127 126 Principal Scientist Profiles Ralf Röhlsberger Ralf Röhlsberger Principal Scientist Profiles PROFESSOR FOR X-RAY SCIENCE, INSTITUTE OF OPTICS AND QUANTUM ELECTRONICS AND HELMHOLTZ INSTITUTE JENA Prof. Röhlsberger is member of the directorate of the Helmholtz Institute Jena and head of the x-ray Science Group at this institute. He also leads a research group on Magnetism and Coherent Phenomena at the Deutsches Elektronen-Synchrotron DESY in Hamburg, and he is member of the Cluster of Excellence CUI – AIM (Centre for Ultrafast Imaging – Advanced Imaging of Matter) at the University of Hamburg as well as a principal investigator in the SFB-TRR QuCoLiMa (Quantum Cooperativity of Light and Matter) of the DFG. Moreover, he currently is the chair of the International Board on the Applications of the Mössbauer Effect. RALF RÖHLSBERGER RESEARCH AREAS Prof. Röhlsberger’s research is focused on the interaction of x-rays with matter to reveal its fundamental aspects and to develop new applications of x-ray scattering and spectroscopy with accelerator driven sources of hard X-rays. In particular these are: • Development of photonic nanostructures for controlling the interaction of x-rays and matter • Pump-probe experiments to study the non-equilibrium dynamics of matter • Applications of high-purity polarimetry for nuclear quantum optics and materials science • Experiments towards establishing a nuclear clock based on the isotope Scandium-45 TEACHING FIELDS Prof. Röhlsberger’s teaching activities are focused on the applications of highly brilliant x-rays from synchrotrons and x-ray lasers in solid state physics, materials science and quantum optics. He gives lectures and seminars in: • Introduction to modern x-ray physics and advanced x-ray spectroscopies with synchrotron radiation • Fundamentals and applications of Mössbauer spectroscopy in the natural sciences RESEARCH METHODS Prof. Röhlsberger’s research methods comprise laboratorybased methods for preparation and characterization of multilayers and thin films, as well as the use of advanced instrumentation and x-ray scattering methods at synchrotrons and x-ray lasers. • Development and applications of high-purity polarimeters at synchrotrons and x-ray lasers • Laser pump – x-ray probe spectrometers • Advanced sputter deposition setup for preparation of new functional materials • High-resolution x-ray diffractometry for characterization of thin films and multilayers RECENT RESEARCH RESULTS Polarization analysis of x-rays bears an enormous potential for fundamental studies of anisotropies in nature. We have recently achieved record extinction ratios of 10-10 that allow for probing tiniest optical acitivies, surpassing any kind of polarimetry in the optical regime. At these extreme levels of polarization purities, it is indispensable to distinguish between the contributions of dichroism and birefringence to the total optical activity. In a recent study we have demonstrated for the first time how x-ray dichroism and x-ray birefringence can be disentangled via high-purity polarimetry. Both effects play an important role in fundamental studies of condensed matter, especially in correlated materials. A striking example are spectroscopic measurements in the vicinity of atomic absorption edges to reveal the electronic occupancy of selected orbitals, e.g., in materials like CuO and La2CuO4, that are highly relevant as mother compounds for high-Tc superconductors [1]. An important branch of our group is to employ nuclear resonances of Mössbauer isotopes in materials science as well as in quantum optics at energies of hard x-rays. In the latter field we could recently realize a photonic control scheme for quantum systems embedded in a solid-state host that works at room temperature. Efficient control schemes for such systems are one of the major goals of contemporary condensed matter quantum technologies. Our scheme contrasts with established laser control schemes. Its striking feature is that it relies on the excitation of a solid’s quasi-particle to tune the interactions of the solid with the quantum system to achieve a precise quantum phase control of the scattered x-rays. This constitutes a completely new approach in the field of photonic quantum technologies. We could demonstrate the coherent manipulation of a collective nuclear excitation and show that this new control scheme does not destroy its coherence, a prime prerequisite for quantum technologies [2]. In another recent experiment we could achieve coherent control of atomic nuclei via suitably shaped near-resonant x-ray fields. By tuning the phase between two x-ray pulses we could switch the nuclear exciton dynamics between coherent enhanced excitation and coherent enhanced emission. This was facilitated by shaping single pulses delivered by state-ofthe-art x-ray facilities into tunable double pulses, for which we could demonstrate a temporal stability of the phase control on the few-zeptosecond timescale [3]. Our results unlock coherent optical control for nuclei, which should not only advance nuclear quantum optics, but also enable time-resolved studies of nuclear out-of-equilibrium dynamics, which is a longstanding challenge in Mössbauer science. RESONANT X-RAY EXCITATION OF THE NUCLEAR CLOCK ISOMER SCANDIUM-45 Resonant oscillators with stable frequencies and large quality factors help us to keep track of time with high precision. The search for more stable and convenient reference oscillators is continuing. Nuclear oscillators have significant advantages over atomic oscillators because of their naturally higher quality factors and their higher resilience against external perturbations. One of the most promising cases is an ultra-narrow (1.4 femto-eV) nuclear resonance transition in the isomer Scandium-45 between the ground state and the 12.4-keV isomeric state with a long lifetime of 0.47 s. High-brightness x-ray sources for direct excitation of the resonance have become available only recently. In a recent experiment we could excite the Scandium-45 isomeric state by irradiation with 12.4-keV photon pulses from a state-of-the-art x-ray free-electron laser, the European XFEL near Hamburg. This enabled us to determine the transition energy as 12,389.59 eV with an uncertainty that is two orders of magnitude smaller than the previously known values [4]. Our findings now open applications of this isomer in extreme metrology, nuclear clock technology, ultra-high-precision spectroscopy and similar fields. [1] Schmitt et al., Optica 8, 56 – 61 (2021). [2] Bocklage et al., Science Advances 7, eabc3991 (2021). [3] Heeg et al, Nature 590, 401 – 404 (2021). [4] Shvyd’ko et al., Nature 622, 471 – 475 (2023). Contact: Phone: + 49 3641 9-47900 Email: ralf.roehlsberger@uni-jena.de
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