Abschlussarbeiten

Ptychographic imaging based on HHG in the EUV spectral range enables spatial resolutions approaching the illumination wavelength while providing quantitative, label-free imaging capabilities on a laboratory scale [72, 152, 243]. Despite these advantages, the application of EUV ptychography to biological specimens remains challenging, as such samples are typically structurally fragile, highly radiation-sensitive, and weakly scattering. This dissertation presents the development, implementation, and quantitative evaluation of a compact EUV ptychographic microscope driven by a fiber-laser-based HHG source operating at a wavelength of 13.5nm (Section 3.1). A central methodological contribution is a novel purity-based approach for axial distance calibration in ptychography, which enables robust determination of the object–detector distance under partially coherent illumination and significantly improves the quantitative reliability of reconstructed images (Chapter 4). In addition, a correlative multi-wavelength imaging concept was realized to enable efficient navigation and enhanced experimental stability within a single compact microscope platform(Section 3.2). Building on these methodological advances, the developed system was successfully applied for the first time to quantitative EUV imaging of biological specimens with sub-50nm spatial resolution. Studies on eukaryotic microorganisms and bacterial model systems yielded detailed morphological and compositional information. By combining thesemeasurements with multivariate statistical analysis, quantitative nanoscale phenotyping of biological specimens was achieved. In particular, the analysis of antibiotic-induced structural changes in Bacillus subtilis demonstrates the potential of EUV ptychography for quantitative and statistically robust biological investigations. Furthermore, this work presents a quantitative analysis of dose-limited resolution for biologically relevant cellular models. By explicitly accounting for material-dependent contrast mechanisms in XDM, together with an established dose–resolution framework, the required radiation doses for imaging hydrated and dehydrated cellular states are systematically compared. The results identify an EUV energy window with particularly efficient protein contrast, enabling high-resolution quantitative imaging of dehydrated biological specimens at substantially reduced dose. These findings provide quantitative design guidelines for future EUV and SXR microscopy. In summary, this work demonstrates that HHG-based EUV ptychography enables quantitative, label-free nanoscale imaging of biological specimens on a laboratory scale, providing a practical and versatile framework for future biological imaging.

Laser–plasma interactions are determined both by plasma dynamics and by the properties of the driving laser pulse. While stimulated Raman scattering has been extensively studied, the influence of spatio-temporal couplings on such instabilities remains largely unexplored. This thesis investigates the impact of pulse-front tilt, one of the most common spatio-temporal couplings, on stimulated Raman side-scattering. Experiments were performed at the JETi-200 laser facility (Helmholtz Institute Jena/Institute of Optics and Quantum Electronics Jena) and directly observed using few-cycle microscopy. The measurements revealed asymmetric Raman scattering, propagation-dependent behavior, and larger scattering angles than predicted by conventional Raman theory, indicating a strong influence of pulse-front tilt. To explain these observations, the theory of Gaussian beam propagation with spatio-temporal couplings was extended to converging and diverging beams. Based on this, an analytical model combining conventional Raman scattering theory with pulse-front tilt was developed and successfully reproduced the experimental dependence on tilt, propagation distance, and electron density. Complementary two-dimensional particle-in-cell simulations provided insight into the microscopic dynamics of the instability and showed quantitative agreement with both experiment and theory. The results demonstrate that the observed scattering behavior is governed by the interplay of pulse-front tilt, laser propagation dynamics, and resonance conditions determined by the plasma electron density.

This work presents a new experimental approach for precision x-ray spectroscopy of highly-charged heavy ions in a storage ring. lt deploys novel metallic magnetic microcalorimeters (MMCs) at 0° and 180° on either side of the electron cooler of the recently commissioned CRYRING@ESR at the FAIR/GSI facility. These detectors of the maXs-type combine the broad spectral coverage of semiconductor detectors with an energy resolution approaching that of crystal spectrometers. The new approach represents a breakthrough in two key aspects. First, it enables the elimination of hard to-control systematic uncertainties that are imposed on the Doppler-shift of measured x-rays, and were always hampering heavy ion storage ring experiments. Second, it achieves an energy resolution that is improved by an order of magnitude compared to semiconductor detectors, which are typically used for x-ray spectroscopy over a broad spectral range. The measurement presented in this work was conducted in 2021, and investigated N, M->L and L->K -+K transitions of U90+ ions, following radiative recombination of cooler electrons into U91+ ions decelerated at 10.225 MeV/u. lt was the first successful coincidence measurement using MMC detectors at a storage ring, and achieved an energy resolution below 100 eV FWHM for photon energies ranging from a few keV to over 100 keV. Thus, the new experimental approach enabled, for the first time, an intrinsic Doppler correction in a heavy ion storage ring experiment, decoupling the correction from the hard-to-control systematic uncertainties, which are not expected to improve in the foreseeable future. In contrast, the systematic uncertainties of the new spectroscopy technique show potential of being reduced to below 1 eV with further investigation and refinement of the approach. Despite the low photon counts in the presented measurement - owing to it being the first time uranium was injected in the CRYRING, and to it being a first that U91+ was decelerated to 10.225 MeV/u and stored in a storage ring - the ion beam velocity, on which the Doppler correction depends, determined through the intrinsic method yields a statistical uncertainty that is of similar magnitude than the systematic uncertainty of the conventional method (deducing the term from the electron cooler voltage). Moreover, the new experimental approach provided the first theory-supported experimental insight into relative populations of excited states, as well as the branching ratio E1/M2 of the 1s2p 3P2 state in U90+.

This thesis presents a rigorous theoretical investigation of strong-field ionization (SFI), focusing on above-threshold ionization (ATI). The study examines SFI driven by few-cycle laser pulses within the strong-field approximation, comparing dipole and nondipole regimes. While the dipole approximation captures quantum interference structures, nondipole effects induce significant momentum shifts and asymmetries, with predictions showing enhanced experimental agreement. Two analytical approaches are employed: the Jacobi-Anger expansion, which decomposes the ionization amplitude into photon orders, and the saddle-point method, which isolates dominant quantum orbits for physical insight and computational efficiency. The analysis reveals how electron dynamics are governed by concealed fundamental frequencies and quantifies the influence of laser parameters on photoelectron distributions. Finally, the framework is generalized to structured twisted Bessel pulses carrying orbital angular momentum. Analytical expressions for the ionization amplitude are derived, systematically investigating the impact of the pulse's opening angle and OAM as tunable parameters for shaping electron emission.

We investigate vacuum polarization effects in collisions of two high-intensity laser pulses. According to the theory of QED in strong fields, the classical Maxwell equations in vacuum receive nonlinear corrections due to quantum vacuum fluctuations. The resulting violation of the superposition principle enables indirect interactions between macroscopic electromagnetic fields, which manifest as small modifications of the fields. Building on the Heisenberg–Euler effective action, which captures these nonlinear corrections, we describe the resulting changes in the macroscopic laser fields in terms of a photonic vacuum emission process. After introducing the fundamentals of strong-field QED, we compute the one-loop contribution to the zero-to-single-photon transition amplitude in weak and slowly varying electromagnetic background fields. We then analyze vacuum birefringence in the context of laser pulse collisions. Modeling the lasers as paraxial Gaussian beams, we demonstrate that the leading-order paraxial approximation can significantly overestimate the signal photon yield for both parallel and perpendicular laser polarization configurations. Furthermore, we present an experimental setup designed to detect signatures of light-by-light scattering in laser pulse collisions, currently being implemented at CALA (Garching, Germany), along with corresponding quantitative signal predictions. Our setup is based on the concept of an annular transverse beam profile far away from the focus, that causes a portion of the signal photons to be scattered into the central dark region. These photons can then be detected with a single-photon-sensitive detector under minimal background conditions. Our predictions, based on a numerical algorithm that propagates the laser fields according to Maxwell's equations in vacuum, yield approximately 0.2 polarization-flipped signal photons and about 4.3 signal photons in a polarization-insensitive measurements per shot under optimal conditions.

The electric field experienced by bound electrons in highly charged ions ranks among the strongest available to experimental studies. For moderate atomic numbers, the electric field in hydrogen-like ions is equivalent to current laser systems. Quantitative interaction experiments with ions require a single-species, well-controlled target. In particular, the target’s ion distribution and density must be characterised. This thesis addresses these requirements by producing and characterising a suitable ion target through dynamic capture in a Penning trap as part of the High-Intensity Laser-Ion Trap Experiment. Three key developments have been implemented. First, the ion optics were upgraded to control the initial conditions of the incoming ion bunches. Second, a phosphor screen detector was introduced to measure the radial distribution of the captured ion ensembles. Finally, a new Penning trap setup featuring a dedicated dual-electrode resonator was developed and integrated into the experiment. The presented experimental results demonstrate that the ion ensemble reaches maximum radial density when the incoming ions are aligned with the central axis. Under these conditions, the radial thermalisation process occurs within 50 μs; resulting in a Gaussian-shaped radial distribution characteristic of a weakly coupled ion ensemble in thermal equilibrium. In the axial direction, the applied resonator facilitates the resistive cooling process of the captured ions, reducing the centre-of-mass energy by more than 99 % within 30 to 100 ms. A model is developed to describe the time-resolved induced signal during resistive cooling. From this model, key parameters such as the ion number and axial equilibrium energy are extracted. The combination of these radial and axial results yields an estimated peak density of 50 000 per cubic millimetre. For an upcoming tunnel ionisation experiment, ionisation yields are estimated, with more than 100 ionisations expected per laser pulse.

Current HHG experiments often face at least one of the following problems: low photon flux, long integration times, and a fixed harmonic comb structure that may not cover all desired photon energies. Tunable HHG sources have been developed, but suffer from moderate photon flux, incomplete spectral coverage, high complexity and low tuning speed, or a combination of these. This work addresses all these challenges by demonstrating a state-of-the-art photon flux between 50 eV and 70 eV, and 80 eV and 120 eV. The presented EUV source achieves full tunability with low losses and allows fast tuning by adjusting the pulse energy of the driving laser. By deliberately changing the pulse energy, both dispersion and plasma-induced effects can be used to change the instantaneous wavelength of the laser pulse and thus the generated EUV radiation. The experiments showed that managing the heat generated by the high average power of the driving laser is challenging and affects the efficiency of the HHG at high repetition rates. The experimental design already included active cooling of the fiber mount and a fluorine-doped cladding structure to direct stray light away from the required gas seals. However, ionization of the gas in the interaction zone caused significant heating of the gas, which reduced particle density, disturbed phase matching, and degraded the sealing rubbers. To address the heat-induced problems, the work proposes two strategies: active cooling of the fiber or a side-slit fiber geometry. To reduce reabsorption, the fiber core diameter can be increased or an axially drilled fiber can be used. All of these strategies have been investigated in gas flow, phase-matching, or temperature-based simulations and shown to be potentially promising. With existing fiber laser systems capable of delivering more than 1 kW of average power with mJ and fs pulses, this work represents an important step toward the generation of higher power from waveguide-based HHG sources.

A circularly polarized light possesses spin angular momentum. However, a twisted light carries orbital angular momenthe theoretical physics of the interaction of vector light with atoms, which has prospects for application in fields such as quantum information, quantum metrology, atomic clocks,these interesting properties, linear combination of two or more twisted light results in vector light which does not have uniform polarization pattern across their beam profile. In this thesis, interaction between these interesting light fields and atomic targets are considered. First of all, we consider the differences between scalar and vector light fields in driving non-dipole atomic transitions in single trapped ionic target. In the second example, we consider interaction between vector light with an ensemble of atoms in the presence of an external constant and oscillating magnetic fields. We propose that this setup can be used to detect properties of the external (oscillating) magnetic fields by monitoring the intensity of the transmitted vector light through the target atom. Thus, this thesis sketches theoretical physics of the interaction of vector light with atoms which has the prospcets of application in the fields such as quantum information, quantum metrology, atomic clocks and many more.

Dielectronic recombination (DR) is the resonant capture of a free electron by an ion. It is a fundamental process in determining the charge-state balance of low-temperature plasma environments, which can be found in a wide range of astrophysical objects such as planetary nebulae, active galactic nuclei and supernova remnants. Accurate rates of DR are therefore required to model these objects and correctly interpret astronomical observations. While theory can nowadays predict many DR transitions quite reliably, in particular for high centre-of-mass energies, transitions at low energies (ie a few eV and below) are less easy to predict in terms of their position and strength. In order to provide reliable data for plasma modelling, DR experiments are carried out at the electron coolers of heavy-ion storage rings which provide cooled merged-beam electron targets that allow for precise measurements of DR spectra. At the CRYRING@ESR storage ring at GSI/FAIR a new setup for DR measurements has been installed and taken into operation in the past few years. Making use of the ultra-cold electron beam and the wide variety of available ions in different charge states, we have implemented DR experiments to explore various physics cases. This includes both recombination rate measurements with an astrophysical motivation as described above, as well as the use of DR as a spectroscopic tool to probe the atomic structure of heavy highly-charged ions in order to benchmark atomic theory. The focus of this work is on DR measurements on neon ions: The first DR experiment conducted at CRYING@ESR was a test measurement on Li-like neon, which reproduced data obtained in a previous CRYRING experiment in Stockholm. As part of a new programme of astrophysically motivated DR campaigns, low-energy DR of O-like neon was then measured and the merged-beam recombination rate coefficient (MBRRC) and plasma recombination rate coefficient (PRRC) were calculated.

The GSI Helmholtzzentrum für Schwerionenforschung GmbH (GSI) in Darmstadt is a large-scale facility for basic research on and with highly charged ions in various physics fields, such as atomic and quantum physics. The amount of beamtime requested for experiments at the GSI accelerator complex significantly exceeds the amount available. At the research installations for low energetic ions, such as HITRAP and CRYRING@ESR, local ion sources can help to reduce this discrepancy. S-EBIT II is an electron beam ion trap (EBIT), which is planned to be used as a local ion source at HITRAP. EBITs are versatile tools, as they can provide ions for other experiments and can be used as independent experiments for spectroscopic measurements with highly charged ions. In the course of this work, S-EBIT II was set up and put into operation at the HITRAP platform at GSI. Successful commissioning was demonstrated through the measurement of an x-ray spectrum, which exhibits the K-Alpha and K-Beta emissions of trapped highly charged argon ions. Subsequently, measurements of KLL dielectronic recombination resonances of carbon-like to helium-like argon were carried out, based on which charge state abundances within the EBIT could be estimated.

The acceleration of electron bunches reaching GeV energies within a centimeter-scale device exemplifies the remarkable advancements achieved in the field of laser-plasma acceleration. One essential by-product of such acceleration process is the production of highly energetic X-ray photons. In this thesis, I will detail an experimental research centered on Laser Wakefield Acceleration (LWFA). The main focus of this research is directed towards exploiting LWFA as compact sources of brilliant, hard synchrotron radiation, commonly referred to as betatron radiation. The primary result of the thesis follows the production of hard X-ray photons in keV regime using gas cell as the target for LWFA through ionisation injection scheme. The gas cell length was kept much longer than the electron dephasing length with an anticipation of the overlap of the laser fields with the charge trapped inside the plasma wave. This overlap could result in stronger transverse oscillation of the trapped electron bunches and an increase in the total emission of the X-rays produced by the LWFA. Hydrogen and Helium were used as the background gas with Nitrogen as the dopant. The resulting X-rays showed high critical energy, peak brilliance and source size at par with the results shown by other groups [1–4]. Additionally, the X-rays produced boast of high degree of shot to shot stability and reproducibility paving way for the implementation of single shot imaging set-up at JETi200 laser system in Jena, Germany. Another result discusses the production of quasi-monoenergetic electron beams from LWFA by implementing shock-front injection mechanism [5, 6]. The X-ray beam measured from such quasi-monoenergetic beams were found to have critical energy similar to [7]. However, the X-ray beam had lower critical energy and photon yield than the beams produced using ionization injection mechanism.

Laser-driven plasma accelerators (LPAs) offer an efficient and highly compact alternative to conventional radio-frequency (RF) accelerators. This technology provides the potential to extend the application range of accelerators to a wider community, including science, industry, and healthcare. However, significant research and development is necessary to achieve the beam quality, stability, average power, and energy levels required for these applications. One of the key advancements needed to reach this operation regime is the development of a suitable driving laser source that can operate at kHz repetition rates while providing pulses at peak powers in or close to the TW range. Terawatt-class Ti:Sa lasers, representing the most common driving lasers for LPA sources to date, are limited to low repetition rates due to thermal issues. In contrast, Ytterbium-doped Yttrium Aluminum garnet (Yb:YAG) lasers are capable of supporting multi-millijoule energies at high average powers and repetition rates. However, they typically fall short of the peak power requirements for LPAs due to their narrow gain bandwidth, which limits the pulse duration to hundreds of femtoseconds. Combining Yb:YAG lasers with efficient postcompression methods like multi-pass cells (MPCs) could provide a promising solution to this challenge, enabling high repetition rates and TW peak powers through extreme-scale post-compression. In this work, we aim to explore this new LPA-driving laser approach with the main focus on developing a suitable laser source using a relatively compact Yb:YAG Innoslab laser that delivers 10 mJ pulses with 1.2 ps pulse duration at a 1 kHz repetition rate. This dissertation further addresses application-tailored optimization approaches and delivery of the generated pulses for first electron acceleration tests, aiming at demonstrating the first Yb-laser-driven LPA source with expected electron energies in the few-MeV regime.

This thesis focuses on two strong field ultrafast processes: non-sequential double ionization (NSDI) and high-order above-threshold ionization (HATI). Using an improved quantitative rescattering (QRS) model, we investigate the complex electron dynamics involved in these processes. We first investigate the drastic variations in the correlated two-electron momentum distributions (CMDs) during the transition from near-single-cycle to multi-cycle driving laser pulses. Using QRS model, we reproduce the CMDs for the NSDI of argon. We find that the transition from near-single-cycle to multi-cycle driving laser pulses depends strongly on the details of the pulse envelope. In particular, the cross-shaped structure observed in the CMD for near-single-cycle pulses can be traced back to two main factors: the strong backward scattering of the recolliding electron, and the narrow momentum distributions of the tunnel-ionized electrons which stand in contrast to those for long pulses. This contrast also explains why the cross-shaped distributions collapse. Furthermore, since strong-field ionization can induce electron motion in both the continuum and the valence shell of the parent ion, we explore their interplay by studying laser-induced electron diffraction (LIED) patterns arising from interaction with the potentials of two-hole states of the xenon cation. QRS model is used to calculate the corresponding photoelectron momentum distributions for HATI, providing evidence that the spin-orbit dynamics could be detected by LIED. We identified the contribution of these time-evolving hole states to the angular distribution of the rescattered electrons, particularly noting a distinct change along the backward scattering angles. We benchmark numerical results with experiments using ultrabroad and femtosecond laser pulses centered at 3100 nm.

In this dissertation, we explore two major themes related to intense laser-matter interaction. Firstly, we present a comprehensive characterization of the intense THz light and charged particle emission from the rear surface of thin targets during the interaction with ultrashort laser pulses. Secondly, we report the first direct visualization of the Coulomb field of relativistic electron bunches from laser-thin solid interactions on a sub-picosecond timescale. We introduce a novel non-destructive single-shot detection scheme based on the electrooptic principle. Our time-resolved measurements reveal a complex temporal structure with multiple electron bunches propagating at nearly the speed of light. Moreover, our observations confirm the contraction of the electric field of the relativistic electron bunches under the Lorentz transformation. Further, we demonstrate the spatiotemporal evolution of the Coulomb field wavefronts as the electron bunches propagate away from the target. This work paves the way for non-invasive measurements of fast dynamics of charged particles on sub-picosecond timescales.

Spectroscopy of highly charged ions and nuclei is a field with great potential to open new frontiers for precision metrology, act as a scientific test bed for quantum electrodynamics, and contribute to the advancement of technology in applied physics. However, precision spectroscopy of these species typically requires laser sources with high photon energies and narrow line widths in the vacuum ultraviolet (VUV) part of the electromagnetic spectrum. The aim of this dissertation is to develop key methods which will enable to create a VUV laser source tailored to the demands set by this spectroscopy application. Laser pulses with a duration of few optical cycles have a key potential for VUV generation, and therefore for spectroscopy of highly charged ions or nuclei. Furthermore, high average power lasers play a key role in addressing narrow transitions. However, laser sources supporting high average power such as Ytterbium-based laser systems, have a pulse duration of a few hundred femtoseconds. It is therefore essential to compress the pulses to a short duration. In this dissertation, we address temporal pulse compression of such high average power laser sources to few cycles. A well-known demanding objective for VUV spectroscopy is the low energy transition of the Thorium 229 (229Th) nucleus. When this dissertation work started, the energy of this transition was known within a range of 149.7 +/- 3.1 nm. However, a laser with a narrow linewidth, high-power and wavelength tunability covering this range is not yet available. This dissertation addresses the development of a high-power frequency comb laser to support tunable VUV generation to drive the low energy nuclear transition of 229Th. Finally, we discuss a preliminary experiment to investigate the low energy VUV nuclear transition of highly charged 229Th89+ ions. This experiment has the potential to locate the low energy nuclear transition of 229Th at a precision two orders of magnitude higher than the currently known uncertainty range.

Engineered photons from spontaneous parametric down-conversion (SPDC) are a valuable tool for studying and applying photonic entanglement, as well as serving as an effective source of single photons. In SPDC, a nonlinear crystal converts a high-energy photon from a laser field into a photon pair, commonly known as signal and idler. Both the theoretical and experimental research conducted by SPDC has primarily focused on the paraxial regime, where the transverse momentum of photons is referred to as the spatial degree of freedom (DOF), and frequency is considered as the spectral DOF. Hence, this dissertation also considers the paraxial regime. Photon pairs generated through SPDC inherently exhibit spatio-spectral coupling, which implies that photons with different spatial DOFs possess varying spectra. While quantum optics applications often focus on either spatial or spectral DOFs independently, the correlation between them poses a fundamental challenge in protocols involving entangled photon sources or single-mode photon states. Theoretical studies on SPDC, that address both space and spectrum together, are mostly limited to approximate wave functions of photon pairs or involve numerical computations. Such theoretical studies usually consider either monochromatic signal and idler photons (the narrowband approximation), loosely focused pump and collection beams (the plane wave approximation), or infinitesimally thin crystals (the thin crystal approximation). This dissertation aims to bridge the gap between the fundamental theory of SPDC and its practical applications. In particular, we have developed a comprehensive theory that does not rely on a specific pump beam or nonlinear crystal and goes beyond the common narrowband, plane wave, and thin crystal approximations. The developed approach accurately describes the inseparability of spatial and spectral DOF and applies to a wide range of experimental setups. Furthermore, we show that the origin of the spatio-spectral coupling is closely related to the Gouy phase of the interacting beams. We utilize the developed theory, taking into account the spatio-spectral coupling insights, to control the entanglement of photon pairs from SPDC. As an application, we shape the spatial distribution of the pump beam to design an efficient source of high-dimensional entangled states in the spatial DOF. In our second application, we tailor simultaneously the effective nonlinearity of the crystal and spatial distribution of the pump, to engineer single-mode photons.

The dynamics of quantum information lies at the heart of future technologies that aim to utilize the laws of quantum mechanics for practical purposes. Beyond that, it provides a unifying language that shines new light on longstanding problems home to historically separate fields of theoretical physics. Considering how quantum information propagates and spreads over the degrees of freedom of a quantum many-body system far from equilibrium has proven particularly helpful for various subjects, ranging from the emergence of statistical mechanics in isolated quantum systems to the black hole information paradox. Crucial for these developments are impressive experimental advances that nowadays allow us to explore the nonequilibrium physics of paradigmatic, simple, and (almost) isolated quantum many-body systems in the laboratory. In this thesis, we investigate the dynamics of quantum information in one-dimensional systems of interacting qubits, i.e., spin-chains, where we particularly consider systems that embody nonlocal interactions. The latter are ubiquitous in many experimental platforms for quantum simulation. Our results reveal an interesting connection between two complementary probes of quantum information dynamics, i.e., entanglement growth and operator spreading. This connection allows us to characterize different dynamical classes and underlines that nonlocal interactions induce rich behavior, such as slow thermalization accompanied by superballistic information propagation. In particular, we show that the famous slowdown of entanglement growth in systems with powerlaw interactions implies a slowdown of operator dynamics. The latter clearly distinguishes a system with powerlaw interactions from a system possessing fast scrambling, a characteristic property of black holes and holographic duals to theories of quantum gravity.