Theses

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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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.

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.

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.

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.

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.