Theses

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.

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.

One of the many astonishing predictions of quantum field theory is the nonlinear nature of the quantum vacuum. In the context of quantum electrodynamics this enables classically forbidden, nonlinear interactions between electromagnetic fields in the vacuum. One much pursued idea to test this prediction is to excite the quantum vacuum with high-intensity laser pulses to emit signal photons that can be distinguished from the driving laser photons, for example, by their polarization or photon energy. A major experimental challenge is the small number of signal photons compared to the background photons. With the aim of identifying a potential discovery experiment of quantum vacuum nonlinearity, we provide theoretically firm predictions for two different quantum vacuum signatures with an emphasize on experimentally feasible setups. This covers on the one side the much studied effect of vacuum birefringence, for which we analyze an innovative setup using a single X-ray free-electron laser. Special attention is paid to the influence of the optical components in the setup on the laser pulse properties. On the other side, we perform an in-depth study of a signature of quantum vacuum nonlinearity that has received less attention so far, namely laser photon merging. The signal photons here have the outstanding property that they differ in photon energy from the background photons and can thus be isolated efficiently. We show that, using state-of-the-art technology, the merging signal can compete with the vacuum birefringence signal in terms of its suitability for a potential discovery experiment of quantum vacuum nonlinearity.

In this dissertation, the experiments are conducted at the Jenaer Titanium: Sapphire 200 Terawatt Laser System (JETi200) located in Jena, Germany. With its excellent temporal contrast, the few-nanometer freestanding target can remain in a solid state for a few picoseconds before the main pulse arrives, greatly reducing the pre-expansion of the target. The resulting proton beams exhibit distinctive features in terms of cut-off energy and energy spectrum distribution. The proton beams in the presented experiment show a more than 30 MeV monoenergetic peak under the circularly polarized laser, and the highest peak particle kinetic energy per Joule of laser energy is around 20MeV/J. As opposed to the circularly polarized driving light, the cut-off energy shows weak dependence on the target thickness when irradiated with linearly polarized light. Moreover, the implementation of a transmission light diagnostic in the experiment indicates that the transmission light of the main pulse is significantly weaker than that in other similar experiments. The energy and energy spectrum of the protons provide the potential to conduct in vivo research and proton skin therapy using the Terawatt-level laser system. Laser contrast significantly impacts laser-driven ion acceleration, as low contrast can lead to premature expansion of thin targets. The evolution of premature expansion, caused by pre-pulses, is primarily based on numerical calculations research. However, in this paper, I conduct a comprehensive experimental study of pre-pulse-induced pre-plasma evolution, including the measurement of pre-plasma evolution time and comparison with a previous numerical model. This investigation is especially beneficial for the latest generation of laser ion accelerators, as it enables the precise quantification of temporal contrast requirements in the Petawatt laser driver era.

Starke Laserfelder sind für die Erforschung der Laser-Atom-Dynamik in der modernen Physik unerlässlich. Die Erzeugung von Harmonischen höherer Ordnung (HHG) ist ein bedeutender Starkfeldprozess, bei welchem ein ionisiertes Elektron durch das elektrische Feld des einfallenden Lasers beschleunigt wird und anschließend mit seinem Mutterion rekombiniert. Das beschleunigte Elektron emittiert bei der Rekombination schlussendlich ein Photon, welches auf makroskopischer Ebene höherer harmonische Strahlung entspricht. Jüngste Fortschritte in der Lasertechnologie ermöglichen die Erzeugung intensiver Laserfelder im mittleren Infrarotbereich. Diese neuen Laserquellen erweitern den Parameterbereich der HHG erheblich bezüglich des schwach relativistischen Bereichs, in welchem das Magnetfeld des einfallenden Laserfeldes zu diesem Prozess beiträgt. In dieser Dissertation wird ein theoretisches Modell der HHG vorgestellt, welches eine formale Erweiterung der bekannten Starkfeld-Näherung auf den schwach relativistischen Bereich darstellt. Generell kann das Modell im Gegensatz zu aktuellen Ansätzen beliebig räumlich strukturierte Lichtfelder berücksichtigen und bietet somit die Möglichkeit, verdrillte Lichtstrahlen im schwach relativistischen Regime zu untersuchen. Das hier entwickelte Modell betrachtet explizit einen elliptisch polarisierten ebenen Laserstrahl als Beispiel. Darüber hinaus werden auch komplexere Laserfelder betrachtet und anschließend kurz diskutiert. Darüber hinaus wird in dieser Dissertation die Phasenanpassung im Kontext der HHG untersucht, die mit einer geringen Konversionseffizienz von weniger als 0, 1 % behaftet ist. Die Suche nach geeigneten externen Parametern, unter denen die Konversionseffizienz entsprechend hoch ist, ist daher von großem In- teresse für die Starkfeldgemeinde. Es wird ein analytischer Ausdruck für die kritische Intensität abgeleitet, der die Bedingung der Phasenanpassung für einen beliebigen Satz von Anfangsparametern erfüllt. Der Ansatz ist auf wasserstoffähnliche Edelgase und linear polarisierte Gauß-Laserpulse mit beliebigen Laserfeldparametern beschränkt, die die üblicherweise verwendeten Konfigurationen umfassen. Der analytische Fehler im Vergleich zu numerischen Berechnungen ist kleiner als 1 %, während die Berechnungszeit um vier bis sechs Größenordnungen verbessert wird.

Every interesting quantity to be investigated in the realm of bound-state quantum electrodynamics (BSQED), such as, for example, the Lamb shift, the (hyper)fine-splitting or the g-factor, is closely or remotely connected to the energy levels of the considered system. Therefore, as a prerequisite, it is mandatory to have the ability to accurately assess energy levels of increasingly sophisticated electronic configurations of atoms or ions. BSQED predictive powers are nowadays limited to either simple light systems, where an αZ expansion is justified, or heavy few-electron highly-charged ions, where specialized all-order methods in αZ are required, to reliably capture interelectronic interactions. The redefined vacuum state approach, which is frequently employed in the many-body perturbation theory, proved to be a powerful tool allowing analytical insights. This thesis elaborates on this approach within BSQED perturbation theory, based on the two-time Green’s function method. In addition to a rather formal formulation, the particular example of a single-particle (electron or hole) excitation with respect to the redefined vacuum state is considered. Starting with simple one-particle Feynman diagrams, characterized by radiative corrections to identical single incoming and single outgoing state, first- and second-order many-electron contributions are derived, namely screened self-energy, screened vacuum-polarization, one-photon-exchange, and two-photon-exchange. The redefined vacuum state approach provides a straightforward and streamlined derivation facilitating its application to any electronic configuration. Moreover, based on the gauge invariance of the one-particle diagrams, various gauge-invariant subsets within analysed many-electron QED contributions are identified. The identification of gauge-invariant subsets in the framework of the proposed approach opens a way to tackle more complex diagrams, where the decomposition into simpler subsets is crucial.

Intense laser-matter interactions are generally determined by the instantaneous electric field of the laser pulse. For light-matter interactions with few-cycle pulses, the carrier-envelope phase (CEP) plays a critical role as the temporal variation of the electric field depends on the phase. This has a profound impact on many scientific applications. More importantly, controlling the CEP provides an additional degree of freedom to control field-driven processes in atomic, molecular and solid-state systems. In this thesis, we will demonstrate the development and implementation of a single-shot CEP measurement technique, i.e., the so-called carrier-envelope phasemeter based on stereo-above-threshold ionization in Xe and operating at short-wave infrared (SWIR) wavelengths, which allows for simultaneous pulse duration measurement. The experimental results are compared to simulations with two different theoretical models. Next, we will demonstrate the significance of the phase-volume effect, i.e. the reduction of the CEP-dependence due to the spatial distribution of the CEP in focused few-cycle pulsed beams. We formulate a general description of the impact of the focal phase for laser-matter interactions of different nonlinear orders to answer the general question: if, when, and how much should one be concerned about the phase-volume effect? At last, CEP-dependent strong-field ionization of Xe using 3.2um few-cycle pulses as a benchmark will be studied. In order to find an alternative target for a single-shot CEPM at mid-infrared (MIR) wavelengths, Cs will be investigated. We observed an anomalous CEP-dependence in Cs, particularly at high intensities, which can be interpreted as the interference of two backscattered quantum orbits from adjacent optical cycles. Viewed from a higher perspective, this thesis demonstrates a precise characterization of the CEP and an accurate analysis of CEP-dependent light-matter interaction from the NIR, via the SWIR to the MIR range.

In this work, we report on the first x-ray spectroscopy study associated with the RR processes for bare lead ions at the electron cooler of the CRYRING@ESR, as storage rings are currently the only facilities routinely delivering hydrogen-like ions at high-Z in large quantities. With ultra-cold electron beam temperatures and near zero electron-ion collision energies, the effective production of characteristic projectile x-rays was well demonstrated at 0 deg and 180 deg observation geometries in our experiment by decelerated 10 MeV/u hydrogen-like lead ions. To reveal the role of radiative feeding transitions in the formation of observed intense Lyman and Balmer lines, an elaborate theoretical model describing the radiative decay dynamics and each (n, l, j)-state population varying over time is put to a test. As a result, the presented rigorous treatment reproduces observed x-ray spectroscopy really well in terms of the RR transitions and characteristic x-ray lines. In addition, we found a strong enhancement for l = n − 1 states in inner shells due to radiative Yrast-cascades from high Rydberg states, that finally contribute strikingly to the observed intensities of characteristic x-ray lines. Further on the current thesis lays the basis for a successful effort to push the experimental resolution of x-ray spectroscopy for L → K ground-state transitions at high-Z of below 80 eV at about 100 keV. This was done in an RR experiment of free electrons into the bound states of initially hydrogen-like uranium ions by adopting low temperature x-ray detectors, namely metallic magnetic calorimeters. Such an experiment allowed us for the first time to resolve the substructure of the Kα2 line and partially the Kα1 line in helium-like uranium ions. The preliminary data again prove the unique potential of the experimental method based on x-ray spectroscopy at the electron cooler of the CRYRING@ESR.

One of the most intriguing physics processes that remain untested is the pure photon electron-positron pair production via quantum-vacuum fluctuations described by the nonlinear Breit-Wheeler theory. These fluctuations generate virtual pairs that can be turned into observable particles by applying strong electric fields above the Schwinger critical limit of \num{1.3d18}~V/m~\cite{Schwinger.1951, Ritus.1985}. Despite the advent of high-intense lasers, the critical limit is still far beyond achievable. However, such fields can be achieved on the rest frame of the real particles after the collision of a high-energy $\gamma$-ray photons with the laser beam. To diagnose the created pairs, this thesis describes the design of a particle detection system capable of successfully probing the single leptons created from strong-field quantum electrodynamics (SF-QED) interactions at the upcoming SF-QED experiments E-320 at FACET-II and FOR2783 at CALA. The designed detection system is composed of tracking layers made of LYSO:Ce scintillating screens and a Cherenkov calorimeter that, having their signals combined, can identify a positive event with a confidence level above 99%. At the E-320 experiment, electron beams generated by the FACET-II linear accelerator with an energy of 13~GeV collide with an intense laser beam of $\anot \approx 10$, and nonlinear Breit-Wheeler pairs are produced in the nonperturbative full quantum regime of SF-QED interaction ($\chie > 1$ and $\anot > 1$). About 100 electron-positron pairs per shot are expected to be created. According to Monte-Carlo simulations of the experimental layout, the detection system will be placed on a region permeated by a shower of x-rays and few-MeV $\gamma$-photons, however, a signal-to-noise ratio of $\SNRsig \approx 18$ on the detectors is achieved.