiation and back-reflected
seeded MC-THz generation. Both methods improved the efficiency of the
MC-THz generation, compared to a single stack. In particular, for the backreflected seeded MC-THz generation, pulse energies of 280 mJ with a CE of
0.29% was achieved; thus demonstrating the potential of seeded MC-THz
generation. These achievements are an important step for the realization of
next-generation, THz-driven electron accelerators.

F. Tuitje
Diffraction-based metrology in the extreme ultraviolet
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2020)

Abstract: The growing numerical development of Coherent Diffraction Imaging (CDI) towards ptychography allows for the first time the separate reconstruction of object and the wavefront illuminating it. This work is dedicated to the investigation of further possibilities resulting from the complex reconstruction of object and illumination.
In this thesis, gold structures buried in silicon are reconstructed and examined with regard to their surface morphology in reflection geometry. This completely non-destructive method allows metrology on structures of embedded circuits and otherwise hidden defects.
The increasing demand for easily accessible and compact high-performance light sources around the silicon and water window opens the question regarding their suitability for lensless imaging. In the following chapters a method is introduced which allows an almost complete source analysis by means of a single long time exposed diffraction pattern. The knowledge gained in this way allows an improvement of the source with respect to water window CDI and provides insight into dynamic processes within the source.
The complex-valued reconstruction of the wavefront allows an insight into the plasma and the ionization states prevailing there. The XUV seed pulse of a seeded Soft-X-Ray laser (SXRL), which passes the pumped plasma and changes its properties with respect to the states in the plasma, is reconstructed ptychographically. Adapted Maxwell-Bloch simulations allow by comparison with the measurement to restore the ionization states during the passage of the seed pulse.
Previous experiments showed artifacts during reconstruction, which were directly related to the periodicity of the objects. Simulation of periodic objects of different sizes and with the addition of intentional defects showed a dependence of the reconstruction of the object on the illumination function. Various criteria were derived from this simulation and are presented in this thesis.

M. Beyer
Characterization of optical componentsof a laser amplifier via spectral interferometry
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2020)

Abstract: The fundamentals of ultrafast optics based on Maxwell’s equations are presented, Gaussian beams, optical pulses and their propagation in dispersive media are introduced. The method of spectral interferometry (SI) is fundamentally introduced and explained in section 3, different possibilities for characterizing the spectral phase are presented. The experimental setup for the characterization and a referencing measurement to well characterized materials is done in section 4. It is also investigated in section 4 which experimental issues can occur, how large their influences on the measurement are and how they can be resolved. The derived methods of spectral phase characterization are used in section 5 to specify the optical components of an amplifier in a CPA laser system. The components of the laser amplifier are categorized and their effects on the spectral phase are compared and discussed. It is then summarized why dispersion measurements are important and how the method of SI can be utilized to select suitable components for a laser amplifier.

M. Nolte
Charakterisierung expandierter ultradünner DLC-Folien für die Laser-Protonenbeschleunigung
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2020)

Abstract: In dieser Arbeit werden zunächst die physikalischen Grundlagen vorgestellt, welche
für das Verständnis der Simulationen und deren Auswertung notwendig sind. Das Plasma, welches
sich vor dem Erreichen der maximalen Laserintensität ausgebreitet hat, kann den TNSA-Prozess und
dessen Effektivität beeinflussen. Es ist daher wichtig die genaue Form und den Zustand des
Targets zum Zeitpunkt des Eintreffens des Hauptpulses zu charakterisieren. Dafür wird der Computercode
MULTI-fs verwendet, welcher noch einmal genauer in Abschnitt 3 diskutiert wird, um
die Interaktion eines relativistischen Laserpulses mit einem dünnen Target zu simulieren. Die zeitliche
Struktur des in der Simulation verwendeten Laserpulses wurde bei Experimenten am POLARIS-Laser
in Jena gemessen. Betrachtet wird dabei die ansteigende Flanke der Laserintensit¨at bis zu dem Zeit-punkt, an dem die Laserintensität 10^17W/cm^2 ¨uberschreitet, für verschiedene Targetdicken
und verschiedene maximale Laserintensitäten. Aus diesen Simulationen wird die Verteilung der Elektronendichte
gewonnen und parametrisiert, um die Form der Plasmaverteilung
systematisch beschreiben zu können. Die aus der Simulation gewonnenen Ergebnisse
werden vorgestellt und diskutiert.

R. Hollinger
Extreme nonlinear optics in highly excited semiconductors
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2020)

Abstract: This thesis studies extreme nonlinear optical phenomena in highly excited ZnO semiconductor samples. ZnO with a band gap of 3.2 eV, in the near-ultraviolet spectral range, is irradiated with far-off resonance strong light fields in the near (0.8 µm, 1.5 eV) to the far-infrared (10 µm, 0.13 eV). Specifically, the coherent conversion of laser light into high orders of the fundamental frequency, also known as high harmonic generation (HHG) and optically pumped lasing were investigated.

J. Krause
Oberflächendynamik eines Plasmas im Bereich des steilen Dichtegradienten bei Wechselwirkung mit relativistischen Intensitäten
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2020)

Abstract: In this thesis the spectra of light reflected back from a laser plasma are analyzed with respect to the surface dynamics in the region of the increasing density gradient. In addition, the indentation movement as a function of energy, polarization and foil thickness was investigated.

B. Böning
Above-threshold ionization driven by spatially structured laser fields
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2020)

Abstract: Strong laser fields are a valuable tool to study the electron dynamics in atoms and molecules. A prominent strong-field process is the above-threshold ionization (ATI), where the momentum distributions of emitted photoelectrons encode not only details about the laser-atom interaction, but also properties of the driving laser field. Recent advances in the generation of intense laser beams at mid-infrared wavelengths enable the investigation of ATI in a new parameter range. Moreover, laser beams with a sophisticated spatial structure as a result of an orbital angular momentum (twisted light) have found applications in the strong-field regime. In this dissertation, we theoretically investigate ATI driven by mid-infrared and twisted light beams. We show that not only the temporal but also the spatial dependence of such beams has a pronounced impact on the ionization dynamics due to nondipole interactions. Therefore, we develop a quite general theoretical approach to ATI that incorporates this spatial structure: in order to extend the widely used strong-field approximation (SFA), we construct nondipole Volkov states which describe the photoelectron continuum dressed by the laser field. The resulting nondipole SFA allows the treatment of ATI and other strong-field processes driven by spatially structured laser fields and is not restricted to plane-wave beams. We apply this nondipole SFA to the ATI driven by mid-infrared plane-wave laser beams and show that peak shifts in the photoelectron momentum distributions can be computed in good agreement with experiments. As a second application, we consider the ATI driven by standing light waves, known as high-intensity Kapitza-Dirac effect. Here, we calculate the momentum transfer to photoelectrons for elliptically polarized standing waves and demonstrate that low- and high-energy photoelectrons exhibit markedly different angular distributions, which were not observed previously. Finally, we investigate the ATI of localized atomic targets driven by intense few-cycle Bessel pulses. Based on a local dipole approximation, we demonstrate that the photoelectrons can also be emitted along the propagation direction of the pulse owing to longitudinal electric field components. Moreover, when measured in propagation direction, the ATI spectra depend on both the opening angle and the orbital angular momentum of the Bessel pulse. To conclude, we also discuss the extension of this work towards long pulses, which can be treated within the above nondipole SFA.

S. Ringleb
The HILITE Setup for High-Intensity-Laser Experiments with Highly Charged Ions: Design and Commissioning
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2020)

Abstract: Quantitative studies of the interaction of atomic and molecular ions with laser radiationat high laser intensities and/or high photon energies are a novel area in the field of laser-matter-interaction. They are facilitated by precise knowledge of the properties of the ions as a target for the laser. This refers to the location, composition, density and shape of the ion cloud as a target, as well as to the capability of characterising the ion target before and after the laser interaction. Ion traps are versatile instruments when it comes to localising ions with a defined particle composition, density and state within a specific and small volume in space. They allow in particular the combination of ions in well-defined quantum states with intense photon fields. The present thesis contains the detailed description of the setup and commissioning of the HILITE (High-Intensity Laser Ion-Trap Experiment) Penning trap, which is dedicated to providing a well-defined cloud of highly charged ions for a number of different experiments with intense lasers. Various experimental procedures are necessary to create such an ion cloud, starting with the production of highly charged ions, their transport, selection, capture, storage, cooling, compression and detection. In the present thesis, the experimental setup is described in detail and the components required for ion target preparation, characterisation and non-destructive ion detection inside the trap are characterised. Special attention is paid to the counting limits of the detection electronics, because knowledge of the exact number of stored ions is essential for the planned experiments. Highly charged ions are produced in an electron-beam ion trap (EBIT), selected with respect to their mass-to-charge ratio, decelerated, and injected into the trap, where they are dynamically captured and stored. For the preparation of a well-defined ion cloud, the initially high energetic ions must be slowed and cooled to an energy of less than 1 meV. This thesis describes the applied methods of active-feedback cooling and resistive cooling and examines their potential cooling efficiencies.

J. Hofbrucker
Two-photon ionization of many-electron atoms
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2020)

Abstract: Until recently, the nonlinear interaction between light and matter has been restricted to only low photon energies produced by optical lasers. However, about a decade ago, the rise of free-electron laser facilities revolutionized the field of nonlinear light-matter interaction by delivering intense high-energy light pulses. Today, such lasers are used for research in materials science, chemical technology, biophysical science, solid-state physics as well as fundamental research. It is the new experimental possibilities provided by free-electron lasers that motivated the work presented in this thesis. Two-photon ionization process is one of the simplest nonlinear interactions in which absorption of two photons by an atom (or a molecule) leads to promoting one of its bound electrons to continuum. This work presents studies of two-photon ionization of neutral atoms. After a brief historical introduction to the topic of nonlinear light-matter interaction, the density matrix describing the state of an atom and a photoelectron following two-photon ionization is derived. The density matrix contains the complete information about the overall system consisting of a photoion and a photoelectron. In each successive chapter, part of this density matrix is used to obtain characteristic quantities such as total two-photon ionization cross section, photoelectron angular distributions, ion polarization or even degree of polarization of fluorescence photon produced by subsequent decay of the photoion. Physical properties of these quantities are studied and intriguing phenomena, such as elliptical dichroism, polarization transfer as well as relativistic and screening effects are investigated. In one-photon ionization, the photon energy for which the dominant ionization channel vanishes is called the Cooper minimum. This concept is extended to nonlinear ionization of atoms and the effect of the minimum on all above mentioned quantities is studied. In this work it is shown, that the nonlinear Cooper minimum leads to strong variation in practically all observables of the two-photon ionization process. For example, the polarization transfer from the incident to fluorescence photon can be maximized and so can be the elliptical dichroism in photoelectron angular distributions. Furthermore, it is theorized, that detection of the energy position of the nonlinear Cooper minimum could lead to comparison of experimental measurements and theoretical calculations at hitherto unreachable accuracy.

W. Paufler
High-Harmonic Generation with Laguerre-Gaussian Beams
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2020)

Abstract: High-harmonic generation is a versatile process, for one thing, useful to explore the structure of atoms or molecules during the generation itself and apart from that a source of bright, short, coherent extreme ultraviolet radiation. Thereby the harmonic radiation can be controlled by the shape of the driving laser with respect to its polarization or frequencies. Recent advances show that Laguerre-Gaussian beams, which carry in addition to their spin also orbital angular momentum, can be utilized for high-harmonic generation. In this thesis, we analyze high-harmonic generation with Laguerre-Gaussian beams in the framework of the strong-field approximation and show that this requires both the interaction of a single atom with the driving laser and the macroscopic superposition of all single atom contributions. We first investigate high-harmonic generation with linearly polarized Laguerre-Gaussian beams. There, we show how the orbital angular momentum of the driving laser is transferred to the generated harmonics. Here, we developed vivid photon diagrams to explain the conservation of orbital angular momentum. We then consider phase matching of the generated radiation in order to increase the conversion efficiency. In particular, we analyze the coherence length at different positions in the generating beam. Furthermore, we investigate high-harmonic generation with a pair of counter-rotating circularly polarized Laguerre-Gaussian beams. Here, we derive selection rules that take account of the conservation of energy, spin and orbital angular momentum. In addition, we show that the orbital angular momentum of the generated harmonics can be precisely controlled by the orbital angular momentum of the driving beam.

I. Tamer
Petawatt-Class Laser Optimization and Ultrashort Probe Pulse Generation for Relativistic Laser-Plasma Interactions
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2020)

Abstract: Advancements in high peak power laser development have resulted in laser systems capable of accelerating charged particles in a plasma to nearly the speed of light. For a comprehensive understanding and optimization of such interactions towards higher experimental yields, further enhancements in the laser system performance are required, along with a method that enables a direct view into the laser-induced plasma with a high spatial and temporal resolution. The work presented in this thesis details the results of multiple investigations regarding upgrades to the petawatt-class POLARIS laser and the development of a multi-beam ultrashort laser system for probing relativistic laser-plasma interactions at Friedrich Schiller University and Helmholtz Institute in Jena, Germany.
As laser pulse intensities are improved worldwide, the spatial, temporal, and temporal intensity contrast profiles of the pulses become increasingly crucial to the experimental performance and future scalability of the laser system. Where possible, an optimization of these parameters should be accomplished using simple, robust methods to avoid large-scale changes to the operational petawatt-class system. To improve the fluence homogeneity of the POLARIS laser pulse, a comprehensive spatio-temporal model of the pump-induced wavefront aberrations was constructed and the results of the verified model were applied to correct the heavily aberrated amplified beam profile in a joule-class multi-pass amplifier through a precise adjustment the pump distribution. Furthermore, the pulse duration post-CPA could be further compressed by a factor of 3 after near field SPM in a highly nonlinear material. In parallel to the spatial and temporal profile improvements, the temporal intensity contrast of the POLARIS laser pulse was enhanced 1000-fold using a plasma mirror.
An insight into the complex dynamics of relativistic laser-plasma interactions produced by the enhanced POLARIS laser can be achieved by employing an additional ultrashort laser system as an optical probe. For this purpose, a multi-beam ultrashort optical probing system, seeded by the POLARIS oscillator and pumped by a dedicated Yb:FP15-based CPA system, has been developed and installed within the petawatt-class laser system. The probing setup simultaneously offers two millijoule-level, nearly 100 fs laser pulses, along with a few-cycle laser pulse for high precision optical probing. Here, noncollinear optical parametric amplification (NOPA) is utilized to generate 20 µJ, 230 nm FWHM bandwidth pulses centered at 820 nm. The nonlinear BBO crystal is employed not only as the gain medium, but also as the pulse compressor, delivering near-FTL 11 fs pulses in a setup smaller than 40 cm × 40 cm. The temporal synchronization of the ultrashort probe pulses with the main POLARIS pulse are characterized using a live diagnostic system that monitors several orders of magnitude of delay. With the enhanced petawatt-class laser pulse, now equipped with a few-cycle optical probe, the intricate details of relativistic laser-plasma interactions can be revealed at the POLARIS laser system.


B. Lei
High energy radiation from compact plasma-based sources
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2019)

Abstract: Throughout the current century, compact, high-energy radiation sources have become critically important for many advanced applications in medicine, industry, education, and scientific research. In contrast to conventional radiation sources mainly produced in huge facilities, plasma-based radiation sources with centimetre lengths can provide great flexibility and drive science forward.
In this thesis, several plasma wakefield-based undulator schemes have been developed in parallel. First, the guiding of laser beams, including a single Gaussian pulse, Hermite-Gaussian (HG) modes, and Laguerre-Gaussian (LG) modes, is studied through the Schrödinger-like wave equation for a harmonic oscillator with paraxial and quasistatic approximations in a parabolic plasma density channel. If the laser pulse is injected into the plasma channel with a transverse offset or an angle with respect to the propagation axis, it will undergo centroid oscillation. Special conditions are found to control the interesting properties of such oscillation: frequency, amplitude, and polarisation.
Second, wakefield excitation driven by the oscillating laser pulse is theoretically and numerically studied in the linear/nonlinear regime. The specific field structure of each scheme is demonstrated. For a short, wide laser pulse, the wakefield provides a linear focusing force near the propagation axis that drives the betatron oscillation of the injected electrons. The extra driving force is introduced by the centroid oscillation of the laser pulse. Surprisingly, the undulator field generated by beating several different HG modes becomes monochromatically sinusoidal when the strength of laser pulses matches a special condition. This is very beneficial for the generation of a narrow radiation spectrum.
Third, the dynamics of both a single electron and an electron beam are studied in these generated undulator fields. Generally, an electron undergoes the combined motion of betatron and undulator oscillations. However, the weak betatron oscillation could be totally removed if certain injection conditions for an electron can be satisfied. Further theoretical work on the dynamics of an accelerated electron indicates that there is a resonance between the betatron oscillation of the electrons and centroid oscillation of the laser pulse. This resonance can be used to increase the oscillation amplitude and strength for the electron rapidly within the first several Rayleigh lengths of propagation. While being accelerated in the wakefield, the resonance is broken and results in a semi-steady oscillation with large amplitude and strength, which enables the generation of strong γ-ray radiation.
Ultimately, the radiation spectrum from the oscillation of an electron beam is calculated. The proposed schemes are capable of generating an x-ray radiation spectrum with a narrow bandwidth or synchrotron-like x/γ-ray radiation of high energy. The energy and brightness are comparable with currently available conventional radiation sources. It is also demonstrated that these flexible schemes can be tuned to generate radiation carrying well-defined angular momentum.

M. Bilal
High precision many-electron calculations for multiply-charged ions
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2019)

Abstract: Recent advances in measurements/observations have made it possible to test small and minute fundamental physical eff ects for transition rates and line strengths in many-electron atomic systems with unprecedented accuracies. This thesis provides high-precision calculations of line strengths and lifetimes for diff erent atomic systems where we accurately account for various higher-order eff ects. In all these systems, systematically enlarged multiconfi guration Dirac-Hartree-Fock (MCDHF) wave functions are employed for calculation of the atomic states involved in the transitions to account for the relativistic correlation corrections.
Firstly, the QED sensitive magnetic dipole (M1) line strengths between the fi ne-structure levels of the ground confi gurations in B-, F-, Al- and Cl-like ions are calculated for the four elements argon, iron, molybdenum and tungsten. For these transitions, in addition to relativistic correlation corrections, the QED corrections are evaluated to all orders in αZ utilizing an eff ective potential approach. As a result, our calculations have reached an accuracy of 10−4 for the M1 line strengths. These accurate theoretical predictions provide the prerequisite for a test of QED by lifetime measurements at diff erent frequencies and timescales. This will help to find a reason for the present discrepancies between theory and experiment for B-like Ar and Al-like Fe.
Secondly, the line strength of the 1s 2 2s2p 1 P 1 – 1s 2 2s 2 1 S 0 spin allowed E1 transition in Be-like carbon is calculated. For this highly correlated transition, different correlation models are developed to account for all major electron-electron correlation contributions. The fi nite nuclear mass eff ect is accurately calculated taking into account the energy, wave functions as well as operator contributions. As a result, a reliable theoretical benchmark of E1 line strength with a relative accuracy of 1.5×10−4 is provided to support high precision lifetime measurement at GSI Darmstadt for the 1s 2 2s2p 1 P 1 state in Be-like carbon.
Finally, large-scale calculations are performed for all allowed (E1) and forbidden (M1, E2, M2) transitions among the fi ne structure levels of the 3s 2 3p 5 , 3s3p 6 and 3s 2 3p 4 3d confi gurations for Ni XII. Here, we validate all recently identifi ed tentative experimental lines with one exception. Moreover, we present ab initio lifetimes that are better than previously reported ab initio and semi-empirical values as compared to available experimental data. Thus, we provide reliable predictions in the prospects of future experiments.

M. Mäusezahl
Untersuchung lasergetriebener Protonenbeschleunigung bezüglich Vorplasmaerzeugung und räumlicher Protonendetektion
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2019)

Abstract: Die lasergetriebene Beschleunigung von Protonen mittels TNSA hat ein erhebliches Potential, die physikalische Grundlagenforschung um ein weiteres Instrument zur Untersuchung hochenergetischer Wechselwirkungen zu ergänzen. Um die erreichten Protonenenergien und die Stabilität für derartige Anwendungen weiter zu steigern, ist ein grundlegendes Verständnis der innerhalb weniger Pikosekunden ablaufenden Prozesse nötig. Im Rahmen dieser Masterarbeit wurde ein Teil der Diagnostik für die Entstehung solcher Protonenstrahlen untersucht. Dadurch stehen in Zukunft weitere Instrumente zur Charakterisierung von Protonen am POLARIS-System zur Verfügung.

G. Tadesse
Nanoscale Coherent Diffractive Imaging using High-harmonic XUV Sources
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2019)

Abstract: Imaging using sources in the XUV and X-ray spectral range combines high resolution with longer penetration depth (compared to electron/ion microscopy) and found applications in many areas of science and technology. Coherent diffractive imaging (CDI) techniques, in addition, lift the performance limitation of conventional XUV/X-ray microscopes imposed by image forming optics and enable diffraction limited resolutions. Until recently, CDI techniques were mainly confined to large scale facilities e.g. synchrotrons and X-ray free electron lasers due to unavailability of suitable table-top XUV/X-ray sources. Tabletop sources based on high-order harmonic generation (HHG) nowadays offer high and coherent photon flux which widened the accessibility of CDI techniques. First imaging experiments already showed the potential of HHG-based setups albeit with limited resolution on features much larger than the illuminating wavelength. So far, table-top CDI systems were not able to resolve sub-100 nm features using performance metrics that can qualify these systems for real world applications. The huge progress in scaling the coherent flux of HHG sources driven by high power femtosecond fiber laser systems presented unique opportunities for reaching new regimes in imaging performance. However, experimental issues with power handling and the onset of so-far-unexplored resolution limits for wavelength-scale features were some of the challenges that needed to be addressed. In this work, CDI experiments with the highest resolutions in different modalities using a highnflux fiber laser driven HHG source are presented. In conventional CDI, a record-high resolution of 13 nm is demonstrated together with the possibility of high speed acquisition with sub-30 nm resolution. In a holographic implementation of CDI, features with a half-distance of 23 nm are resolved which are the smallest features to ever be resolved with a table-top XUV/X-ray imaging system. In addition, waveguiding effects are shown to affect image quality and limit the achievable resolution in these wavelength-sized features. Ptychographic imaging of extended samples is also performed using a reliable Rayleigh-like resolution metric and resolving of features as small as 2.5 λ (sub-50 nm) is demonstrated. Together with the significant reduction in measurement times, the imaging results presented push the performance of table-top CDI systems a step closer to that required for real world applications. The scalability of the HHG flux at higher photon energies (soft X-rays) with the power of the driving fiber laser system promises to deliver imaging setups with few nanometer resolutions in the near future. These systems can find applications in material and biological sciences, study of ultrafast dynamics, imaging of semiconductor structures and EUV lithographic mask inspection.

D. Würzler
Untersuchung und Simulation der Ionisations- und Streudynamik von Photoelektronen mithilfe von Zwei-Farben-Feldern
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2019)

Abstract: If atoms or molecules are exposed to strong laser fields, various processes can occur after ionization, and the dynamics of these processes depend on the trajectory of the emitted electrons. Both the ionization rates and the electrons trajectory depend strongly on the shape of the laser field. Thus, tailoring strong laser fields on the sub-cycle and sub-femtosecond time scale, the insight and control of the underlying dynamics of these processes has been significantly increased in the last two decades. Here, orthogonal and parallel two-color laser fields represent an effective approach to manipulate the ionization rates and the subsequent electron movement in the laser-dressed continuum. This is achieved by varying the relative phase, ϕrel , between both field components ( ω and 2 ω ). In this thesis orthogonal and parallel two-color laser fields are used to study the ionization and scattering dynamics of noble gases. Further, phases-dependent photoelectron spectra- captured by a velocity map imaging spectrometer, are studied by applying the recently introduced phase-of-the-phase analysis [1].
The measured results are compared with three dimensional semi-classical calculations, which can be performed for arbitrarily polarized laser fields, while taking higher order scattering events into account. These simulation also allows for the separation and investigation of different classes of photoelectrons (e.q. direct and scattered electrons), which alows for analysis of the underlying dynamics.
In one vmi measurement in this thesis, an orthogonal two-color laser field ( λω = 800 nm, λ2ω = 400 nm)with an unconventional orientation, i.e. with the polarization of the ionizing laser field perpendicular to the detector surface and the steering field parallel to it, is used. This allows for the investigation of the phase-dependent photoelectron spectra, as the deflections of photoelectrons due to the 2 ω -field are directly mapped onto the detector. The phase dependence of the photoelectron spectra of neon and xenon shows clear phase shifts between scattered and direct electrons. When comparing the phase dependency of neon and xenon, a strong target dependency is observed. Namely xenon show vastly more complex phase dependence then neon. Further investigations of xenon where perfomed using parallel two-color field within the short-wave infrared range ( λω = 1800 nm, λ2ω = 900 nm). To measure electrons with high energy, which are created during ionization with these long wavelengths, a high-energy VMI spectrometer was developed based on the design presented in [2]. Using this device, electron energies up to 320 eV can be detected. The intention of this measurement is to retrieve the ionization time of the photoelectrons contributing to the characteristic fork structure [3] based on the phase dependencies of the contributing photoelectrons. Using these wavelengths, the fork structure can be easily detected and provides a well-suited benchmark for this study. Based on the semi-classical model it is shown that phase-dependent photoelectron signal, which encodes information about the contributing ionization times, is convoluted with the phase dependencies resulting from perturbation of the electron trajectories propagating in the laser-dressed continuum. Independent on the degree of the perturbation this can mislead assignment of the ionization time by up to 80 as.

R. A. Müller
Investigation of Atomic Nuclei via Electronic Processes
Technische Universität Carolo-Wilhelmina zu Braunschweig, Fakultät für Elektrotechnik, Informationstechnik, Physik (2019)

Abstract: In atomic physics, nuclei are often described as a point-like charges with an infinite mass that binds the electrons. With more and more precise experimental techniques, however, this approximation is no longer sufficient and it is necessary to develop a better theoretical understanding of the ways atomic nuclei interact with the electron shell. We do observe for example small shifts in the lines of spectra of different isotopes of the same atomic species. In this thesis, we present calculations for these isotope shifts and use them to derive the difference between the nuclear charge radii of two thorium isotopes, ²³²Th and ²²⁹Th as well as ²²⁹Th and the isomeric state ²²⁹mTh. These results are of particular interest for the development of a future
nuclear clock and coherent high-energy light sources. Moreover, we discuss precise isotope shift calculations for singly charged barium and compare them with a recent experiment. We motivate the relevance of such studies for the search for physics beyond the Standard Model.
Spectral lines, however, do not only shift but also split due to the non-point-like nature of atomic nuclei. From the spectroscopy of this hyperfine splitting, it is possible to extract the multipole moments of the nuclear electromagnetic field. As a part of this thesis, we present the first value of the nuclear magnetic dipole moment of the ²²⁹mTh nuclear isomer that does not rely on previous calculations or measurements.
Having extracted several important properties of the ²²⁹Th nucleus and the isomer ²²⁹mTh using atomic theory we invert our view in the second part of this thesis. Namely, we want to use processes in the electron shell to populate the ²²⁹mTh isomeric state. Preparatory to our calculations for the actual excitation of the isomer, we discuss the atomic structure of thorium. Of particular experimental interest is the level structure of singly charged thorium. In a recent study, we show the results of atomic structure calculations that help to interpret measured thorium spectra and can be used to estimate the probability of a nuclear excitation via the electron shell in this system. A deeper and more accurate discussion is performed for the comparably simple triply charged thorium ion. This study helps to test the various approximations necessary to discuss systems with a more complicated electronic structure. Bringing everything together the final publication presented in this thesis proposes an experimental setup to excite the ²²⁹Th nucleus in a controlled way depending on the yet to be found energy of the nuclear isomeric state. This method is currently applied in an experiment at the German National Metrology Institute.

R. Beerwerth
Electron Correlation in Relativistic Multiconfiguration Calculations of Isotope Shift Parameters, Hyperfine Coupling Constants and Atomic Processes
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2019)

Abstract: Electron correlation denotes the corrections to central field approximations applied in Hartree--Fock methods that arise from the electron-electron interaction. As a consequence, wave functions for atomic states are represented as a mixture of different electronic configurations. Corresponding highly correlated multiconfiguration wave functions allow precise computations of atomic parameters such as energy levels, transition rates, isotope shift parameters and hyperfine coupling constants.
In this work, multiconfiguration Dirac–Hartree–Fock computations are utilized to compute precise isotope shift parameters and hyperfine coupling constants for actinium, nobelium and iron. As a prerequisite, extensive computations of the atomic level structure for actinium were performed to assign the computed energies to measured transitions, and as a consequence several unknown levels are predicted. In order to estimate uncertainties of the computed results, systematically enlarged configuration spaces are utilized and the results of several model computations that probe different correlation effects are compared.
Furthermore, electron correlation is crucial to describe higher order processes such as shake transitions that accompany photoionization or Auger processes. These processes are in addition caused by the non-orthogonality of the single electron orbitals obtained in Hartree–Fock computations. The latter can be transformed into electronic correlation by a biorthonormal transformation and we evaluate its application to the efficient computation of Auger transition rates. With this approach, large scale calculations for complex atoms with multiple open shells can be extended to include shake transitions. These transition rates are utilized in Auger cascade models that describe the ionization or excitation of core electrons from atoms or ions into highly excited states and the subsequent decay of these inner-shell holes by the emission of a cascade of Auger electrons.

D. Hoff
Elektronendynamik in fokussierten Einzelzyklenpulsen
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2019)

Abstract: This work investigates light-driven electron re-scattering from atomic gases and metal nanotips in focused few-cycle laser pulses. In particular, the work concentrates on the investigation of the evolution of the electric field of few-cycle pulses during focussing. Electrons emitted from a Tungsten nanotip are used to probe the electric field. With this insight the differences between the noble gas Xenon and nanotips made of Tungsten and Gold can be understood.
To measure such fast processes, ultra-short laser pulses consisting of merely a few optical cycles (<2) are employed. When dealing with pulses as short as this, the relative position between the optical carrier wave and envelope becomes important. This value is called the carrier-envelope phase and is responsible for how the re-scattering takes place. Having control over this phase means being able to control the re-scattering process. As determining this value at the site of interaction is extremely difficult, measurements have been almost exclusively determining the “relative” carrier-envelope phase dependence, i.e. the effects of the change in carrier-envelope phase without an absolute reference. As examination of the phenomena investigated herein requires a knowledge of the “absolute” carrier-envelope phase, a method for determining this value is proposed and implemented. To this end, the phase dependencies of the photo-electron spectra of Xenon are compared to those of atomic Hydrogen, which can in turn be calibrated with ab initio calculations. This insight makes it possible to use the relatively easy determination of the carrier-envelope phase dependence of Xe-spectra as a ruler in other measurements. For instance, further photo electron spectra of Argon and Krypton are shown.
Because the carrier-envelope phase shifts through the focus it is necessary to know these changes in order to understand local interactions. The metal nanotip, being an extremely localized electron emitter, serves splendidly as a tool to quantify the focussing of the electric field of few-cycle pulses. For the first time the carrier-envelope phase of a wide range of the focus, both on and off axis, was scanned without complications from volume averaging. Significant deviations from the often assumed arcustangent-shaped evolution described previously by Gouy on the optical axis for the monochromatic case were observed. The behaviour is well reproduced with an analytic model calculated by Porras and can be drawn back to the spectral geometry of the laser beam, which can be easily accessed experimentally and used for a coarse estimation of the focusing properties. The insight into the relationship between input beam properties and focussing behaviour allows for better interpretation and design of light-matter interactions in the future.
Here, this technique is utilised to compare the absolute carrier-envelope phase dependence of electron re-scattering at metal nanotips, i.e. Tungsten and Gold, and in Noble gasses. We find that the observed shift can be attributed to the shape of the ionization potential of the different species and that in case of the nanotips the optical near-field due to the geometry of the tip causes an additional phase shift.

P. Luckner
Entwicklung, Aufbau und Charakterisierung eines optischen, hochgenauen Target-Positioniersystems
Ernst-Abbe-Hochschule Jena, Fachbereich Feinwerktechnik (2019)

Abstract: Die Bachelorarbeit wurde am Institut für Optik und Quantenelektronik Jena erstellt. Die Arbeitsgruppe der relativistischen Laserphysik untersucht die Wechselwirkung hochintensiver Laserstrahlung mit Materie. Eines der aktuellen Projekte ist der Aufbau, die Entwicklung und die Anwendung des POLARIS-Lasersystems. POLARIS steht für Petawatt Optical Laser Amplifier for Radiation Intensive ExperimentS und ist das derzeit leistungsstärkste, vollständig dioden-gepumpte Hochleistungslasersystem der Welt mit Pulsspitzenleistungen von bis zu 170 TW. Hintergrund des POLARIS-Projektes ist zum einen die Entwicklung von dioden- gepumpten Lasersystemen und zum anderen die Untersuchung von lasergetriebenen Beschleunigungsmechanismen. Ziel der Bachelorarbeit ist die Entwicklung, der Aufbau und die Charakterisierung eines hochgenauen optischen Target-Positioniersystems für das Hochleistungslasersystem POLARIS. Aufgrund der sehr kleinen Fokusgröße, ist eine hochgenaue Positionierung der Targets notwendig. Das Target soll somit möglichst präzise innerhalb der Rayleigh-Länge des POLARIS Lasers positioniert werden. Hierfür wird das Target mit einem Laser-basierten optischen Aufbau vermessen. Momentan erfolgt das Vermessen der Targets noch manuell durch einen Mitarbeiter, der vor jedem Experiment ca. zwei Stunden für diesen Vorgang benötigt. Um diesen Prozess nicht nur deutlich schneller, sondern auch genauer zu gestalten, soll dieser weitestgehend automatisiert werden. Zunächst erfolgt in Kapitel 2 eine kurze Einführung in die Grundlagen des POLARIS Lasers und es werden verschiedene Methoden der Positionsbestimmung und Bildverarbeitung diskutiert. In Kapitel 3 wird der optische Versuchsaufbau charakterisiert. Hierbei liegt der Schwerpunkt auf der Target-Positionierung und dem Weglängenmesssystem. In Kapitel 4 wird das herausgearbeitete Konzept zum Autofokussystem näher erläutert und aufgetretene Probleme analysiert. Anschließend erfolgt die Umsetzung der Ansätze, wo das Autofokussystem auf seine Genauigkeit und Reproduzierbarkeit überprüft wird. In Kapitel 5 werden schließlich die Ergebnisse diskutiert und ein kurzer Ausblick gegeben. Die Idee ist, dass das Target - nach Eingabe weniger Parameter - vermessen und anschließend nach jedem Schuss positioniert werden soll. Hierzu wird über den selbst entwickelten Auto-Fokus eine Referenzstelle für den Laserfokus auf dem Target scharf gestellt und die zu beschießenden Stellen mit einem konfokal-chromatischen Sensor entlang der optischen Achse vermessen.

Z. Samsonova
Relativistic interaction of ultra-short laser pulses with nanostructured solids
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2019)

Abstract: Relativistic interaction of ultra-intense laser pulses with nanostructured solids is widely considered to be one of the most promising directions for research in high energy density physics. This thesis investigates the influence of the target morphology on the plasma parameters and produced hard X-ray emission. The study is rather broad and covering a range of emerging applications such as a development of efficient X-ray sources and generation of the extreme states of matter for laboratory astrophysics.
We have performed a sequence of experimental campaigns starting from a benchmark experiment at moderate laser intensities and continuing with measurements at relativistic intensities (Iλ^2 ≥1.3 × 10^18 Wcm−2μm2). A set of fundamental questions regarding the laser energy absorption and morphology dependent plasma dynamics were addressed. Measurements of the bremsstrahlung emission and K-shell emission helped to draw some very important conclusions. First of all, nanowire targets are impractical for the generation of the cold line emission since they demonstrate essentially the same photon flux as the flat targets. However, according to the detected emission from the highly charged ion states (He- and H-like), nanowire morphology enables an effective generation of hot dense plasmas. Spectroscopic analysis of the produced X-ray emission, as the main diagnostic tool, revealed keV temperatures and solid density (≥10^23 cm−3) plasmas. In fact, such plasmas can be generated also with a planar target, however only in a thin top layer since the laser cannot deposit energy deeper. The use of NW arrays, on the other hand, increases the laser energy absorption and the interaction volume, resulting in an effective plasma heating, which does not take place for the flat targets. We have also experimentally observed higher flux and higher energies of the ions accelerated away from the front surface of the target matching with the other observations.
The experimental results were supported by numerical simulations. For the chosencases, we have synthesized X-ray line spectra using the plasma parameters provided by the Particle-in-Cell (PIC) and Hydrodynamic (HD) simulations. A good correlation between the measured and synthetic spectra has been achieved. The plasma dynamics for the case of flat and nanostructured solids is strikingly different. For hot high-density plasmas, the collisional rates (e.g., ionization, excitation) are high and, therefore, radiative cooling of the plasmas may overrun hydrodynamic cooling, as it happens for nanowire targets. This naturally causes a great increase in the X-ray yield.
The response of the flat and nanowire targets was investigated in the interaction with short- and long-wavelength laser pulses (0.4 μm and 3.9 μm), corresponding to completely different regimes of interaction. While ultra-short laser pulses in UV, visible and near-infrared are commonly used in laser-induced plasma studies, femtosecond mid-infrared pulses have not been yet extensively applied. In this thesis, we highlight the potential of such long-wavelength drivers to generate hot and dense plasmas. We demonstrate that this becomes feasible only with nanowire targets.

P. Wustelt
Atome und Moleküle fundamentaler Bedeutung in intensiven Laserfeldern: He, He+ und HeH+
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2019)

Abstract: This work focuses the control of fundamental single- and two-electron systems using intense, ultra-short laser fields and includes new measurements, novel data evaluation techniques, and interpretation using various theoretical techniques. The measurements were carried out using an ion-beam apparatus that produces a beam of atomic or molecular ions, which is exposed to the controlling laser pulses causing fragmentation and/or ionization. The three-dimensional momenta of these fragments are then detected in coincidence, which allows for reconstruction of the interaction dynamics.

In this thesis, to understand the fundamental timing of the laser-induced electron tunneling, the attoclock method was applied to the helium ion, a single-electron system with twice the charge of hydrogen. This serves to test and refine models of tunneling ionization and the larger intensity required for ionization enables the investigation of the tunneling process close to the ideal case - in the quasi-static tunnel regime. Evaluation of the measured electron-emission angle as a function of the radial momentum for He+ is significantly smaller than for, the typically used, atoms with lower ionization potential. Moreover, using He+ results in a much lower Keldysh parameter, which significantly reduces the importance of nonadiabatic effects that can complicate interpretation. The results are in good agreement with TDSE solutions as well as semiclassical simulations that do not include tunneling times.

Further, double ionization of the helium atom by nearly circularly polarized few-cycle laser pulses was investigated. The dependence of the sequential double ionization on the subcycle shape of the ionizing few-cycle laser field was demonstrated by comparing measured ion momentum distributions with classical Monte Carlo simulations. Simulations based on a purely sequential ionization model show a remarkable good agreement with the experimental observations and reproduce the characteristic 6-peak structure of the measured ion momentum distribution after double ionization with few-cycle laser pulses.

In addition to laser-induced ionization of fundamental atomic systems with strong laser fields, in this work the first experimental investigation of the simplest asymmetric molecule, the helium hydride ion, in strong laser fields was performed. Helium hydride is only stable as an ion and, therefore, an ion beam apparatus is required for its investigation. This study focused on how the asymmetric structure, and the resulting permanent dipole moment of the HeH+, influence laser-induced fragmentation. Both experiment and theory for dissociation, single ionization and double ionization of HeH+ and the isotopologue HeD+ reveal, that for the asymmetric molecule, direct vibrational excitation, with almost no electronic excitation as the initial process, dictates the fragmentation process. The dynamics of this extremely asymmetric molecule contrasts the symmetric molecules and gives new and fundamental insights into the behavior of molecular systems in general.

E. Menz
A Scintillation Particle Detector for Recombination Experiments at CRYRING@ESR
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2019)

Abstract: The following work describes the implementation of a single-particle detector based on a YAP:Ce scintillation crystal at the CRYRING heavy-ion storage ring at GSI. YAP:Ce is a durable and non-hygroscopic crystal that is bakeable to a certain degree and is thus suitable for installation directly in the ultra-high vacuum of the storage ring. The photons produced by the scintillator are detected by a photomultiplier tube. The detector is located downstream from a dipole magnet and is used to detect reaction products that undergo a change of their charge-to-mass ratio in the preceding straight section of the ring which houses the electron cooler. This positioning facilitates a number of applications for the setup that include the observation of beam losses both from interaction with residual gas atoms and molecules and with electrons in the cooler section. It can also be used for future recombination studies in the cooler section, providing detailed insight into the atomic structure of highly charged ions. The detector has been assembled and installed at CRYRING and was used during two beamtimes in August and November of 2018 to test its functionality and gather first experimental data. During these tests a number of issues concerning the detector itself and the signal read-out were identified and solved and the setup demonstrated its suitability for detecting single ions even at low energies of ∼300 keV. Moreover for the November beamtime a data acquisition system was implemented and tested.

H. Bernhardt
Hochpräzise Röntgenpolarimetrie mit Diamantkristallen
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2019)

Abstract: The dissertation describes the development and application of several diamond crystal x-ray polarizers. The polarizers are based on the channel-cut principle, in which an X-ray beam is diffracted several times under a Bragg angle of 45° and linearly polarized. The diamond crystals were characterized and the effect of defects (dislocations and stacking faults) on X-ray polarimetry were investigated. Since the diamonds were unsuitable for the fabrication of monolithic channel-cut crystals, special quasi-channel cuts (QCC's) out of invar alloy and mirror mounts were developed. With these QCC's up to four diamonds could be adjusted parallel to each other with a precision of sub-μrad. These diamond QCC’s were used in experiments at the European synchrotron in Grenobel, where an unprecedented polarization purity of 1.3 x 10^(-10) was achieved. As a further result, it was proved that the polarization purity is limited by the divergence of the synchrotron and that a better purity can be measured with reduced divergence. Thus, even better polarization purity can be achieved at x-ray sources with lower divergence, e.g. Synchrotron 4th generation and X-ray lasers. This is an important result for the measurement of vacuum birefringence in future. Al in al the dissertation shows that even diamond crystals with dislocation densities in the range of 10^4 to 10^6 cm^-2 are suitable for high-precision X-ray polarimetry and the production of highly pure linear polarized X-ray beams.


B. Arndt
Time-of-flight Measurements at HILITE
Johann Wolfgang Goethe-Universität Frankfurt, Fachbereich Physik (2018)

Abstract: -

A. Massinger
Aufbau und Charakterisierung eines zeitaufgelösten 2D Plasma Anrege-Abfrage-Systems
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2018)

Abstract: Im Rahmen dieser Arbeit wurde ein Anrege-Abfrage-System mit zwei Probepulsen entwickelt. Mithilfe dieses Systems kann ein Plasma, das dem Vorplasma des Polaris-Lasers gleicht, erzeugt und untersucht werden. Das Vorplasma besitzt einen wichtigen Einfluss auf die Effizienz der TNSA Laser- Protonenbeschleunigung. Da das diese Prozesse auf sehr kurzen Zeitskalen von ca. 1 ps stattfinden, muss der Aufbau eine vergleichbare zeitliche Auflösung bieten. Dies ist Elektronisch nicht möglich. Dafür wurde eine rein optisches System zur zeitlichen Separation eines Pulses in mehrere Einzelpuse entwickelt, das Pulse mit einer Pulsdauer von 400 fs und einen zeitlichen Versatz zwischen 0 ps und 333 ps mit einer Genauigkeit von 67ps erzeugt.

S. Fuchs
Optische Kohärenztomographie mit extrem ultravioletter Strahlung
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2018)

Abstract: In this thesis, the concept and the realization of laboratory-based optical coherence tomography in the extreme ultraviolet (XUV) spectral range is presented. XUV coherence tomography (XCT) is a three-dimensional imaging technique with an axial resolution down to a few nanometer. A theoretical XCT model has been developed for the reconstruction of the sample structure, which includes the interaction between the XUV light and the sample. It is valid for absorbing samples illuminated under arbitrary angles of incidence and thus extends a common model of optical coherence tomography (OCT). As the information about the absorption and dispersion of the sample is contained in the XCT model, an additional reconstruction of material properties of the sample will be enabled.
The demonstration of laboratory-based XCT, which before has only been implemented at synchrotron facilities, was a major gaol of this thesis. Using high harmonic generation (HHG) of a femtosecond infrared laser pulse, a broadband laboratory-based XUV source with sufficient photon flux (approximately 0,2 nW/eV over the full bandwidth) in the so-called silicon transmission window between 30 eV − 100 eV was realized. A revised XCT microscope has been designed, constructed and adapted to the new laser-based XUV source, which routinely facilitates XCT measurements in the laboratory. The microscope is a three meter long vacuum beamline consisting of XUV source, focusing mirror, and sample chamber.
A comparison between laser-based and synchrotron-based measurements shows good agreement. With laser-based XCT, an axial resolution of approximately 30 nm has been achieved. This is comparable to the achieved synchrotron-based axial resolution of approximately 20 nm. Accordingly, the axial resolution of XCT is 2-3 orders of magnitude higher than in conventional OCT.
Unlike conventional OCT, the realized XCT setup does not use a beamsplitter for the generation of a reference wave. Instead, the surface of the sample serves as a reference. Therefore, the interferometric stability is intrinsically achieved and simplifies the experimental setup significantly. However, such a setup has the disadvantage that the reconstruction is ambiguous, since autocorrelation artifacts appear. A non-ambiguous reconstruction of the axial structure was so far not possible. In this thesis, a novel one-dimensional phase-retrieval algorithm is presented, which is able to remove the artifacts from the signal and allows a non-ambiguous reconstruction of the structure. Three-dimensional structured silicon-based samples have been investigated and processed with the new algorithm, which is referred to as PR-XCT. With the removal of artifacts and thus the possibility to use XCT on samples, whose inner structure is unknown before the measurement, a further goal of this thesis was achieved.
In fact, during laser-based PR-XCT measurements, an unexpected nanometer-thin layer was found inside the sample, which was not intentionally planned in the production process. The existence of this layer and thus the XCT measurement could only be confirmed by a transmission electron microscope. To this end, a thin slice was cut out of the sample, which was thus destroyed. The resolution of a scanning electron microscope was not high enough to resolve the layer. Later it turned out, that the vacuum chamber was vented for a short amount of time during the production process and a 1-2 nm layer of SiO2 was formed. Hereby, a striking advantage of XUV microscopy becomes apparent. Lighter elements like oxygen produce a high contrast in the XUV albeit they are almost indistinguishable from surrounding light elements like silicon in an electron microscope.
In this work, XCT is realized using optics with low numerical aperture (NA) since the fabrication of high NA optics in the XUV is technically extremely demanding. Therefore, the lateral resolution of the laboratory-based XCT setup is limited to approximately 23 μm. At least, the lateral resolution has been improved by a factor of 10 compared to the synchrotron-based measurements. However, the axial resolution of XCT is still orders of magnitudes better than the lateral resolution. Even with this technical limitation of the current XCT setup, several applications are within reach, e.g., threedimensional investigation of (multilayer-)coatings of optical mirrors or even XUV-mirrors, axial structured devices like solar cells or axial-structured semiconductor devices like graphene-based electronics. In addition, imaging of laterally homogeneous biological membranes might be possible. XCT with high numerical aperture and thus high lateral resolution could even have further applications, e.g., non-destructive three-dimensional imaging of semiconductor devices, lithographic masks, and biological structures. A combination of XCT with lensless imaging techniques like „Coherent Diffraction Imaging“ or Ptychography might be a promising approach to improve the lateral resolution of XCT. Furthermore, the intrinsic time resolution of the HHG source in the range of femto- or even attoseconds may allow time-resolved imaging of ultrafast processes in solids.

F. Irshad
Single-Shot Optical Probing of Laser-Generated Plasmas
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2018)

Abstract: TNSA process is an important means to generate energetic ion beams. The understanding of the pre-plasma is an important step towards optimizing the TNSA process. In this work, a complete system to generate and characterize different kinds of plasma was assembled. Generating two pulses and using them to probe the plasma in a single shot increases the utility of such a step since it eliminates the shot to shot variations. Different absorption mechanisms were considered while investigating the plasma and their dominance evaluated in the context of current work.

Two devices named temporal separation and spatial separation devices were used to generate the probe pulses. An imaging system to focus, collect and relay the pulses at a large distance was built and optimized to generate near diffraction limited spatial resolution (≈3.5μm). The pulses also give a sufficient temporal resolution with 330 fs pulses to study the hydrodynamic evolution of the plasma. The plasma was created with pulses ranging in intensity from 0.67E16 to 3E16 W/cm^2 with a pulse duration of 120 fs at a central wavelength of 1.03 μm. An intensity as well as a time scan was done to evaluate the plasma based on the scale lengths and plasma electron temperature. Both linear and circular polarization of pump pulse was used to create the plasma. A custom LABVIEW program was used to analyze the phase and generate scale lengths from it by Fourier transform. To gain access to the 3-D information, a cylindrical symmetry was assumed, and Abel inversion was applied on the 2-D chord phase integrals. From this, plasma scale lengths were calculated, and utilizing the single-shot pulses at different time steps, plasma velocity and plasma electron temperatures were calculated. Both the linear and circular polarized pump pulses generated plasma scale lengths in the range of 4-10 μm with an electron temperature of 50-280 eV. This data was also compared with MULTI-fs simulation data and possible reasons of deviations discussed. Dominant absorption mechanisms identified are the Normal and Anomalous Skin Effects under normal incidence. The
similarity in the plasma scale lengths and the plasma electron temperature for both polarization implies the absence of vacuum heating and resonance absorption. This is also confirmed by underlying physics of these two absorption processes, which require an electric field component in the direction of the plasma electron gradients.

M. Reuter
Characterisation of a Laser Wakefield Accelerator with Ultra-Short Probe Pulses
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2018)

Abstract: Within the frame of this thesis, aspects of the acceleration of electrons with high-intensity laser pulses inside an underdense plasma were investigated. The basic acceleration mechanism, which is referred to as laser wakefield acceleration relies on the generation of a plasma wave by an intense laser pulse. Since the plasma wave co-propagates with the laser pulse, its longitudinally alternating electric field moves with a velocity close to the speed of light and electrons trapped in the accelerating phase of the wave can be accelerated to relativistic energies. While basic principles such as the generation of a plasma wave, the injection of electrons into the accelerating phase of the wave and limits to the acceleration process are known, the exact processes occurring during the nonlinear interaction of laser pulse and plasma wave still need to be explored in more detail. The consequence of those nonlinear processes is a drastic change of the electron parameters – e.g. final electron energy, bandwidth and pointing – through slight changes in the initial conditions. In this context, the position in the plasma at which electrons are injected into the plasma wave plays a key role for the maximum achievable electron energy. Therefore, the injection of electrons at a defined position is a possibility to reduce shot-to-shot fluctuations and might make the electron pulses applicable, e.g. as a stable source of secondary radiation for temporally and spatially highly resolving imaging techniques. The investigation of controlled injection of electrons at an electron density transition demonstrated a correlation of electron pulse parameters such as electron energy gain and accelerated charge to the properties of the transition, and thus, might be a promising method to generate custom designed electron pulses. Nevertheless, shot-to-shot fluctuations in the electron parameters were still observed and are most likely caused by the nonlinear evolution of the laser pulse inside the plasma. To further reduce instabilities, deeper insight into these nonlinear processes is required and hence, a method to observe the plasma wave and the laser pulse. Combining an ultra short probe pulse with a highly resolving imaging system as successfully implemented at the institute of Optics and Quantumelectronics in Jena, more light can be shed on these processes, which take place on femtosecond and micrometer scales. With that system, characteristics of the magnetic fields inextricably connected to the acceleration process could be studied in unprecedented detail. This deeper insight allowed to observe signatures of the magnetic field of the driving laser pulse for the first time, which paves the way for the indirect observation of the main laser pulse during the interaction.

A. Peshkov
Interaction of atoms with twisted light
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2018)

Abstract: Twisted photons are particles which carry a nonzero projection of the orbital angular momentum onto their propagation direction. During the last years, the interaction between twisted photons and atoms became an active area of fundamental and applied research. In the present work, we show how the “twistedness” of Bessel and Laguerre-Gauss photons may affect a number of fundamental light-matter interaction processes in comparison with the results for standard plane-wave radiation. In particular, we perform an analysis of the photoionization of hydrogen molecular ions by twisted photons. It is shown that the oscillations in the angular and energy distributions of photoelectrons are affected by the intensity profile of twisted photons. We also investigate the excitation of atoms by these twisted photons. We demonstrate here that the orbital angular momentum of light leads to the alignment or specific magnetic sublevel population of excited atoms. Apart from these studies, we explore the elastic Rayleigh scattering of twisted photons by hydrogenlike ions. Our results indicate that the “twistedness” of incident photons may significantly influence the polarization properties of scattered light.

F. Kröger
Charge State Tailoring for Relativistic Heavy Ion Beams
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2018)

Abstract: In this work charge state distributions of heavy ions have been calculated for the production of effective stripper foils for heavy ion acceleration facilities. In this context, the FAIR facility at GSI and the proposed Gamma Factory at CERN are presented, where the use of partially stripped, relativistic ions will be of special interest for upcoming experiments. To determine the charge state distribution as a function of penetration depth, various programmes have been applied, depending on the respective energy regime. For stripping scenarios in the lower energy regime, the GLOBAL code was applied, that allows to take into account up to twenty-eight projectile electrons for energies up to 2000 MeV/u. Since the GSI/FAIR facility can accelerate even low-charged uranium ions up to 2700 MeV/u, and the Gamma Factory at CERN considers a stripping scenario at 5900 MeV/u, another programme was needed. This is why for the stripping scenarios in the high energy regime, first the well-known CHARGE code was used. However, even though it can operate in the very high energy regime, it only takes into account bare, hydrogen- and heliumlike projectile charge states. To overcome this limitation, the recently developed BREIT code was verified and used for stripping scenarios in the high energy regime. As this code has no built-in treatment of the various charge-changing processes, it needs a multitude of information about the electron capture and loss cross sections as input parameters. Thus, for the calculation of charge state distributions with the BREIT code, cross sections were computed by well-tested theories and codes. The BREIT code together with the codes for the cross section computation were then applied for two studies: first for an exemplification study for the upcoming GSI/FAIR facility to show the practicability of the BREIT code together with the cross section programmes, and then for a study to find optimal stripper foils for the Gamma Factory study group at the CERN facility, in order to efficiently produce Pb⁸⁰⁺ and Pb⁸¹⁺ ions from a Pb⁵⁴⁺ beam before entering the LHC. Furthermore, experimental data of a beam time at ESR at GSI in 2016 was analysed, where a Xe⁵⁴⁺ ion beam of several MeV/u was colliding with a hydrogen gas target. The data allowed the derivation of experimental NRC cross sections, and it was shown that the predictions of the EIKONAL code are in good agreement with these cross sections in an energy range most relevant for upcoming experiments at CRYRING@GSI.

S. Kuschel
Erzeugung dichter Elektronenpulse mit Laser-Plasma-Beschleunigern für QED-Experimente in hohen Feldern
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2018)

Abstract: Quantum electrodynamics (QED) is widely considered to be one of the most accurately tested theories. Nevertheless fundamental processes such as pair production from the vacuum or the motion of the electron in extreme fields have not been measured in the laboratory to date. Their measurement requires a high intensity laser together with a high intensity electron or γ-beam, which can be produced by a high density electron bunch.

A recent development within the last two decades are plasma based accelerators. The high fields that can be sustained by a plasma are used to deliver extremely short and dense electron bunches while shrinking size and costs of the device. Importantly, they are automatically co-located with and synchronized to a high intensity laser pulse, providing an ideal basis for investigating QED in high fields.The availability of generating dense electron bunches brings new QED experiments within reach. However, the quality and stability of laser wake field accelerated (LWFA) electron beams still has to be improved to make these experiments possible. Beyond the tests of QED, the stability and quality of the electron beam is also crucial for highly demanding applications such as LWFA-driven free-electron lasers.

The first part of this thesis is devoted to the LWFA process and its improvements with a particular emphasis on improving the stability of laser plasma accelerators. It is shown that the gas dynamics on a 10 μm scale plays an important role in LWFA, which has not been fully appreciated yet. Density modulations on a 10 μm scale were measured in a gas jet using a few-cycle probe pulse. It is shown that self-injection can be triggered by these modulations. Particle-in-cell (PIC) simulations and analytical modeling confirm the experimental results. A gas cell providing a homogeneous plasma density has been developed in order to reduce self-injection. Using this gas cell, it was possible to suppress self-injection. The experiments show that self-injection was suppressed in the gas cell. Using ionization injection and the gas cell, the beam shape as well as the pointing stability were strongly improved. This finding paves the way towards self-injection free acceleration in a plasma based accelerator. It also establishes a new requirement on the homogenouity of the plasma density – not only for LWFA, but also in a broader context, for example in particle driven plasma wake field acceleration (PWFA).

In the second part of this, the possibility of focusing the ultra-short electron bunch by passive plasma lensing is studied. LWFA-beams typically have a very small source size and a divergence of the order or a few mrad, resulting in a rapid drop in electron beam density during free-space propagation. Many of the envisioned experiments, however, require intense focused electron bunches. Therefore, the concept of passive plasma lensing has been applied to ultra-short LWFA-bunches for the first time. The passive plasma lens effect was demonstrated experimentally by using a second gas target with predefined density. PIC simulations and analytical modeling describe the measured effect. Notably, the observed focusing strength of the passive plasma lens is larger compared to a conventional magnetic quadrupole lens. The analytical model predicts that the focusing strength can be further enhanced by increasing the bunch charge.


A. K. Arunchalam
Investigation of laser-plasma interactions at near-critical densities
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2017)

Abstract: During the high-intensity laser-plasma experiments conducted at the high-power laser system JETI40 at IOQ, the two qualitatively different laser side-scattering processes have been observed. The side-scattering observed during the first experiment was found to be non-symmetric in nature with respect to the laser’s propagation direction and it was estimated to occur from under-dense to quarter critical plasma densities. The scattering angle was found to gradually decrease, as the laser pulse propagates towards regions of higher densities (i.e. the gas jet centre). For increasing nozzle backing pressures, the scattering was also found to gradually change from upward to downward directions. In this thesis, this side-scattering process is shown to a consequence of the laser propagation in non-uniform plasma, where the scattering angle was found to be oriented along the direction of the plasma gradient. In the second experiment, a symmetric side-scattering process with respect to the laser’s propagation direction was observed from the intense central laser-plasma interaction region. This scattering process was found to originate from a longitudinally narrow laser-plasma interaction region and vary over +-50° with respect to the laser’s transverse direction. It was found to primarily occur in the nearcritical plasma density regime (0.09 n_c to 0.25_nc, where n_c is the plasma critical density). In contrast to the previous experiment, Raman scattering has been shown to be the cause of this symmetric scattering process, where the scattering occurs as the result of the wave vector non-alignment between the main laser pulse and the resulting plasma wave.

P. Pfäfflein
Entwicklung und Aufbau eines Teilchendetektors für erste Experimente am Ionenspeicherring CRYRING
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2017)

Abstract: This thesis describes the development of a particle counter based on a Cerium activated yttrium aluminium perovskite (YAP:Ce) scintillator. The detector is designed for charge exchange experiments at the ion storage ring CRYRING at the GSI Helmholtz Zentrum für Schwerionenforschung in Darmstadt. It will be used for charge exchange experiments. The suggested detector design was tailored for the requirements set by the desired ultra-high vacuum conditions of up to 1E-12 mbar at CRYRING in combination with a high radiation hardness against ion irradiation. The design was kept as simple as possible, offering an easy exchange of the scintillator (not limited to YAP:Ce) if necessary.
For an estimation of the detector lifetime the radiation hardness was systematically investigated for hydrogen, oxygen and iodine irradiation in the energy regime of 1–10 MeV. The measurement took place at the JULIA tandem accelerator operated by the Institute of Solid State Physics at the University of Jena. As the measurement of detector degradation the light yield was used. Values determined for the critical fluence, defined as fluence at half the initial light yield, varied from 1E15/cm2 in the case of hydrogen down to 1.7E12/cm2 for iodine irradiation.
Prior to the hardness investigation, the used photomultiplier tube (PMT) was tested for temporal drifts of the output signal and whether the signal depends on the position of illumination on the sensitive surface. To the limit of the experimental uncertainties, no such dependencies could be observed. It was concluded that the investigated PMT was well suited for the use in the experiment as well as in the particle counter.

M. Kienel
Power Scaling of Ultrashort Pulses by Spatial and Temporal Coherent Combining
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2017)

Abstract: Ytterbium-doped solid-state lasers are versatile tools for the generation of intense ultrashort pulses, which are the key for many industrial and scientific applications. The performance requirements on the driving laser have become very demanding. High pulse-peak powers and high average powers are desired at the same time, e.g. to initiate a physical process of interest while providing fast data-acquisition times. Although sophisticated state-of-the-art laser concepts have already demonstrated remarkable performance figures, their working principles hamper the simultaneous delivery of both high peak power and high average power. Coherent combination of pulses provided by an amplifier array constitutes a novel concept for scaling both the average power and the peak power. Although this technique is applicable to any laser concept, it is especially well suited for fibers due to their high single-pass gain and their reproducible, excellent beam quality. As the number of amplifier channels may become too large for the ambitious energy levels being targeted, divided-pulse amplification (DPA) – the coherent combination of a pulse burst into a single pulse – can be applied as another energy-scaling approach, which is the focus of this thesis. In this regard, the energy-scalability of DPA implementations as an extension to well established chirped-pulse amplification (CPA) is analyzed. In a first experiment, high-energy operation is demonstrated using an actively-controlled DPA implementation and challenges that occurred are discussed. Next, in a proof-of-principle experiment, the potential of merging spatial and temporal coherent combining concepts in a power- and energy-scalable architecture has been demonstrated. Furthermore, phase stabilization of actively-controlled temporal and spatio-temporal combination implementations is investigated. Based on the findings, the layout of a state-of-the-art high-power fiber-CPA system is improved and extended by eight parallel main-amplifier channels, in which bursts of up to four pulse replicas are amplified that are eventually stacked into a single pulse. With this technique < 300 fs pulses of 12 mJ pulse energy at 700 W average power have been achieved, which is an order of magnitude improvement in both energy and average power compared to the state-of-the-art at the beginning of this work.

M. Möller
Probing Strong-field Photoionization of Atoms and Diatomic Molecules with Short-wave Infrared Radiation
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2017)

Abstract: The availability of pico- and femtosecond laser pulses, which can be focused to peak intensities in the range between 10^12 and 10^16 W/cm2, allows the investigation of the interaction between atoms or diatomic molecules with strong laser fields. It has revealed fascinating phenomena such as above-threshold ionization (ATI), high energy above-threshold ionization (HATI), non-sequential ionization (NSDI), high-harmonic generation (HHG) and, most recently, frustrated tunnel ionization (FTI). Today, these characteristic strong-field phenomena are the backbone of the burgeoning field of attosecond science. Derived applications presently mature to standard techniques in the field of ultrafast atomic and molecular dynamics. Examples are HHG as table-top source of coherent extreme ultraviolet radiation with attosecond duration or the application of HATI for the characterization of few-cycle laser pulses. Although experimental and theoretical considerations have shown that using longer laser wavelength is interesting for applications as well as for fundamental aspects, primary due to technological limitations, the vast majority of measurements has been performed at laser wavelengths below 1.0 μm.
In this thesis, an optic parametric amplification laser source of intense femtosecond laser pulses with short-wave infrared (SWIR) and infrared (IR) wavelength is put to operation, characterized and compressed to intense few-cycle pulses. Further, it is applied to investigate strong-field photoionization (SFI) of atoms and diatomic molecules using two different experimental techniques for momentum spectroscopy of laser-induced fragmentation processes.
For SFI of atoms, the velocity map imaging technique is used to measure three-dimensional momentum distributions from strong-field photoionization of Xenon by strong SWIR fields with different pulse duration. Besides observation of the pulse duration dependence of characteristic features, like the low-energy structures, which are particularly pronounced in the SWIR, an eye-catching off-axis low-energy feature, called the “fork”, which appears close to right angle to the polarization axis of the laser, is investigated in detail. The corresponding modeling with an improved version of the semi-classical model, demonstrates that on- and off-axis low-energy features can be traced to rescattering between the laser-driven photoelectron and the remaining ion. They can, thus, be understood on the same footing as HATI, where the electron scatters into high energy states.
SFI of diatomic molecules is investigated using an apparatus for Ion Target Recoil Ion Momentum Spectroscopy (ITRIMS). Besides measuring intensity dependent vector momentum distributions of the protons from SFI of the hydrogen molecular ion, it is shown that momentum conservation can be used to extract the correlated electron momentum from the measured data, although the electron is not detected. The capability of having experimental access to the momenta of all fragments, i.e. two protons and one electron, enables the analysis of correlated electron-nuclear momentum distributions. Together, with a one-dimensional two-level model, this sheds light on correlated electron-nuclear ionization dynamics during SFI of diatomic molecules by SWIR fields.

J. Ullmann
Laserspektroskopie an hochgeladenen Bismutionen zum Test der Quantenelektrodynamik
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2017)

Abstract: Ths dissertation concerns a test of the theory of quantum electrodynamics in strong fields by laser spectroscopy of the ground state hyperfine splitting of highly charged bismuth ions. The experiment was performed and analyzed at the storage ring ESR at Helmholtzzentrum für Schwerionenforschung in Darmstadt. A systematic study of space charge effects was carried out and the laser wavelength measurement was verified by absorption spectroscopy of iodine. The determination of the ion velocity by an in-situ measurement of the electron cooler voltage reduced the main systematic uncertainty of the previous experiment by over an order of magnitude. This indicated the necessity to establish a permanent high voltage measurement at the electron cooler, which was promoted in this work. The measured wavelengths were combined in a specific difference which deviates significantly from the theoretical predictions. None of the investigated systematics has the magnitude to explain this deviation. Apart from doubts regarding theory, the literature value of the nuclear magnetic moment of Bismuth-209 is indicated as a possible explanation. Follow-up experiments to solve this puzzle are described in the outook.