Referierte Publikationen

Microscopy with extreme ultraviolet (EUV) radiation holds promise for high-resolution imaging with excellent material contrast, due to the short wavelength and numerous element-specific absorption edges available in this spectral range. At the same time, EUV radiation has significantly larger penetration depths than electrons. It thus enables a nano-scale view into complex three-dimensional structures that are important for material science, semiconductor metrology, and next-generation nano-devices. Here, we present high-resolution and material-specific microscopy at 13.5 nm wavelength. We combine a highly stable, high photon-flux, table-top EUV source with an interferometrically stabilized ptychography setup. By utilizing structured EUV illumination, we overcome the limitations of conventional EUV focusing optics and demonstrate high-resolution microscopy at a half-pitch lateral resolution of 16 nm. Moreover, we propose mixed-state orthogonal probe relaxation ptychography, enabling robust phase-contrast imaging over wide fields of view and long acquisition times. In this way, the complex transmission of an integrated circuit is precisely reconstructed, allowing for the classification of the material composition of mesoscopic semiconductor systems.

In order to reach the highest intensities, modern laser systems use adaptive optics to control their beam quality. Ideally, the focal spot is optimized after the compression stage of the system in order to avoid spatio-temporal couplings. This also requires a wavefront sensor after the compressor, which should be able to measure the wavefront on-shot. At PHELIX, we have developed an ultra-compact post-compressor beam diagnostic due to strict space constraints, measuring the wavefront over the full aperture of 28 cm. This system features all-reflective imaging beam transport and a high dynamic range in order to measure the wavefront in alignment mode as well as on shot.

A key requirement for the correct interpretation of high-resolution X-ray spectra is that transition energies are known with high accuracy and precision. We investigate the K-shell features of Ne, CO2, and SF6 gases, by measuring their photo ion-yield spectra at the BESSY II synchrotron facility simultaneously with the 1s–np fluorescence emission of He-like ions produced in the Polar-X EBIT. Accurate ab initio calculations of transitions in these ions provide the basis of the calibration. While the CO2 result agrees well with previous measurements, the SF6 spectrum appears shifted by ∼0.5 eV, about twice the uncertainty of the earlier results. Our result for Ne shows a large departure from earlier results, but may suffer from larger systematic effects than our other measurements. The molecular spectra agree well with our results of time-dependent density functional theory. We find that the statistical uncertainty allows calibrations in the desired range of 1–10 meV, however, systematic contributions still limit the uncertainty to ∼40–100 meV, mainly due to the temporal stability of the monochromator energy scale. Combining our absolute calibration technique with a relative energy calibration technique such as photoelectron energy spectroscopy will be necessary to realize its full potential of achieving uncertainties as low as 1–10 meV.

Two-stage multipass-cell compression of a fiber-chirpedpulse amplifier system to the few-cycle regime is presented. The output delivers a sub-2-cycle (5.8 fs), 107W average power, 1.07 mJ pulses at 100kHz centered at 1030nm with excellent spatial beam quality (M-2 =1.1, Strehl ratio S = 0.98), pointing stability (2.3 mu rad), and superior long-term average power stability of 0.1% STD over more than 8 hours. This is combined with a carrier-envelope phase stability of 360mrad in the frequency range from 10Hz to 50kHz, i.e., measured on a single-shot basis. This unique system will serve as an HR1 laser for the Extreme Light Infrastructure Attosecond Light Pulse Source research facility to enable high repetition rate isolated attosecond pulse generation

High-power laser beams can be used to accelerate neutron beams. Here the authors demonstrate the application of laser-driven neutron beams to neutron resonance spectroscopy and neutron resonance imaging. Neutrons are a valuable tool for non-destructive material investigation as their interaction cross sections with matter are isotope sensitive and can be used complementary to x-rays. So far, most neutron applications have been limited to large-scale facilities such as nuclear research reactors, spallation sources, and accelerator-driven neutron sources. Here we show the design and optimization of a laser-driven neutron source in the epi-thermal and thermal energy range, which is used for non-invasive material analysis. Neutron resonance spectroscopy, neutron radiography, and neutron resonance imaging with moderated neutrons are demonstrated for investigating samples in terms of isotope composition and thickness. The experimental results encourage applications in non-destructive and isotope-sensitive material analysis and pave the way for compact laser-driven neutron sources with high application potential.

Abstract The Gamma Factory (GF) is an ambitious proposal, currently explored within the CERN Physics Beyond Colliders program, for a source of photons with energies up to ≈400 MeV and photon fluxes (up to ≈1017 photons s-1) exceeding those of the currently available gamma sources by orders of magnitude. The high-energy (secondary) photons are produced via resonant scattering of the primary laser photons by highly relativistic partially-stripped ions circulating in the accelerator. The secondary photons are emitted in a narrow cone and the energy of the beam can be monochromatized, down to 10-3...10-6 level, via collimation, at the expense of the photon flux. This paper surveys the new opportunities that may be afforded by the GF in nuclear physics and related fields.

The bound-electron g factor is a stringent tool for tests of the standard model and the search for new physics. The comparison between an experiment on the g factor of lithiumlike silicon and the two recent theoretical values revealed the discrepancies of 1.7 sigma [Glazov et al. Phys. Rev. Lett. 123, 173001 (2019)] and 5.2 sigma [Yerokhin et al. Phys. Rev. A 102, 022815 (2020)]. To identify the reason for this disagreement, we accomplish large-scale high-precision computation of the interelectronic-interaction and many-electron QED corrections. The calculations are performed within the extended Furry picture of QED, and the dependence of the final values on the choice of the binding potential is carefully analyzed. As a result, we significantly improve the agreement between the theory and experiment for the g factor of lithiumlike silicon. We also report the most accurate theoretical prediction to date for lithiumlike calcium, which perfectly agrees with the experimental value.

Open f-shell elements still constitute a great challenge for atomic theory owing to their (very) rich fine-structure and strong correlations among the valence-shell electrons. For these medium and heavy elements, many atomic properties are sensitive to the correlated motion of electrons and, hence, require large-scale computations in order to deal consistently with all relativistic, correlation and rearrangement contributions to the electron density. Often, different concepts and notations need to be combined for just classifying the low-lying level structure of these elements. With JAC, the Jena Atomic Calculator, we here provide a toolbox that helps to explore and deal with such elements with open d- and f-shell structures. Based on Dirac\textquotesingle s equation, JAC is suitable for almost all atoms and ions across the periodic table. As an example, we demonstrate how reasonably accurate computations can be performed for the low-lying level structure, transition probabilities and lifetimes for Th2+ ions with a 5f6d ground configuration. Other, and more complex, shell structures are supported as well, though often for a trade-off between the size and accuracy of the computations. Owing to its simple use, however, JAC supports both quick estimates and detailed case studies on open d- or f-shell elements.

We analyze the photoexcitation of atoms with a single valence electron by cylindrically polarized Laguerre-Gaussian beams. Theoretical analysis is performed within the framework of first-order perturbation theory and by expanding the vector potential of the Laguerre-Gaussian beam in terms of its multipole components. For cylindrically polarized Laguerre-Gaussian beams, we show that the (magnetic) sub-components of electric-quadrupole field vary significantly in the beam cross section with beam waist and radial distance from the beam axis. We discuss the influence of varying magnetic multipole components in the beam cross section on the sublevel population of a localized atomic target. In addition, we calculate the total excitation rate of electric-quadrupole transition (4s S-2(1/2) -> 3d D-2(5/2)) in a mesoscopic target of a Ca+ ion. These calculations shows that the total rate of excitation is sensitive to the beam waist and the distance between the center of the target and the beam axis. However, the excitation by a cylindrically polarized Laguerre-Gaussian beam is found more efficient in driving electric-quadrupole transition in the mesoscopic atomic target than the circularly polarized beams.

A theoretical investigation of the elastic resonant scattering of photons by ultra-relativistic and partially stripped ions is presented. Particular attention in the study is given to the angular distribution and polarization of scattered photons as \textasciigrave \textasciigrave seen" in both the ion-rest and laboratory reference frames. In order to evaluate these angular and polarization properties, the irreducible polarization tensor approach is combined with the density matrix theory. If, furthermore, the ion-photon coupling is treated within the electric dipole approximation, this framework enables one to obtain simple analytical expressions for both the emission pattern and the polarization Stokes parameters of the outgoing radiation. These (analytical) expressions for the nS0 -> n \textasciigrave P1 -> nS0\textbackslash \textdollar n S_0 \textbackslash rightarrow n<\^>\textbackslash \textbackslash prime \textbackslash P_1 \textbackslash rightarrow n S_0\textbackslash \textdollar , nS1/2 -> n \textasciigrave P1/2 -> nS1/2\textbackslash \textdollar n S_\textbackslash 1/2\textbackslash \textbackslash rightarrow n<\^>\textbackslash \textbackslash prime \textbackslash P_\textbackslash 1/2\textbackslash \textbackslash rightarrow n S_\textbackslash 1/2\textbackslash \textbackslash \textdollar , and nS1/2 -> n \textasciigrave P3/2 -> nS1/2\textbackslash \textdollar n S_\textbackslash 1/2\textbackslash \textbackslash rightarrow n<\^>\textbackslash \textbackslash prime \textbackslash P_\textbackslash 3/2\textbackslash \textbackslash rightarrow n S_\textbackslash 1/2\textbackslash \textbackslash \textdollar transitions are displayed and analyzed , that are of interest for the Gamma Factory project and whose realization is currently under discussion at CERN. Based on the performed analysis, it is demonstrated that the resonantly scattered photons can be strongly (linearly or circularly) polarized, and that this polarization can be well controlled by adjusting either the emission angle and/or the polarization state of the incident radiation. Moreover, the potential of the photon scattering for measuring the spin-polarization of ion beams is also discussed in detail.

In recent years, high-precision x-ray polarimeters have become a key method for the investigation of fundamental physical questions from solid-state physics to quantum optics. Here, we report on the verification of a polarization purity of better than 8×10−11 at an x-ray free-electron laser, which implies a suppression of the incoming photons to the noise level in the crossed polarizer setting. This purity provides exceptional sensitivity to tiny polarization changes and offers intriguing perspectives for fundamental tests of quantum electrodynamics.

Abstract The perspectives of studying vacuum birefringence at the Gamma Factory are explored. To this end, the parameter regime which can be reliably analyzed resorting to the leading contribution to the Heisenberg?Euler effective Lagrangian is assessed in detail. It is explicitly shown that?contrary to naive expectations?this approach allows for the accurate theoretical study of quantum vacuum signatures up to fairly large photon energies. The big advantage of this parameter regime is the possibility of studying the phenomenon in experimentally realistic, manifestly inhomogeneous pump and probe field configurations. Thereafter, two specific scenarios giving rise to a vacuum birefringence effect for traversing gamma probe photons are analyzed. In the first scenario the birefringence phenomenon is induced by a quasi-constant static magnetic field. In the second case it is driven by a counter-propagating high-intensity laser field.

Among the existing techniques for measuring ultrashort pulse durations, the two classical second-order methods - interferometric and the background-free autocorrelation - are distinguished due to their simplicity and reliability. In this work we report on a technique that allows realignment-free switching between these two modes of autocorrelation. It is based on a collinearly aligned inverted-field interferometer and an optical vortex plate that is added/removed in front of the device in order to switch between both modes. Experiment and theoretical modeling confirm the effectiveness of the technique down to the 10-fs range.

High power short pulse lasers provide a promising route to study the strong field effects of the quantum vacuum, for example by direct photon-photon scattering in the all-optical regime. Theoretical predictions based on realistic laser parameters achievable today or in the near future predict scattering of a few photons with colliding Petawatt laser pulses, requiring single photon sensitive detection schemes and very good spatio-temporal filtering and background suppression. In this article, we present experimental investigations of this photon background by employing only a single high power laser pulse tightly focused in residual gas of a vacuum chamber. The focal region was imaged onto a single-photon sensitive, time gated camera. As no detectable quantum vacuum signature was expected in our case, the setup allowed for characterization and first mitigation of background contributions. For the setup employed, scattering off surfaces of imperfect optics dominated below residual gas pressures of 1 x 10(-4) mbar. Extrapolation of the findings to intensities relevant for photon-photon scattering studies is discussed.

We demonstrate a 41.6 MHz, 1.3 ps, 140 pJ Ho:fiber oscillator using a nonlinear amplifying loop mirror (NALM) as saturable absorber. The oscillator is constructed entirely with polarization-maintaining (PM) fibers, is tunable with a center wavelength between 2035 nm and 2075 nm, and can be synchronized to an external RF reference. For our application of Ho:YLF amplifier seeding for dielectric electron acceleration, the laser is tuned to 2050 nm and synchronized to a stable RF reference with 45 fs rms timing jitter in the integration interval [10 Hz, 1 MHz]. We show long term synchronized operation and characterize the relative intensity noise (RIN) and timing jitter of the oscillator for two different Tm-fiber pump lasers.

Ultrafast lasers reaching extremely high powers within short fractions of time enable a plethora of applications. They grant advanced material processing capabilities, are effective drivers for secondary photon and particle sources, and reveal extreme light-matter interactions. They also supply platforms for compact accelerator technologies, with great application prospects for tumor therapy or medical diagnostics. Many of these scientific cases benefit from sources with higher average and peak powers. Following mode-locked dye and titanium-doped sapphire lasers, broadband optical parametric amplifiers have emerged as high peak- and average power ultrashort pulse lasers. A much more power-efficient alternative is provided by direct post-compression of high-power diode-pumped ytterbium lasers-a route that advanced to another level with the invention of a novel spectral broadening approach, the multi-pass cell technique. The method has enabled benchmark results yielding sub-50-fs pules at average powers exceeding 1 kW, has facilitated femtosecond post-compression at pulse energies above 100 mJ with large compression ratios, and supports picosecond to few-cycle pulses with compact setups. The striking progress of the technique in the past five years puts light sources with tens to hundreds of TW peak and multiple kW of average power in sight-an entirely new parameter regime for ultrafast lasers. In this review, we introduce the underlying concepts and give brief guidelines for multi-pass cell design and implementation. We then present an overview of the achieved performances with both bulk and gas-filled multipass cells. Moreover, we discuss prospective advances enabled by this method, in particular including opportunities for applications demanding ultrahigh peak-power, high repetition rate lasers such as plasma accelerators and laser-driven extreme ultraviolet sources.

Laser-driven light sources in the extreme ultraviolet range (EUV) enable nanoscopic imaging with unique label-free elemental contrast. However, to fully exploit the unique properties of these new sources, novel detection schemes need to be developed. Here, we show in a proof-of-concept experiment that superconducting nanowire single-photon detectors (SNSPD) can be utilized to enable photon counting of a laser-driven EUV source based on high harmonic generation (HHG). These detectors are dark-count free and accommodate very high count rates-a perfect match for high repetition rate HHG sources. In addition to the advantages of SNSPDs for classical imaging applications with laser-driven EUV sources, the ability to count single photons paves the way for very promising applications in quantum optics and quantum imaging with high energetic radiation like, e.g., quantum ghost imaging with nanoscale resolution.

We investigate the two-color two-photon K-shell ionization of neutral atoms based on the relativistic second-order perturbation theory and independent particle approximation. Analytical expressions for the relativistic and nonrelativistic total cross sections are derived in terms of radial transition amplitudes and Stokes parameters. Particular attention is paid especially to how the two-photon ionization total cross section depends on the energy sharing and polarization of the two incident photons. We construct the nonrelativistic expressions of cross section ratios for different polarization combinations of the two incident photons. The numerical results of total cross section and cross section ratios show that the energy sharing of the two incident photons plays an essential role in two-photon K-shell ionization. Particularly, if the energies of the two incident photons are identical, the total cross section and cross section ratios will reach the minimum or maximum value. Moreover, due to the strong screening effects, we find strong deviations of the cross section ratios near the two-photon ionization threshold of the Ne atom.

We present an experimental and theoretical study of symmetric Xe54++Xe collisions at 50, 30, and 15 MeV/u, corresponding to strong perturbations with vK/vp=1.20, 1.55, and 2.20, respectively (vK is the classical K-shell orbital velocity and vp is the projectile velocity), as well as Xe53++Xe collisions at 15 MeV/u. For each of these systems, x-ray spectra are measured under a forward angle of 35∘ with respect to the projectile beam. Target satellite and hypersatellite radiation Kαs2,1 and Kαhs2,1, respectively, are analyzed and used to derive cross-section ratios for double-to-single target K-shell vacancy production. We compare our experimental results to relativistic time-dependent two-center calculations.

We present a coherently combined femtosecond fiber chirped-pulse-amplification system based on a rod-type, ytterbium-doped, multicore fiber with 4 × 4 cores. A high average power of up to 500 W (after combination and compression) could be achieved at 10 MHz repetition rate with excellent beam quality. Additionally, łess 500 fs pulses with up to 600 µJ of pulse energy were also realized with this setup. This architecture is intrinsically power scalable by increasing the number of cores in the fiber.

Extreme ultraviolet microscopy and wavefront sensing are key elements for nextgeneration ultrafast applications, such as chemically-resolved imaging, focal spot diagnostics in pump-and-probe experiments, and actinic metrology for the state-of-the-art lithography node at 13.5 nm wavelength. Ptychography offers a robust solution to the aforementioned challenges. Originally adapted by the electron and synchrotron communities, advances in the stability and brightness of high-harmonic tabletop sources have enabled the transfer of ptychography to the laboratory. This review covers the state of the art in tabletop ptychography with high harmonic generation sources. We consider hardware options such as illumination optics and detector concepts as well as algorithmic aspects in the analysis of multispectral ptychography data. Finally, we review technological application cases such as multispectral wavefront sensing, attosecond pulse characterization, and depth-resolved imaging.

On-demand generation and reshaping of arrays of focused laser beams is highly desired in many areas of science and technology. In this work, we present a versatile approach for laser beam structuring in the focal plane of a lens by triple mixing of square and/or hexagonal optical vortex lattices (OVLs). In the artificial far field the input Gaussian beam is reshaped into ordered arrays of bright beams with flat phase profiles. This is remarkable, since the bright focal peaks are surrounded by hundreds of OVs with their dark cores and two-dimensional phase dislocations. Numerical simulations and experimental evidences for this are shown, including a broad discussion of some of the possible scenarios for such mixing: triple mixing of square-shaped OVLs, triple mixing of hexagonal OVLs, as well as the two combined cases of mixing square-hexagonal-hexagonal and square-square-hexagonal OVLs. The particular ordering of the input phase distributions of the OV lattices on the used spatial light modulators is found to affect the orientation of the structures ruled by the hexagonal OVL. Reliable control parameters for the creation of the desired focal beam structures are the respective lattice node spacings. The presented approach is flexible, easily realizable by using a single spatial light modulator, and thus accessible in many laboratories.

Multi-pass cells (MPCs) have emerged as very attractive tools for spectral broadening and post-compression applications. We discuss pulse energy limitations of standard MPCs considering basic geometrical scaling principles and introduce a novel energy scaling method using a MPC arranged in a bow tie geometry. Employing nonlinear pulse propagation simulations, we numerically demonstrate the compression of 125 mJ, 1 ps pulses to 50 fs using a compact 2 m long setup and outline routes to extend our approach into the Joule-regime.