Research group of Prof. Dr. Stefan Mathias

Ultrafast Dynamics in Quantum Materials

In this research area we study optically induced electron and exciton dynamics in two-dimensional (2D) quantum materials. Typical materials are graphene, transition metal dichalcogenides, their heterostructures or hybrid structures of 2D materials and organic semiconductors. For our experiments we use our new setup for time-resolved momentum microscopy, which makes this kind of research possible. The questions in this area are very diverse, ranging from exciton dynamics and relaxation processes to exciton transport and the possible formation of new correlated phases.

In the field of strong light-matter coupling, we use time-resolved band-structure measurements to directly visualize optically induced new states. In recent years, we have focused on so-called 'Floquet engineering' in graphene and have recently been able to show that Floquet engineering is indeed possible in graphene. Building on this, we are now investigating other light-matter-coupled phases in various quantum materials.

In this project we develop the tomographic reconstruction of molecular orbitals from time-resolved momentum microscopy data. The method of molecular orbital tomography from ARPES data was discovered in 2009 and has been continuously developed since then. Together with our partners in mathematics, our research focuses on time-resolved three-dimensional orbital tomography and the extension of the technique to more complex ('three-dimensional') molecules, to electronic states in heterostructures, and the necessary reconstruction algorithms.

In the field of ultrafast magnetization dynamics, we investigate how spin structures, e.g. skyrmions, can be identified, controlled and modified by optical methods. We also study femtosecond magnetization dynamics and ultrafast spin transfer processes in magnetic heterostructures and ferromagnetic 2D quantum materials. In the field of strongly correlated electron systems, we investigate how laser excitation can control the material properties of magnetic oxides, i.e. so-called photo-induced phase transitions.

The extension of ultrafast angle-resolved photoelectron spectroscopy (ARPES) towards ultrafast momentum microscopy, enabling access to the 3D electronic structure dynamics of a material in a single measurement, currently revolutionizes dynamical band-structure mapping across various fields of condensed matter research. This technique has been developed by us and others in recent years, and we keep further developing and discovering new approaches and extensions of the method. Current projects involve ultrafast dark field techniques, measurements on gated sample structures, and space charge suppression modes.