Scientific Motivation

Real-time investigations of dynamic interactions in many-body quantum systems are essential for understanding the microscopic origins of emergent phenomena in condensed matter physics. A major challenge in these studies arises from the strong coupling between electronic carriers and atomic nuclei, with dynamics spanning from attoseconds to picoseconds. The Ultrafast Core-Level Spectroscopy Group aims to uncover the fundamental processes governing light–matter interactions in those systems and investigate phenomena such as electronic and structural phase transitions, non-adiabatic coupling between electronic states, and Mott and excitonic physics.

Ultrafast Core-Level Spectroscopy

Core-level spectroscopy provides element- and orbital-specific sensitivity to the electronic structure of materials. When combined with ultrafast pump–probe techniques, it enables the investigation of nonequilibrium processes on their intrinsic timescales. By accessing K and L absorption edges in the soft X-ray range, ultrafast core-level spectroscopy allows direct tracking of transient changes in charge, spin, and bonding at specific atomic sites following photoexcitation. In this spectral region, the L edges of transition metals such as Ti, V, Cr, Mn, Fe, Co, Ni, and Cu probe transitions into partially filled 3d states, which are central to magnetism, electron correlations, and collective phenomena in quantum materials. At the same time, K edges of light elements including C, N, O, and F are highly sensitive to the local chemical and bonding environment, providing insight into charge redistribution and electron–lattice coupling. Combining element specificity with femtosecond-to-attosecond time resolution makes ultrafast core-level spectroscopy a powerful tool for disentangling coupled electronic and structural dynamics in complex materials.

Our expertise and capabilities

High-energy CEP-stable 2200 nm OPCPA

Traditionally, table-top soft X-ray attosecond beamlines have been driven by Ti:sapphire lasers combined with a TOPAS optical parametric amplifier, delivering short-wave infrared (SWIR) pulses with pulse energies limited to the millijoule range. This limitation restricts both the flux and the cutoff energy of the generated soft X-rays.

Recent advances in thin-disk Yb:YAG laser technology have opened new opportunities to significantly increase the energy of SWIR pulses and, consequently, the brightness of table-top soft X-ray beamlines. Here we use a Dira 200-1 thin-disk Yb:YAG laser operating at 1 kHz from TRUMPF Scientific Lasers, delivering 200 mJ pulses at 1030 nm, to pump our home-built 2200 nm CEP-stable optical parametric chirped-pulse amplifier (OPCPA). The 2200 nm CEP-stable OPCPA is designed to deliver 40 fs pulses with up to 20 mJ of energy at a repetition rate of 1 kHz. A key challenge is the compression of picosecond-duration pulses to the few-cycle regime in order to produce self-CEP-stable SWIR pulses. The nearly picosecond pulses produced by the thin-disk Yb:YAG laser require substantial nonlinear broadening, which can rapidly lead to instabilities such as pulse splitting.

To obtain robust CEP-stable SWIR pulses, the spectrum must therefore be broadened gradually through self-phase modulation. For this purpose, we employ krypton-filled antiresonant reflection photonic crystal fibres (ARR-PCFs) developed by our collaborators at the Max Planck Institute for the Science of Light. These fibres enable controlled spectral broadening while maintaining low loss and minimising spatio-temporal coupling. The krypton-filled ARR-PCFs facilitate controlled white-light generation at 1030 nm, followed by pulse compression using chirped mirrors to a duration of 9 fs. These pulses are ideally suited for generating a self-CEP-stable 2200 nm seed via intra-pulse difference frequency generation (IP-DFG). The CEP-offset of our 2200 nm pulses can be tuned by adjusting the insertion of standard fused silica wedges, with a measured CEP jitter of only 217 mrad—an essential requirement for driving next-generation attosecond soft X-ray sources. For more information, see our publication: A high-energy self-CEP-stable OPCPA seeder using a high-average-power picosecond narrowband Yb thin-disc laser.

Table-top Coherent Soft X-ray Beamline Capabilities

The table-top coherent soft X-ray beamline employs two highly efficient reflection zone plates (RZPs). The first RZP is curved to achieve high energy resolution, while the second RZP utilises the focusing zero-order diffraction from the first element. This configuration enables optimisation of specific spectral regions of interest while maintaining high efficiency and energy resolution, achieving spectrograph efficiencies of up to 5% at 800 eV and resolving powers above 1500. This setup allows in situ monitoring of different absorption edges and enables the detection of electron transport processes that require high energy resolution. Charge transfer processes can be observed as a decrease in signal at one absorption edge accompanied by a corresponding increase at another.