Peer-Review Publications

We combine a recent construction of a Becchi-Rouet-Stora-Tyutin (BRST)-invariant, nonlinear massive gauge fixing with the background field formalism. The resulting generating functional preserves background -field invariance as well as BRST invariance of the quantum field manifestly. The construction features BRST-invariant mass parameters for the quantum gauge and ghost fields. The formalism employs a background Nakanishi-Lautrup field which is part of the nonlinear gauge-fixing sector and thus should not affect observables. We verify this expectation by computing the one-loop effective action and the beta function of the gauge coupling as an example. The corresponding Schwinger functional generating connected correlation functions acquires additional one-particle reducible terms that vanish on shell. We also study off-shell one -loop contributions in order to explore the consequences of a nonlinear gauge fixing scheme involving a background Nakanishi-Lautrup field. As an application, we show that our formalism straightforwardly accommodates nonperturbative information about propagators in the Landau gauge in the form of the so-called decoupling solution. Using this nonperturbative input, we find evidence for the formation of a gluon condensate for sufficiently large coupling, whose scale is set by the BRST-invariant gluon mass parameter.

We present a theoretical investigation of the elastic Rayleigh scattering of X-rays by atomic targets. Special attention is paid to the question of how the polarization of the scattered photons is affected if the incident light is itself polarized. In particular, we found that the circular polarization of the incoming X-rays may lead to a remarkable modification of the linear polarization of the scattered photons. Based on this \textasciigrave circular-to-linear-polarization-transfer\textquotesingle and on the fact that the linear polarization of X-rays can be conveniently observed by solid-state Compton detectors, we argue that Rayleigh scattering may be used as a tool for circular polarimetry of hard X-rays. To illustrate our proposal, we performed detailed calculations of 145 and 500 keV circularly polarized photons scattered by lead atoms. Based on these calculations, we found that the photon scattering under large angles with respect to the incident beam direction is most favorable for the circular polarimetry of hard X-rays. In particular, for 500 keV photon energy and scattering angles around 70 deg we found a remarkable modification of the linear polarization of scattered light for the case when the incident radiation is circularly polarized.

We probe the energy-loss enhancement in preequilibration and the charge-equilibration time for highly charged Xe ions (qin = 15-26) at near-Bohr velocity impinging on an Al target by measuring the dependence of the Xe L-shell x-ray yield on the initial charge state of the ions. The present results are found to be very consistent with those obtained in ion-transmission thin-foil experiments. The charge-equilibrium time is measured in the near-Bohr velocity region of highly charged ions in solids. Moreover, for the present collision system it is demonstrated by measuring the inner-shell x rays that the ionization of the inner-shell electrons of the projectile ions starts to contribute to ion energy loss at near-Bohr velocity.

The low-energy heavy ion storage ring CRYRING was transported from Stockholm to Darmstadt, modernized and reconfigured, and recommissioned as CRYRING@ESR. The machine is now in operation with all installations in service and is available as a user facility for experiments proposed through the SPARC collaboration. During the 2020-2022 period, we brought a number of experimental installations into service and used them to measure first data: the ultra-cold electron cooler for merged-beam electron-ion collisions, the gas jet target for atomic collisions, a next-generation microcalorimeter-based X-ray spectroscopy setup, and others. Ions can be injected either in low charge states from a local ion source through a 300 keV/u RFQ linac, or in high charge states from the GSI accelerator chain through ESR. This allows for very broad access to ions across the entire periodic table. CRYRING@ESR is able to de- or accelerate ions and cool and store beams of isotopically pure species in a desired charge state. While the analysis is still largely ongoing, the first experimental data already show that the machine reached its expected performance level, and our high expectations regarding achievable resolution in spectroscopy experiments have been fulfilled. With access to new classes of ions available through ESR injection and a new generation of experimental instrumentation, CRYRING@ESR is a unique facility for experiments with heavy, highly charged ions. Here, we will review our present setup and machine performance, discuss the data from our first commissioning experiments and briefly preview the upcoming new installations for the coming years.

Spontaneous parametric down-conversion (SPDC) is a widely used source for photonic entanglement. Years of focused research have led to a solid understanding of the process, but a cohesive analytical description of the paraxial biphoton state has yet to be achieved. We derive a general expression for the spatio-temporal biphoton state that applies universally across common experimental settings and correctly describes the nonseparability of spatial and spectral modes. We formulate a criterion on how to decrease the coupling of the spatial from the spectral degree of freedom by taking into account the Gouy phase of interacting beams. This work provides new insights into the role of the Gouy phase in SPDC, and also into the preparation of engineered entangled states for multidimensional quantum information processing.

This article studies the impact of mechanical deformations on the performance of a coaxial-type cryogenic current comparator (CCC). Such deformations may become a concern as the size of the CCC increases (e.g., when used as a diagnostic device in a particle accelerator facility involving beamlines with a large diameter). In addition to static deformations, this article also discusses the effect of mechanical vibrations on the CCC performance.

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.