Energy conversion in correlated electron materials
In materials with strong correlations, the interactions of spin, charge, and lattice determine the path of energy conversion after optical excitation. Depending on the dominant interaction, the deposited energy is directed into different forms of work inducing electronic, magnetic, and structural changes. Often, however, the main interaction that would be responsible for the pathway of energy flow after an excitation is hard to determine in thermal equilibrium. Here, ultrafast time-resolved spectroscopies are a powerful way to overcome this problem and to investigate non-equilibrium energy flow in correlated materials. In particular, time-resolved photoemission techniques are well suited, since they can follow in a direct manner the optically induced redistribution of charge carriers. Hence, ultrafast time-resolved mapping of the electron's energies, spins, and momenta after a strong optical excitation sheds light on band-structure formation, its relaxation to equilibrium, and the pathways of energy flow that are determined by the material's spin-charge-lattice interactions.
Ultrafast magnetization dynamics in nanostructures
The speed at which a magnetic state can be manipulated and, hence, data can be magnetically stored depends ultimately on the elementary spin-photon interaction, spin-scattering, and spin-transport processes. Until the mid-1990s, dynamics in magnetic systems were believed to occur on time scales of ~100 picoseconds or longer, determined by the interaction of the spins with the lattice. However, studies using femtosecond laser pulses starting from 1996 revealed the presence of other processes beyond this simple spin-lattice relaxation picture.
In this research field, we use novel methods based on the combination of coherent ultrafast X-ray pulses from laser-based high-harmonic generation with a variety of magneto-optical techniques. These combinations allow us to probe ultrafast spin dynamics with element-specificity and highest time-resolution. Highlights of our research are, e.g., elucidating the role of superdiffusive spin-currents in a femtosecond demagnetization process, and probing the timescale of the exchange interaction in a ferromagnetic alloy. Currently, we study cooperative effects of interacting magnetic subsystems in magnetic multilayers, alloys, and nanostructures.
Electron dynamics at interfaces
The idea in this research area is to study photo-stimulated electron dynamics after an optical excitation in real time. In particular, we are interested in the fate of excited electrons, i.e., their decay processes and their according ultrashort lifetimes. In general, the investigation of the dissipation of such "hot electrons" is of relevance, for instance, in femto-chemistry, spin-dynamics, for spin-injection processes or for energy conversion mechanisms. Since the lifetime of excited electrons does play a central role in all photo-stimulated processes, and also depends on a diverse range of physical parameters, our works extend from dynamics in quantum-well nanostructures to molecule/surface hybrid systems and topological materials with high spin-orbit coupling. As an experimental method to access the relevant ultrafast dynamical processes, we employ time-resolved two-photon photoemission spectroscopy. This real-time pump-probe technique is then combined with different photoemission methods which include angular- ("ARPES"), spin-, and/or real-space resolution.
Method development in ultrafast materials science
Rapid progress in ultrafast X-ray science worldwide, both in high-harmonic generation (HHG) and X-ray free electron laser (FEL) sources, has paved the way for a new generation of light-matter experiments investigating ultrafast electronic, magnetic, and structural dynamics in materials. Here, we developed in recent years several ultrafast material science experiments that are based on the use of table-top HHG lightsources. By the virtue of the short wavelength pulses produced by high-harmonic generation lightsources, we could show that HHG is an ideal probe for even the fastest dynamics in matter. Using elemental absorption edges, site-specific magnetic, electronic, structural, and chemical dynamics can be captured, providing unique capabilities for the study of complex emerging materials.
In our activities, the development of such novel tools for the study of ultrafast dynamics in materials is an integral part of our research.