Peer-Review Publications

Spontaneous parametric down-conversion (SPDC) is widely used in quantum applications based on photonic entanglement. The efficiency of photon pair generation is often characterized by means of a sinc(L delta k/2) function, where L is the length of the nonlinear medium and delta k is the phase mismatch between the pump and down-converted fields. In theoretical investigations, the sinc behavior of the phase mismatch has often been approximated by a Gaussian function exp (-alpha x(2)) in order to derive analytical expressions for the SPDC process. Different values have been chosen in the literature for the optimization factor alpha, for instance, by comparing the widths of sinc and Gaussian functions or the momentum of down-converted photons. As a consequence, different values of alpha provide different theoretical predictions for the same setup. Therefore an informed and unique choice of this parameter is necessary. In this paper, we present a choice of alpha which maximizes the validity of the Gaussian approximation. Moreover, we also discuss the so-called super-Gaussian and cosine-Gaussian approximations as practical alternatives with improved predictive power for experiments.

The design and performances of a newly built electrostatic charge state analyzer constructed to act as a spectrometer for keV/u ions are reported. It consists of two 90 & LCIRC; curved electrodes enclosed by Matsuda electrodes. This setup was recently tested using Ar9+ and Ar12+ ion beams at an energy of 10 keV per charge unit. This spectrometer achieves a good separation of different charge states formed by electron capture processes during collisions between primary ions and the residual gas. Thanks to these first tests, we have identified up to three different background contributions on the detector that need to be reduced or suppressed.

A Raman-based label-free analytical method was developed to detect the antibiotic ciprofloxacin (CIP) in various pharmaceutical formulations in the presence of different matrices (e.g., ear drops, eye drops and infusion solutions). The Raman spectral analysis is performed for semiquantification in the case of a low background Raman signal, i.e., the signal originating from the excipient and carrier substance of the formulation does not interfere with the fingerprint spectrum of ciprofloxacin. In the case of a background spectrum rich in Raman modes originating from the excipient and carrier substance of the formulation, the pharmaceutical formulation is diluted 1:5000, and thus, the background signal is undetectable. Due to the high affinity of ciprofloxacin towards metallic surfaces, surface-enhanced Raman spectroscopy (SERS) is applied to allow for the sensitive detection of ciprofloxacin. Moreover, the developed measurement routine can be applied to monitor the degradation of the active component ciprofloxacin within the pharmaceutical formulation. The developed assay can therefore be extended to the pharmaceutical industry for quality control in assay applications or the preparation of individualized medicines.

Metastable states of ions can be sufficiently populated in absorbing and emitting astrophysical media, enabling spectroscopic plasma-density diagnostics. Long-lived states appear in many isoelectronic sequences with an even number of electrons, and can be fed at large rates by various photonic and electronic mechanisms. Here, we experimentally investigate beryllium-like and carbon-like ions of neon and iron that have been predicted to exhibit detectable features in astrophysical soft X-ray absorption spectra. An ion population generated and excited by electron impact is subjected to highly monochromatic X-rays from a synchrotron beamline, allowing us to identify K alpha transitions from metastable states. We compare their energies and natural line widths with state-of-the-art theory and benchmark level population calculations at electron densities of 10(10.5) cm(-3).

In this article, we report on the latest investigations and achievements in proton beam shaping with our laser-driven ion beamline at GSI Helmholtzzentrum fiir Schwerionenforschung GmbH. This beamline was realized within the framework of the Laser Ion Generation, Handling, and Transport (LIGHT) collaboration to study the combination of laser-driven ion beams with conventional accelerator components. At its current state, the ions are accelerated by the high-power laser PHELIX via target normal sheath acceleration, and two pulsed high-magnetic solenoids are used for energy selection, transport, and transverse focusing. In between the two solenoids, there is a rf cavity that gives the LIGHT beamline the capability to longitudinally manipulate and temporally compress ion bunches to sub-nanosecond durations. To get optimal results, the rf cavity has to be synchronized with the PHELIX laser and therefore a reliable measurement of the temporal ion beam profile is necessary. In the past, these measurements showed unexpected correlations between the temporal beam profile and the phase as well as the electric field strength of the cavity. In this article, we present a numerical simulation of the beam transport through the LIGHT beamline which explains this behavior by a beam filamentation. We also report on our latest experimental campaigns, in which we combined transverse and longitudinal focusing for the first time. This led to proton bunches with a peak intensity of (3.28 +/- 0.24) x 10(8) protons/ (ns mm(2)) at a central energy of (7.72 +/- 0.14) MeV. The intensity refers to a circle with a diameter of (1.38 +/- 0.02) mm that encloses 50% of the protons in the focal spot at the end of the beamline. The temporal bunch width at this position was (742 +/- 40) ps (FWHM).

In the present contribution, we use x-rays to monitor charge-induced chemical dynamics in the photoionized amino acid glycine with femtosecond time resolution. The outgoing photoelectron leaves behind the cation in a coherent superposition of quantum mechanical eigenstates. Delayed x-ray pulses track the induced coherence through resonant x-ray absorption that induces Auger decay. Temporal modulation of the Auger electron signal correlated with specific ions is observed, which is governed by the initial electronic coherence and subsequent vibronic coupling to nuclear degrees of freedom. In the time-resolved x-ray absorption measurement, we monitor the time-frequency spectra of the resulting many-body quantum wave packets for a period of 175 fs along different reaction coordinates. Our experiment proves that by measuring specific fragments associated with the glycine dication as a function of the pump-probe delay, one can selectively probe electronic coherences at early times associated with a few distinguishable components of the broad electronic wave packet created initially by the pump pulse in the cation. The corresponding coherent superpositions formed by subsets of electronic eigenstates and evolving along parallel dynamical pathways show different phases and time periods in the range of (-0.3 +/- 0.1 ) pi <= phi <= ( 0.1 +/- 0.2 ) pi and 18.2(-1.4)(+1.7) <= T <= 23.9(-1.1)(+1.2 )fs. Furthermore, for long delays, the data allow us to pinpoint the driving vibrational modes of chemical dynamics mediating charge-induced bond cleavage along different reaction coordinates. (C) 2022 Author(s).

The strong-field approximation (SFA) is a widely used theoretical framework that describes the process of high-order harmonic generation of atoms and molecules. Here, we propose a generalization of the dipole SFA towards weakly relativistic contributions to the laser-electron interaction. These weakly relativistic contributions are closely related to the spatial structure of the light field and imply a correction of the relativistic order 1/c. Within this generalized nondipole SFA (GN-SFA), we demonstrate how to obtain explicit results and discuss their physical aspects. This approach enables one to investigate the nondipole effects of linear polarized plane waves as well as the influence of structured light fields, such as twisted light, that have not yet been captured by the currently available models. In order to utilize our generalized nondipole SFA, we consider a linearly polarized plane wave and demonstrate the decrease of the harmonic yield that is directly related to the nondipole effects of the laser field. Furthermore, we discuss the GN-SFA with regard to other nondipole SFA approaches by determining their physical and mathematical context. Finally, the GN-SFA is a powerful theoretical framework that extends the nonrelativistic SFA rigorously to the weakly relativistic regime and therefore will be a useful model for further theoretical investigations.

High-order harmonic generation (HHG) in gases leads to short-pulse extreme ultraviolet (XUV) radiation that is useful in a number of applications, such as attosecond science and nanoscale imaging. However, this process depends on many parameters, and there is still no consensus on how to choose the target geometry to optimize the source efficiency. We review the physics of HHG with emphasis on the macroscopic aspects of the nonlinear interaction, discussing the influence of length of medium, pressure, and intensity of the driving laser on the HHG conversion efficiency. Efficient HHG can be realized over a large range of pressures and medium lengths, if these follow a certain hyperbolic equation. This explains the large versatility in gas target designs for efficient HHG and provides design guidance for future high-flux XUV sources.

We investigate the influence of the pump wavelength on the high-power amplification of large-mode area, thulium-doped fibers which are suitable for an ultrashort pulsed operation in the 2 mu m wavelength region. By pumping a standard, commercially available photonic crystal fiber in an amplifier configuration at 1692 nm, a slope efficiency of 80 % at an average output power of 60 W could be shown. With the help of simulations we investigate the effect of cross-relaxations on the efficiency and the thermal behavior. We extend our investigations to a rod-type, large-pitch fiber with very large mode area, which is exceptionally suited for high-energy ultrafast operation. Pumping at 1692 nm leads to a slope efficiency of 74% with a average output power of 67 W, instead of the 38 % slope efficiency obtained when pumping at 793 nm. These results pave the way to highly efficient 2 mu m fiber-based CPA systems.

We report on the first integration of novel magnetic microcalorimeter detectors (MMCs), developed within SPARC (Stored Particles Atomic Physics Research Collaboration), into the experimental environment of storage rings at GSI(6), Darmstadt, namely at the electron cooler of CRYRING@ESR. Two of these detector systems were positioned at the 0 degrees and 180 degrees view ports of the cooler section to obtain high-resolution x-ray spectra originating from a stored beam of hydrogen-like uranium interacting with the cooler electrons. While previous test measurements with microcalorimeters at the accelerator facility of GSI were conducted in the mode of well-established stand-alone operation, for the present experiment we implemented several notable modifications to exploit the full potential of this type of detector for precision x-ray spectroscopy of stored heavy ions. Among these are a new readout system compatible with the multi branch system data acquisition platform of GSI, the synchronization of a quasi-continuous energy calibration with the operation cycle of the accelerator facility, as well as the first exploitation of the maXs detectors time resolution to apply coincidence conditions for the detection of photons and charge-changed ions.

In this work, the first proof of the principal of an in situ diagnostics of the heavy-ion beam intensity distribution in irradiation of solid targets is proposed. In this scheme, x-ray fluorescence that occurs in the interaction of heavy-ions with target atoms is used for imaging purposes. The x-ray conversion to optical radiation and a transport-system was developed, and its first test was performed in experiments at the Universal Linear Accelerator in Darmstadt, Germany. The Au-beam intensity distribution on thin foils and Cu-mesh targets was imaged using multiple x-ray pinholes (polychromatic imaging) and 2D monochromatic imaging of Cu Kα radiation by using a toroidally bent silicon crystal. The presented results are of importance for application in experiments on the investigation of the equation of states of high energy density matter using high intensity GeV/u heavy-ion beams of ≥10^10 particles/100 ns.

The biharmonic (omega, 2 omega) photoionization of atomic inner-shell electrons opens up new perspectives for studying nonlinear light-atom interactions at intensities in the transition regime from weak to strong-field physics. In particular, the control of the frequency and polarization of biharmonic beams enables one to carve the photoelectron angular distribution and to enhance the resolution of ionization measurements by the (simultaneous) absorption of photons. Apart from its quite obvious polarization dependence, the photoelectron angular distributions are sensitive also to the (relative) intensity, the phase difference and the temporal structure of the incoming beam components, both at resonant and nonresonant frequencies. Here, we describe and analyze several characteristic features of biharmonic ionization in the framework of second-order perturbation theory and (so-called) ionization pathways, as they are readily derived from the interaction of inner-shell electrons with the electric-dipole field of the incident beam. We show how the photoelectron angular distribution and elliptical dichroism can be shaped in rather an unprecedented way by just tuning the properties of the biharmonic field. Since such fields are nowadays accessible from high-harmonic sources or free-electron lasers, these and further investigations might help extract photoionization amplitudes or the phase difference of incoming beams.

We show through simulation that the improved quantitative rescattering model (QRS) can successfully predict the nonsequential double ionization (NSDI) process by intense elliptically polarized laser pulses. Using the QRS model, we calculate the correlated two-electron and ion momentum distributions of NSDI in Ne exposed to intense elliptically polarized laser pulses with a wavelength of 788 nm at a peak intensity of 5.0 x 10(14) W/cm(2). We analyze the asymmetry in the doubly charged ion momentum spectra observed by Kang et al. [Phys. Rev. Lett. 120, 223204 (2018)] in going from linearly to elliptically polarized laser pulses. Our model reproduces the experimental data well. Furthermore, we find that the ellipticity-dependent asymmetry arises from the drift velocity along the minor axis of the elliptic polarization. We explain how the correlated electron momentum distributions along the minor axis provide access to the subcycle dynamics of recollision. From these findings, we expect that we can extend the QRS model for NSDI toward more complicated laser fields in the future.

Tailoring the properties of the driving laser to the need of applications often requires compromises among laser stability, high peak and average power levels, pulse duration, and spectral bandwidth. For instance, spectroscopy with optical frequency combs in the extreme/visible ultraviolet spectral region requires a high peak power of the near-IR driving laser, and therefore high average power, pulse duration of a few tens of fs, and maximal available spectral bandwidth. Contrarily, the parametric conversion efficiency is higher for pulses with a duration in the 100-fs range due to temporal walk-off and coating limitations. Here we suggest an approach to adjust the spectral characteristics of high-power chirped-pulse amplification (CPA) to the requirements of different nonlinear frequency converters while preserving the low-phase-noise (PN) properties of the system. To achieve spectral tunability, we installed a mechanical spectral shaper in a free-space section of the stretcher of an in-house-developed ytterbium-fiber-based CPA system. The CPA system delivers 100 W of average power at a repetition rate of 132.4 MHz. While gaining control over the spectral properties, we preserve the relative-intensity-noise and PN properties of the system. The high-power CPA can easily be adjusted to deliver either a spectrum ideal for mid-IR light generation (full width at half maximum of similar to 11 nm, compressed pulse duration of 230 fs) or a spectrum ideal for highly nonlinear processes such as high-harmonic generation (-10 dB level of >50 nm, transform-limited pulse duration of similar to 65 fs).

Strong-field atomic processes, driven by long-wavelength laser beams, are known to be affected by magnetic forces. In such beams, the Lorentz force pushes the photoelectrons along the beam direction and prevents their rescattering or recombination with the parent ions. In high-order harmonic generation (HHG), therefore, the yield of energetic photons is markedly suppressed, rendering x-ray radiation sources from high harmonics so far impractical. To compensate these magnetic forces and to reenable HHG at long wavelengths, a setup of two not quite collinear beams has been suggested recently but not much analyzed beyond classical arguments and with respect to accessible laser parameters. Using the nondipole strong-field approximation, we here investigate when the longitudinal momentum of the photoelectrons vanishes and how this noncollinear setup explicitly depends on the wavelength and intensity of the driving beams. We also demonstrate that an optimal crossing angle delta 0 between these beams always exists for which the fraction of the returning electrons is maximized. This rather simple steering of the longitudinal momentum will allow an efficient HHG with driving beams deep in the midinfrared.

We present a tabletop setup for extreme ultraviolet (EUV) reflection spectroscopy in the spectral range from 40 to 100 eV by using high-harmonic radiation. The simultaneous measurements of reference and sample spectra with high energy resolution provide precise and robust absolute reflectivity measurements, even when operating with spectrally fluctuating EUV sources. The stability and sensitivity of EUV reflectivity measurements are crucial factors for many applications in attosecond science, EUV spectroscopy, and nano-scale tomography. We show that the accuracy and stability of our in situ referencing scheme are almost one order of magnitude better in comparison to subsequent reference measurements. We demonstrate the performance of the setup by reflective near-edge x-ray absorption fine structure measurements of the aluminum L2/3 absorption edge in alpha-Al2O3 and compare the results to synchrotron measurements.

The interaction of intense light with matter gives rise to competing nonlinear responses that can dynamically change material properties. Prominent examples are saturable absorption (SA) and two-photon absorption (TPA), which dynamically increase and decrease the transmission of a sample depending on pulse intensity, respectively. The availability of intense soft X-ray pulses from free-electron lasers (FELs) has led to observations of SA and TPA in separate experiments, leaving open questions about the possible interplay between and relative strength of the two phenomena. Here, we systematically study both phenomena in one experiment by exposing graphite films to soft X-ray FEL pulses of varying intensity. By applying real-time electronic structure calculations, we find that for lower intensities the nonlinear contribution to the absorption is dominated by SA attributed to ground-state depletion; our model suggests that TPA becomes more dominant for larger intensities (>1014 W/cm(2)). Our results demonstrate an approach of general utility for interpreting FEL spectroscopies.

Light carrying time-varying orbital angular momentum (OAM) is a recently discovered type of structured electromagnetic field compared with a typical vortex field whose OAM is static. Such so-called self-torqued light is employed for manipulating the fast magnetic, topological, and quantum excitations and increasing its intensity and having access to shorter pulse durations would be of great benefit. Here we theoretically and numerically demonstrate the generation of intense self-torqued harmonics and attosecond pulses in the relativistic regime, driven by two time-delayed relativistic vortex lasers with different OAMs l1 and l2. The OAM of the nth harmonic spans nl1 to nl2, and the OAM of the attosecond pulses changes from l1 to l2. Such intense self-torqued harmonics and attosecond pulses may offer alternative possibilities in ultrafast spectroscopy.

High-harmonic generation (HHG) in solids has been touted as a way to probe ultrafast dynamics and crystal symmetries in condensed matter systems. Here, we investigate the polarization properties of highorder harmonics generated in monolayer MoS2, as a function of crystal orientation relative to the midinfrared laser field polarization. At several different laser wavelengths we experimentally observe a prominent angular shift of the parallel-polarized odd harmonics for energies above approximately 3.5 eV, and our calculations indicate that this shift originates in subtle differences in the recombination dipole strengths involving multiple conduction bands. This observation is material specific and is in addition to the angular dependence imposed by the dynamical symmetry properties of the crystal interacting with the laser field, and may pave the way for probing the vectorial character of multiband recombination dipoles.

The strong-field approximation (SFA) has been widely applied in the literature to model the ionization of atoms and molecules by intense laser pulses. A recent re-formulation of the SFA in terms of partial waves and spherical tensor operators helped adopt this approach to account for realistic atomic potentials and pulses of different shape and time structure. This re-formulation also enables one to overcome certain limitations of the original SFA formulation with regard to the representation of the initial-bound and final-continuum wave functions of the emitted electrons. We here show within the framework of JAC, the Jena Atomic Calculator, how the direct SFA ionization amplitude can be readily generated and utilized in order to compute above-threshold ionization (ATI) distributions for many-electron targets and laser pulses of given frequency, intensity, polarization, pulse duration and carrier-envelope phase. Examples are shown for selected ATI energy, angular as well as momentum distributions in the strong-field ionization of atomic krypton. We also briefly discuss how this approach can be extended to incorporate rescattering and high-harmonic processes into the SFA amplitudes.

Scattering of intense laser pulses on high-energy electron beams allows one to produce a large number of x and gamma rays. For temporally pulsed lasers, the resulting spectra are broadband which severely limits practical applications. One could use linearly chirped laser pulses to compensate for that broadening. We show for laser pulses chirped in the spectral domain that there is the optimal chirp parameter at which the spectra have the brightest peak. Additionally, we use catastrophe theory to analytically find this optimal chirp value.

Many applications of two-dimensional materials such as graphene require the encapsulation in bulk material. While a variety of methods exist for the structural and functional characterization of uncovered 2D materials, there is a need for methods that image encapsulated 2D materials as well as the surrounding matter. In this work, we use extreme ultraviolet coherence tomography to image graphene flakes buried beneath 200 nm of silicon. We show that we can identify mono-, bi-, and trilayers of graphene and quantify the thickness of the silicon bulk on top by measuring the depth-resolved reflectivity. Furthermore, we estimate the quality of the graphene interface by incorporating a model that includes the interface roughness. These results are verified by atomic force microscopy and prove that extreme ultraviolet coherence tomography is a suitable tool for imaging 2D materials embedded in bulk materials.

Quantum field theory predicts that the vacuum exhibits a nonlinear response to strong electromagnetic fields. This fundamental tenet has remained experimentally challenging and is yet to be tested in the laboratory. We present proof of concept and detailed theoretical analysis of an experimental setup for precision measurements of the quantum vacuum signal generated by the collision of a brilliant x-ray probe with a high-intensity pump laser. The signal features components polarized parallel and perpendicularly to the incident x-ray probe. Our proof-of-concept measurements show that the background can be efficiently suppressed by many orders of magnitude which should not only facilitate a detection of the perpendicularly polarized component of the nonlinear vacuum response, but even make the parallel polarized component experimentally accessible for the first time. Remarkably, the angular separation of the signal from the intense x-ray probe enables precision measurements even in the presence of pump fluctuations and alignment jitter. This provides direct access to the low-energy constants governing light-by-light scattering.