Speaker
Description
In condensed matter systems, many dynamical processes driven by strong correlations and non-equilibrium effects occur on ultrashort timescales, ranging from femtoseconds to nanoseconds. The objective of this work is to establish a table-top setup capable of extracting detailed information about these processes in complex materials at ultrafast timescales. X-ray absorption spectroscopy (XAS) has proven to be a powerful technique for obtaining element-specific information of electronic and structural properties of such systems. Here, we present a table-top soft X-ray source based on high-harmonic generation (HHG) in noble gases, which serves as an ideal tool for performing time-resolved XAS experiments. A four-stage optical parametric chirped pulse amplifier (OPCPA) has been developed to deliver intense mid-infrared pulses at 1.5 µm and 3 µm, with broad spectral bandwidths supporting with ~35 fs pulse durations. By employing the 1.5 µm driving laser with ~50 fs duration and 1 mJ pulse energy, and helium as the high-harmonic generation (HHG) medium, photon energies extending up to 350 eV have been achieved. Complementary to this, a thin-disk multipass amplifier capable of producing 100 mJ output pulses is currently under development. Integration of this amplifier with the existing OPCPA setup will enable HHG driven by the 3 µm beam, thereby extending the accessible X-ray photon energy range upto 1 keV. Furthermore, a pump-probe configuration has been realized, utilizing optical pump pulses at 1.2 eV and 2.4 eV to excite materials, followed by ultrafast X-ray to probe the dynamic electronic and structural changes.
Our earlier XAS study at the B K-edge in hexagonal boron nitride (h-BN) revealed well-resolved spectral features attributed to electronic transitions into σ⋆ and π⋆ molecular orbitals [1]. Current work is focused on probing ultrafast electron-phonon coupling and energy relaxation dynamics in borophane (HB) and its different polymorphs [2]. Furthermore, the extented photon energy range will enable access to transition metal L-edges and the oxygen K-edge, significantly broadening the applicability of table-top pump-probe XAS to a wider class of condensed matter systems.
We acknowledge the financial support by the DFG through SFB 1242 (Project No. 278162697) and BO 1823/14–1.