Peer-Review Publications

We report on the experimental realization of a compact, fiber-based, ultrashort-pulse laser system in the 2 μm wavelength region delivering 24 fs pulse duration with 24 MW pulse peak power and 24.6 W average power. This performance level has been enabled by the favorable quadratic wavelength-dependence of the self-focusing limit, which has been experimentally verified to be at approximately 24 MW for circular polarization in a solid-core fused-silica fiber operated at a wavelength around 2 μm. The anomalous dispersion in this wavelength region allows for a simultaneous nonlinear spectral broadening and temporal pulse compression. This makes an additional compression stage redundant and facilitates a very simple and power-scalable approach. Simulations that include both the nonlinear pulse evolution and the transverse optical Kerr effect support the experimental results.

The FOCAL experiment involves a highly accurate twin crystal spectrometer, designed for the measurement of the ground state Lamb shift of stored highly charged ions, like hydrogen-like Au 78+, via spectroscopy in the hard-x-ray regime with an accuracy down to the few-eV level where higher-order QED contributions become accessible. For this level of accuracy all geometrical parameters including the position of the x-ray source are of crucial importance. In this conference proceeding we present our efforts to characterize the internal gas target at the experiment storage ring at GSI Darmstadt where in 2012 the FOCAL experiment was conducted.

A brief review about topical developments in the exploitation of the resonant electron–ion collision process of dielectronic recombination (DR) as a sensitive spectroscopic tool is given. The focus will be on DR storage-ring experiments of few-electron highly charged ions. Among others, the questions addressed in these studies cover diverse topics from the areas of strong-field quantum electrodynamics, of lifetime studies using DR resonances, and of nuclear physics. Examples from the storage rings CRYRING in Stockholm, TSR in Heidelberg, and ESR in Darmstadt are given. In addition, an overview is provided about the ongoing developments and future perspectives of DR collision spectroscopy at the upcoming Facility for Antiproton and Ion Research in Darmstadt, Germany.

Metallic magnetic calorimeters are energy dispersive particle detectors that are operated at temperatures below 100 mK. Applied to x-ray spectroscopy they combine the high energy resolution of crystal spectrometers with the large energy bandwidth of semiconductor detectors. After the absorption of a photon its energy is converted into heat. A paramagnetic alloy converts the temperature change into a change of magnetization that is read out by a sensitive superconducting quantum interference device magnetometer. With such a metallic magnetic calorimeter we performed two successful measurements at the internal gas target of the experimental storage ring at GSI. In the first beamtime lithium-like Au-ions were targeted on a N2 and a Xe gas target, respectively. In the second beamtime we observed a projectile beam of bare Xe ions interacting with a Xe gas target. In both experiments we achieved an energy resolution below 60 eV from 0 keV to 60 keV . We were able to detect K-lines of Xe ions of different charge states, including the Lyman series up to Ly-η and could resolve the Ly-β-doublet in H-like Xe.

The coherent combination of ultra short laser pulses is a promising approach for scaling the average and peak power of ultrafast lasers. Fiber lasers and amplifiers are especially suited for this technique due to their simple singe-pass setups that can be easily parallelized. Here we propose the combination of the well-known approach of spatially separated amplification with the technique of divided-pulse amplification, i.e. an additionally performed temporally separated amplification. With the help of this multidimensional pulse stacking, laser systems come into reach capable of emitting 10’s of joules of energy at multi-kW average powers that simultaneously employ a manageable number of fibers.

The photoionization of H+2 molecular ions is investigated for Bessel beams of twisted light. In particular, the angle-differential photoionization cross sections are evaluated for a macroscopic target of randomly distributed but initially aligned ions by using the nonrelativistic first-order perturbation theory. Detailed calculations of these cross sections and angular distributions are performed for different setups of the electron detectors and for selected opening angles of the Bessel beams and are compared with those for incident plane-wave radiation. It is shown that the modification in the angular distributions of the photoelectrons can be understood quite easily from the variations in the intensity pattern of the Bessel beams, relative to the size of the H+2 molecular ions.

High-harmonic generation (HHG) has been established as an indispensable tool in optical spectroscopy. This effect arises for instance upon illumination of a noble gas with sub-picosecond laser pulses at focussed intensities significantly greater than 1012W/cm2. HHG provides a coherent light source in the extreme ultraviolet (XUV) spectral region, which is of importance in inner shell photo ionization of many atoms and molecules. Additionally, it intrinsically features light fields with unique temporal properties. Even in its simplest realization, XUV bursts of sub-femtosecond pulse lengths are released. More sophisticated schemes open the path to attosecond physics by offering single pulses of less than 100 attoseconds duration. Resonant optical antennas are important tools for coupling and enhancing electromagnetic fields on scales below their free-space wavelength. In a special application, placing field-enhancing plasmonic nano antennas at the interaction site of an HHG experiment has been claimed to boost local laser field strengths, from insufficient initial intensities to sufficient values. This was achieved with the use of arrays of bow-tie-shaped antennas of ∼ 100nm in length. However, the feasibility of this concept depends on the vulnerability of these nano-antennas to the still intense driving laser light.We show, by looking at a set of exemplary metallic structures, that the threshold fluence Fth of laser-induced damage (LID) is a greatly limiting factor for the proposed and tested schemes along these lines.We present our findings in the context of work done by other groups, giving an assessment of the feasibility and effectiveness of the proposed scheme.

We report linear polarization measurements of x rays emitted due to dielectronic recombination into highly charged krypton ions. The ions in the He-like through O-like charge states were populated in an electron-beam ion trap with the electron-beam energy adjusted to recombination resonances in order to produce Ka x rays. The x rays were detected with a newly developed Compton polarimeter using a beryllium scattering target and 12 silicon x-ray detector diodes sampling the azimuthal distribution of the scattered x rays. The extracted degrees of linear polarization of several dielectronic recombination transitions agree with results of relativistic distorted-wave calculations. We also demonstrate a high sensitivity of the polarization to the Breit interaction, which is remarkable for a medium-Z element like krypton. The experimental results can be used for polarization diagnostics of hot astrophysical and laboratory fusion plasmas.

One-loop quantum electrodynamic (QED) corrections are studied for two basic atomic processes, radiative recombination of an electron with a bare nucleus and radiative decay of a hydrogenlike ion. The perturbations of the bound-state wave function and the binding energy due to the electron self-energy and the vacuum polarization are computed in the Feynman and Coulomb gauges. QED corrections induced by these perturbations are calculated for the differential cross section and the polarization of the emitted radiation in the radiative recombination of an electron and a bare uranium nuclei, as well as the corresponding corrections to the ratio of the E1 (electric dipole) and M2 (magnetic quadrupole) transition amplitudes in the 2p3/2→1s radiative decay of hydrogenlike uranium. The results obtained indicate the expected magnitude of the QED effects in these processes on a subpercent level.

During the last decades femtosecond lasers have proven their vast benefit in both scientific and technological tasks. Nevertheless, one laser feature bearing the tremendous potential for high-field applications, delivering extremely high peak and average powers simultaneously, is still not accessible. This is the performance regime several upcoming applications such as laser particle acceleration require, and therefore, challenge laser technology to the fullest. On the one hand, some state-of-the-art canonical bulk amplifier systems provide pulse peak powers in the range of multi-terawatt to petawatt. On the other hand, concepts for advanced solid-state-lasers, specifically thin disk, slab or fiber systems have shown their capability of emitting high average powers in the kilowatt range with a high wall-plug-efficiency while maintaining an excellent spatial and temporal quality of the output beam. In this article, a brief introduction to a concept for a compact laser system capable of simultaneously providing high peak and average powers all along with a high wall-plug efficiency will be given. The concept relies on the stacking of a pulse train emitted from a high-repetitive femtosecond laser system in a passive enhancement cavity, also referred to as temporal coherent combining. In this manner, the repetition rate is decreased in favor of a pulse energy enhancement by the same factor while the average power is almost preserved. The key challenge of this concept is a fast, purely reflective switching element that allows for the dumping of the enhanced pulse out of the cavity. Addressing this challenge could, for the first time, allow for the highly efficient extraction of joule-class pulses at megawatt average power levels and thus lead to a whole new area of applications for ultra-fast laser systems.

The process of recombination of a quasifree electron into a bound state of an initially bare nucleus with the simultaneous creation of a bound-electron–free-positron pair is investigated. This process is called negative-continuum-assisted dielectronic recombination (NCDR). In a typical experimental setup, the initial electron is not free but bound in a light atomic target. In the present work, we study the effects of the atomic target on the single- and double-differential cross sections of positron production in the NCDR process. Calculations are performed within the relativistic framework based on QED theory, accounting for the electron-electron interaction to first order in perturbation theory. We demonstrate how the momentum distribution of the target electrons removes the nonphysical singularity of the differential cross section which occurs for the initially free and monochromatic electrons.

Birefringence is one of the fascinating properties of the vacuum of quantum electrodynamics (QED) in strong electromagnetic fields. The scattering of linearly polarized incident probe photons into a perpendicularly polarized mode provides a distinct signature of the optical activity of the quantum vacuum and thus offers an excellent opportunity for a precision test of nonlinear QED. Precision tests require accurate predictions and thus a theoretical framework that is capable of taking the detailed experimental geometry into account. We derive analytical solutions for vacuum birefringence which include the spatio-temporal field structure of a strong optical pump laser field and an x-ray probe. We show that the angular distribution of the scattered photons depends strongly on the interaction geometry and find that scattering of the perpendicularly polarized scattered photons out of the cone of the incident probe x-ray beam is the key to making the phenomenon experimentally accessible with the current generation of FEL/high-field laser facilities.

FAIR with its intense beams of ions and antiprotons provides outstanding and worldwide unique experimental conditions for extreme matter research in atomic and plasma physics and for application oriented research in biophysics, medical physics and materials science. The associated research programs comprise interaction of matter with highest electromagnetic fields, properties of plasmas and of solid matter under extreme pressure, density, and temperature conditions, simulation of galactic cosmic radiation, research in nanoscience and charged particle radiotherapy. A broad variety of APPA-dedicated facilities including experimental stations, storage rings, and traps, equipped with most sophisticated instrumentation will allow the APPA community to tackle new challenges. The worldwide most intense source of slow antiprotons will expand the scope of APPA related research to the exciting field of antimatter.

A theoretical analysis is presented for the elastic Rayleigh scattering of x-rays by many-electron atoms and ions. Special emphasis is placed on the angular distribution and linear polarization of the scattered photons for the case when the incident light is completely (linearly) polarized. Based on second-order perturbation theory and the independent particle approximation, we found that the Rayleigh angular distribution is strongly affected by the charge state and shell structure of the target ions or atoms. This effect can be observed experimentally at modern synchrotron facilities and might provide further insight into the structure of heavy atomic systems.

This work presents a direct measurement of the ⁹⁶Ru(p,γ)⁹⁷Rh cross section via a novel technique using a storage ring, which opens opportunities for reaction measurements on unstable nuclei. A proof-of-principle experiment was performed at the storage ring ESR at GSI in Darmstadt, where circulating ⁹⁶Ru ions interacted repeatedly with a hydrogen target. The ⁹⁶Ru(p,γ)⁹⁷Rh cross section between 9 and 11 MeV has been determined using two independent normalization methods. As key ingredients in Hauser-Feshbach calculations, the γ-ray strength function as well as the level density model can be pinned down with the measured (p,γ) cross section. Furthermore, the proton optical potential can be optimized after the uncertainties from the γ-ray strength function and the level density have been removed. As a result, a constrained ⁹⁶Ru(p,γ)⁹⁷Rh reaction rate over a wide temperature range is recommended for p-process network calculations.

The efficient generation of redshifted pulses from chirped femtosecond joule level Bessel beam pulses in gases is studied. The redshift spans from a few 100/cm to several 1000/cm corresponding to a shift of 50–500 nm for Nd:glass laser systems. The generated pulses have an almost perfect Gaussian beam profile insensitive of the pump beam profile, and are much shorter than the pump pulses. The highest measured energy is as high as 30 mJ, which is significantly higher than possible with solid state nonlinear frequency shifters.

The bound-bound transition from the 5d²6s² ³F₂e ground state to the 5d6s²6p ³D₁o excited state in negative lanthanum has been proposed as a candidate for laser cooling, which has not yet been achieved for negative ions. Anion laser cooling holds the potential to allow the production of ultracold ensembles of any negatively charged species. We have studied the aforementioned transition in a beam of negative La ions by high-resolution laser spectroscopy. The center-of-gravity frequency was measured to be 96.592 80(10) THz. Seven of the nine expected hyperfine structure transitions were resolved. The observed peaks were unambiguously assigned to the predicted hyperfine transitions by a fit, confirmed by multiconfigurational self-consistent field calculations. From the determined hyperfine structure we conclude that La⁻ is a promising laser cooling candidate. Using this transition, only three laser beams would be required to repump all hyperfine levels of the ground state.

The spectrum of the undulator radiation of beamline P01 at Petra III has been measured after passing a multiple reflection channel-cut polarimeter. Odd and even harmonics up to the 15th order, as well as Compton peaks which were produced by the high harmonics in the spectrum, could been measured. These additional contributions can have a tremendous influence on the performance of the polarimeter and have to be taken into account for further polarimeter designs.

The linear polarization of x-rays, emitted from highly-charged ions, has been studied within the framework of the density matrix theory and the multiconfiguration Dirac-Fock method. Emphasis was placed especially on two-photon cascades that proceed via intermediate overlapping resonances. For such two-step cascades, we here explore how the level-splitting of the resonances affects the linear polarization of the x-rays, and whether modifications in the degree of polarization may help determine small level-splittings in multiply- and highly-charged ions, if carefully analyzed along isoelectronic sequences. Detailed calculations are carried out for the 1s 2p2 J_i = 3/2 → 1s 2s 2p J = 1/2, 3/2 + γ1 → 1s2 2s J_f = 1/2 + γ1 + γ2 radiative cascade of lithium-like W^71+ ions. For this cascade, a quite remarkable increase of the (degree of) linear polarization is found for the second-step γ2 photons, if the level-splitting becomes smaller than Δω ≲ 0.2 a.u. ≈ 5.4 eV. Accurate polarization measurements of x-rays may therefore be also utilized in the future to ascertain small level-splittings in multiply- and highly-charged ions.

In this contribution, we analyze the effect of several preparation methods of Yb3+ doped alumino silicate glasses on their quantum efficiency by using photo-acoustic measurements in comparison to standard measurement methods including the determination via the fluorescence lifetime and an integrating sphere setup. The preparation methods focused on decreasing the OH concentration by means of fluorine-substitution and/or applying dry melting atmospheres, which led to an increase in the measured fluorescence lifetime. However, it was found that the influence of these methods on radiative properties such as the measured fluorescence lifetime alone does not per se give exact information about the actual quantum efficiency of the sample. The determination of the quantum efficiency by means of fluorescence lifetime shows inaccuracies when refractive index changing elements such as fluorine are incorporated into the glass. Since fluorine not only eliminates OH from the glass but also increases the “intrinsic” radiative fluorescence lifetime, which is needed to calculate the quantum efficiency, it is difficult to separate lifetime quenching from purely radiative effects. The approach used in this contribution offers a possibility to disentangle radiative from non-radiative properties which is not possible by using fluorescence lifetime measurements alone and allows an accurate determination of the quantum efficiency of a given sample. The comparative determination by an integrating sphere setup leads to the well-known problem of reabsorption which embodies itself in the measurement of too low quantum efficiencies, especially for samples with small quantum efficiencies.

Electron dynamics in the collision of an electron beam with a high-intensity focused ultrashort laser pulse are investigated using three-dimensional QED particle-in-cell (PIC) simulations, and the results are compared with those calculated by classical Landau and Lifshitz PIC simulations. Significant differences are observed from the angular dependence of the electron energy distribution patterns for the two different approaches, because photon emission is no longer well approximated by a continuous process in the quantum radiation-dominated regime. The stochastic nature of photon emission results in strong signatures of quantum radiation-reaction effects under certain conditions. We show that the laser spot size and duration greatly influence these signatures due to the competition of QED effects and the ponderomotive force, which is well described in the classical approximation. The clearest signatures of quantum radiation reaction are found in the limit of large laser spots and few cycle pulse durations.

Radiative recombination (RR) into the K shell and L subshells of U92+ ions interacting with cooling electrons has been studied in an x-ray RR experiment at the electron cooler of the Experimental Storage Ring at GSI. The measured radiative recombination rate coefficients for electron-ion relative energies in the range 0–1000 meV demonstrate the importance of relativistic effects. The observed asymmetry of the measured K-RR x-ray emission with respect to the cooling energy, i.e., zero average relative velocity (〈vrel〉=0), are explained by fully relativistic RR calculations. With our new approach, we show that the study of the angular distribution of RR photons for different relative energies opens new perspectives for detailed understanding of the RR of ions with cooling electrons in cold magnetized plasma.

A small hollow shell for trapping laser light is proposed. Two-dimensional particle-in-cell simulation shows that under appropriate laser and plasma conditions a part of the radiation fields of an intense short laser pulse can enter the cavity of a small shell through an over-critical density plasma in an adjacent guide channel and become trapped. The trapped light evolves into a circulating radial wave pattern until its energy is dissipated.

Dense cryogenic hydrogen is heated by intense femtosecond infrared laser pulses at intensities of 10^15-10^16 W cm−2. Three-dimensional particle-in-cell (PIC) simulations predict that this heating is limited to the skin depth, causing an inhomogeneously heated outer shell with a cold core and two prominent temperatures of about 25 and 40 eV for simulated delay times up to +70 fs after the laser pulse maximum. Experimentally, the time-integrated emitted bremsstrahlung in the spectral range of 8–18 nm was corrected for the wavelength-dependent instrument efficiency. The resulting spectrum cannot be fit with a single temperature bremsstrahlung model, and the best fit is obtained using two temperatures of about 13 and 30 eV. The lower temperatures in the experiment can be explained by missing energy-loss channels in the simulations, as well as the inclusion of hot, non-Maxwellian electrons in the temperature calculation. We resolved the time-scale for laser-heating of hydrogen, and PIC results for laser–matter interaction were successfully tested against the experiment data.

The necessity for compact table-top x-ray sources with higher brightness, shorter wavelength and shorter pulse duration has led to the development of complementary sources based on laser-plasma accelerators, in contrast to conventional accelerators. Relativistic interaction of short-pulse lasers with underdense plasmas results in acceleration of electrons and in consequence in the emission of spatially coherent radiation, which is known in the literature as betatron radiation. In this article, we report on our recent results in the rapidly developing field of secondary x-ray radiation generated by high-energy electron pulses. The betatron radiation is characterized with a novel setup allowing to measure the energy, the spatial energy distribution in the far-field of the beam and the source size in a single laser shot. Furthermore, the polarization state is measured for each laser shot. In this way, the emitted betatron x-rays can be used as a non-invasive diagnostic tool to retrieve very subtle information of the electron dynamics within the plasma wave. Parallel to the experimental work, 3D particle-in-cell simulations were performed, proved to be in good agreement with the experimental results.