Peer-Review Publications

We have conceived, built, and operated a cryogenic Penning trap with an electrically conducting yet optically transparent solid electrode. The trap, dedicated to spectroscopy and imaging of confined particles under large solid angles, is of “half-open” design with one open endcap and one closed endcap that mainly consists of a glass window coated with a highly transparent conductive layer. This arrangement allows for the trapping of externally or internally produced particles and yields flexible access for optical excitation and efficient light collection from the trapping region. At the same time, it is electrically closed and ensures long-term ion confinement under well-defined conditions. With its superior surface quality and its high as well as homogeneous optical transmission, the window electrode is an excellent replacement for partially transmissive electrodes that use holes, slits, metallic meshes, and the like.

In this article, we provide analytical expressions for the photon polarization tensor in pulsed Hermite- and Laguerre-Gaussian laser beams. Our results are based on a locally constant field approximation of the one-loop Heisenberg-Euler effective Lagrangian for quantum electrodynamics. Hence, by construction they are limited to slowly varying electromagnetic fields, varying on spatial and temporal scales significantly larger than the Compton wavelength/time of the electron. The latter criterion is fulfilled by all laser beams currently available in the laboratory. Our findings will, e.g., be relevant for the study of vacuum birefringence experienced by probe photons brought into collision with a high-intensity laser pulse which can be represented as a superposition of either Hermite- or Laguerre-Gaussian modes.

The possibility of utilizing plasma undulators and plasma accelerators to produce compact ultraviolet and X-ray sources, has attracted considerable interest for a few decades. This interest has been driven by the great potential to decrease the threshold for accessing such sources, which are mainly provided by a few dedicated large-scale synchrotron or free-electron laser (FEL) facilities. However, the broad radiation bandwidth of such plasma devices limits the source brightness and makes it difficult for the FEL instability to develop. Here, using multi-dimensional particle-in-cell (PIC) simulations, we demonstrate that a plasma undulator generated by the beating of a mixture of high-order laser modes propagating inside a plasma channel, leads to a few percent radiation bandwidth. The strength of the undulator can reach unity, the period can be less than a millimeter, and the number of undulator periods can be significantly increased by a phase locking technique based on the longitudinal tapering. Polarization control of such an undulator can be achieved by appropriately choosing the phase of the modes. According to our results, in the fully beam loaded regime, the electron current in the plasma undulator can reach 0.3âEUR%0kA level, making such an undulator a potential candidate towards a table-top FEL.

Ultrafast supercontinuum generation in gas-filled waveguides is an enabling technology for many intriguing applications ranging from attosecond metrology towards biophotonics, with the amount of spectral broadening crucially depending on the pulse dispersion of the propagating mode. In this study, we show that structural resonances in a gas-filled antiresonant hollow core optical fiber provide an additional degree of freedom in dispersion engineering, which enables the generation of more than three octaves of broadband light that ranges from deep UV wavelengths to near infrared. Our observation relies on the introduction of a geometric-induced resonance in the spectral vicinity of the ultrafast pump laser, outperforming gas dispersion and yielding a unique dispersion profile independent of core size, which is highly relevant for scaling input powers. Using a krypton-filled fiber, we observe spectral broadening from 200 nm to 1.7 μm at an output energy of ∼ 23 μJ within a single optical mode across the entire spectral bandwidth. Simulations show that the frequency generation results from an accelerated fission process of soliton-like waveforms in a non-adiabatic dispersion regime associated with the emission of multiple phase-matched Cherenkov radiations on both sides of the resonance. This effect, along with the dispersion tuning and scaling capabilities of the fiber geometry, enables coherent ultra-broadband and high-energy sources, which range from the UV to the mid‐infrared spectral range.

A high-precision, single-shot, and real-time carrier-envelope phase (CEP) measurement at 1.8 μm laser wavelength based on stereographic photoelectron spectroscopy is presented. A precision of the CEP measurement of 120 mrad for each and every individual laser shot for a 1 kHz pulse train with randomly varying CEP is demonstrated. Simultaneous to the CEP measurement, the pulse lengths are characterized by evaluating the spatial asymmetry of the measured above-threshold ionization (ATI) spectra of xenon and referenced to a standard pulse-duration measurement based on frequency-resolved optical gating. The validity of the CEP measurement is confirmed by implementing phase tagging for a CEP-dependent measurement of ATI in xenon with high energy resolution.

Semiconductor nanowire (NW) lasers attract a lot of attention as potential elements of nanophotonic circuits and lab-on-a chip devices. Here, we report on the experimental investigation of stimulated near ultraviolet (NUV) emission, pumped by three-photon absorption from near infrared femtosecond laser pulses, from ZnO NW arrays of different morphologies and compare it to the bulk. The spectrally and temporally resolved measurements of the NUV emission show both strong enhancements in the absorption and emission properties of the nanowire arrays compared to bulk samples. Thus, we determine a many times higher three-photon absorption in the nanostructure morphology compared to the bulk material. Furthermore, the threshold pumping intensity for stimulated emission in a vertically oriented nanowire array is twice lower and the emission onset time is shorter than in randomly oriented arrays, revealing strong influence of the macroscopic nanowire arrangement.

The hyperfine coupling constants of neutron deficient 53Fe were deduced from the atomic hyperfine spectrum of the 3d^6 4s^2 ^5D_4 <-> 3d^6 4s 4p ^5F_5 transition, measured using the bunched-beam collinear laser spectroscopy technique. The low-energy 53Fe beam was produced by projectile-fragmentation reactions followed by gas stopping, and used for the first time for laser spectroscopy. Ground state magnetic-dipole and electric-quadrupole moments were determined as μ = -0.65(1)μ_N and Q = +35(15)e^2 fm^2, respectively. The multiconfiguration Dirac-Fock method was used to calculate the electric field gradient to deduce Q from the quadrupole hyperfine coupling constant, since the quadrupole coupling constant has not been determined for any Fe isotopes. Both experimental values agree well with nuclear shell model calculations using the GXPF1A effective interaction performed in a full fp shell model space, which support the soft nature of the 56Ni nucleus.

We investigate the impact of operators of higher canonical dimension on the lower Higgs-mass consistency bound by means of generalized Higgs—Yukawa interactions. Analogously to higher-order operators in the bare Higgs potential in an effective field theory approach, the inclusion of higher-order Yukawa interactions, e.g., ϕ^3ψ^¯ψ, leads to a diminishing of the lower Higgs-mass bound and thus to a shift of the scale of new physics towards larger scales by a few orders of magnitude without introducing a metastability in the effective Higgs potential. We observe that similar renormalization group mechanisms near the weak-coupling fixed point are at work in both generalizations of the microscopic action. Thus, a combination of higher-dimensional operators with generalized Higgs as well as Yukawa interactions does not lead to an additive shift of the lower mass bound, but it relaxes the consistency bounds found recently only slightly. On the method side, we clarify the convergence properties of different projection and expansion schemes for the Yukawa potential used in the functional renormalization group literature so far.

The in-gas laser ionization and spectroscopy (IGLIS) technique was applied on the 212-215Ac isotopes, produced at the Leuven Isotope Separator On-Line (LISOL) facility by using the in-gas-cell and the in-gas-jet methods. The first application under on-line conditions of the in-gas-jet laser spectroscopy method showed a superior performance in terms of selectivity, spectral resolution, and efficiency in comparison with the in-gas-cell method. Following the analysis of both experiments, the magnetic-dipole moments for the 212-215Ac isotopes, electric-quadrupole moments and nuclear spins for the 214,215Ac isotopes are presented and discussed. A good agreement is obtained with large-scale nuclear shell-model calculations by using a 208Pb core.

A novel practical and efficient way of obtaining intense attosecond pulses is proposed, where the near-critical-density (NCD) plasma target satisfying n₀/a₀n꜀ ≈ 1 is used. The unique interaction dynamics in NCD plasmas have been identified theoretically and by particle-in-cell simulations, which show that three distinct dense electron nanobunches are formed each half a laser cycle and two of them can induce intense attosecond pulses in respectively the reflected and the transmitted directions by the so-called “coherent synchrotron emission” (CSE) mechanism [experimentally confirmed in Nat. Phys. 8, 804 (2012)]. Comparing with CSE in solids, here not only the required stringent conditions on laser and target are relaxed, but also the radiation intensities are enhanced by two orders of magnitude. It is shown that relativistically intense attosecond X-ray pulses with intensity 10¹⁹ W/cm² and duration ~50 as can be robustly obtained in both directions by currently available driving lasers at intensities of 10²⁰ W/cm².

The Noble Prize in physics 2016 motivates one to study different aspects of topological properties and topological defects as their related objects. Considering the significant role of the topological defects (especially magnetic strings) in cosmology, here, we will investigate three dimensional horizonless magnetic solutions in the presence of two generalizations: massive gravity and nonlinear electromagnetic field. The effects of these two generalizations on properties of the solutions and their geometrical structure are investigated. The differences between de Sitter and anti de Sitter solutions are highlighted and conditions regarding the existence of phase transition in geometrical structure of the solutions are studied.

The spatial evolution of the electric field of focused broadband light is crucial for many emerging attosecond technologies. Here the effects of the input beam parameters on the evolution of few-cycle laser pulses in the focus are discussed. Specifically, we detail how the frequency-dependent input beam geometry, chirp and chromatic aberration can affect the spatial dependence of the carrier-envelope phase (CEP), central frequency and pulse duration in the focus. These effects are confirmed by a direct, three-dimensional measurement of the CEP-evolution in the focus of a typical few-cycle pulse laser using electron rescattering at metal nanotips in combination with a CEP-metre. Moreover, we demonstrate a simple measurement technique to estimate the focal CEP evolution by input-beam parameters. These parameters can be used in novel ways in order to control attosecond dynamics and tailor highly nonlinear light–matter interactions.

In strong-field ionization processes, two-color laser fields are frequently used for controlling sub-cycle electron dynamics via the relative phase of the laser fields. Here we apply this technique to velocity map imaging spectroscopy using an unconventional orientation with the polarization of the ionizing laser field perpendicular to the detector surface and the steering field parallel to it. This geometry allows not only to image the phase-dependent photoelectron momentum distribution (PMD) of low-energy electrons that interact only weakly with the ion (direct electrons), but also to investigate the low yield of higher-energy rescattered electrons. Phase-dependent measurements of the PMD of neon and xenon demonstrate control over direct and rescattered electrons. The results are compared with semi-classical calculations in three dimensions including elastic scattering at different orders of return and with solutions of the three-dimensional time-dependent Schrödinger equation.

We investigate the photoionization of atoms by attosecond X waves carrying orbital angular momentum in the presence of a strong, linearly polarized, near infrared (NIR) laser pulse. In the plane-wave case, the streaking of photoelectrons by the NIR pulse has been used to characterize the ionizing pulse. In contrast to plane-wave pulses, X waves have a spatially dependent temporal profile, which modifies the ionization process. Here we explore theoretically the influence of this complex pulse structure on the streaking of photoelectrons for both localized and macroscopically extended targets. On the basis of the strong-field approximation, we find that the streaking spectra of localized targets sensitively depend on the opening angle of the X wave and the position of the atomic target relative to the beam axis. For macroscopically extended targets, we find that the streaking spectra do not depend on the parameters characterizing the twist of the X wave.

The binding energies of the ground 1s2 2s2 2p1/2 and first excited 1s2 2s2 2p3/2 states in boronlike argon are rigorously evaluated. The calculations are performed by the QED perturbation theory in the framework of the extended Furry picture taking into account all the relevant first- and second-order radiative and correlation corrections. The third- and higher-order interelectronic-interaction effects are considered within the Breit approximation. The relativistic nuclear recoil effect is taken into account. In comparison with the previous calculations of the binding energies in boronlike argon the accuracy of the theoretical predictions has been significantly improved.

Angular distribution and linear polarization of photon emission following dielectronic recombination of initially lithium-like ions with spin-polarized electrons are studied. In particular, a general expression is derived for the alignment parameter of the doubly excited states produced via the resonant capture of spin-polarized electrons. By means of the alignment parameter, moreover, the angular distribution and linear polarization of the subsequently emitted photons are further obtained. Detailed computations are performed for the 1s2 2s J0=1/2+εe-→1s2s2 2p1/2 J=1→1s2 2s2 Jf=0+γ resonant electron capture and subsequent radiative decay of iodine ions. It is found that the spin polarization of the incident electrons changes only the q=±1 components of the alignment parameter A2q. As a consequence, the electron spin polarization contributes weakly to the γ photon angular distribution and linear polarization that are dominantly determined by the A20 parameter.

An effective computational method is developed for electronic-structure calculations in few-electron atoms and ions on the basis of the Dirac-Coulomb-Breit Hamiltonian. The recursive formulation of the perturbation theory provides an efficient access to the higher-order contributions of the interelectronic interaction. Application of the presented approach to the binding energies of lithiumlike and boronlike systems is demonstrated. The results obtained are in agreement with the large-scale configuration interaction Dirac-Fock-Sturm method and other all-order calculations.

Abstract We investigate the dielectronic recombination (DR) of an electron and a highly-charged ion with non-zero nuclear spin. We assume that the incident electron is captured into doubly-excited 1s2κκ′J=0,1,2 levels of Be-like ions just above of its autoionization threshold. The angular distribution of the subsequent radiative emission is investigated especially for its dependence upon the nuclear spin and the nuclear magnetic moment. While the hyperfine and even the fine-structure of the ions cannot be resolved in typical DR experiments, we found the photon angular distribution, following the decay of the 1s2 2p3/2nsJ=1,2 DR resonance very sensitive to the nuclear parameters.

In this work, we present a pilot experiment in the experimental storage ring (ESR) at GSI devoted to impact parameter sensitive studies of inner shell atomic processes for low-energy (heavy-) ion-atom collisions. The experiment was performed with bare and He-like xenon ions (Xe54+, Xe52+) colliding with neutral xenon gas atoms, resulting in a symmetric collision system. This choice of the projectile charge states was made in order to compare the effect of a filled K-shell with the empty one. The projectile and target X-rays have been measured at different observation angles for all impact parameters as well as for the impact parameter range of ∼35–70 fm.

Non-perturbative calculations of the relativistic quantum dynamics of electrons in the Bi83+-Xe collisions at 70 AMeV are performed. A method of calculation employs an independent particle model with effective single-electron Dirac-Kohn-Sham operator. Solving of the single-electron equations is based on the coupled-channel approach with atomic-like Dirac-Sturm-Fock orbitals, localized at the ions (atoms). Special attention is paid to the inner-shell processes. Intensities of the K satellite and hypersatellite target radiation are evaluated. The role of the relativistic effects is studied.

We model the multiple ionization of near-neutral core-excited atoms where a cascade of Auger processes leads to the emission of several electrons. We utilize the multiconfiguration Dirac-Fock (MCDF) method to generate approximate wave functions for all fine-structure levels and to account for all decays between them. This approach allows to compute electron spectra, the population of final-states and ion yields, that are accessible in many experiments. Furthermore, our approach is based on the configuration interaction method. A careful treatment of correlation between electronic configurations enables one to model three-electron processes such as an Auger decay that is accompanied by an additional shake-up transition. Here, this model is applied to the triple ionization of atomic cadmium, where we show that the decay of inner-shell 4p holes to triply-charged final states is purely due to the shake-up transition of valence 5s electrons.

Absolute cross-sections for m -fold photoionization (m=1, ... , 6 ) of Fe+ by a single photon were measured employing the photon–ion merged-beams setup PIPE at the PETRA III synchrotron light source, operated by DESY in Hamburg, Germany. Photon energies were in the range 680–920 eV, which covers the photoionization resonances associated with 2p and 2s excitation to higher atomic shells as well as the thresholds for 2p and 2s ionization. The corresponding resonance positions were measured with an uncertainty of ±0.2 eV. The cross-section for Fe+ photoabsorption is derived as the sum of the individually measured cross-sections for m -fold ionization. Calculations of the Fe+ absorption cross-sections were carried out using two different theoretical approaches, Hartree–Fock including relativistic extensions and fully relativistic multiconfiguration Dirac–Fock. Apart from overall energy shifts of up to about 3 eV, the theoretical cross-sections are in good agreement with each other and with the experimental results. In addition, the complex de-excitation cascades after the creation of inner-shell holes in the Fe+ ion were tracked on the atomic fine-structure level. The corresponding theoretical results for the product charge-state distributions are in much better agreement with the experimental data than previously published configuration-average results. The present experimental and theoretical results are valuable for opacity calculations and are expected to pave the way to a more accurate determination of the iron abundance in the interstellar medium.

High-average power laser sources delivering intense few-cycle pulses in wavelength regions beyond the near infrared are promising tools for driving the next generation of high-flux strong-field experiments. In this work, we report on nonlinear pulse compression to 34.4 μJ-, 2.1-cycle pulses with 1.4 GW peak power at a central wavelength of 1.82 μm and an average power of 43 W. This performance level was enabled by the combination of a high-repetition-rate ultrafast thulium-doped fiber laser system and a gas-filled antiresonant hollow-core fiber.

The relativistic treatment of the nuclear magnetic shielding effect in boronlike ions is presented. The leading-order contribution of the magnetic-dipole hyperfine interaction is calculated. Along with the standard second-order perturbation theory expression, the solutions of the Dirac equation in the presence of magnetic field are employed. All methods are found to be in agreement with each other and with the previous calculations for hydrogenlike and lithiumlike ions. The effective screening potential is used to account approximately for the interelectronic interaction.

A method for calculation of electron-positron pair production in low-energy heavy-ion collisions beyond the monopole approximation is presented. The method is based on the numerical solving of the time-dependent Dirac equation with the full two-center potential. The one-electron wave functions are expanded in the finite basis set constructed on the two-dimensional spatial grid. Employing the developed approach the probabilities of bound-free pair production are calculated for collisions of bare uranium nuclei at the energy near the Coulomb barrier. The obtained results are compared with the corresponding values calculated in the monopole approximation.