The ground state properties of complex materials as well as the reaction pathways of photo-excited states are governed by many-body phenomena and the mutual dependence of electron, spin and lattice subsystems. Ultrafast techniques provide access to these fundamental correlations as femtosecond light pulses allow for subsystem-specific excitations and time-resolved observation of the same or another subsystem’s response. In the limit of weak optical excitation, this concept may be seen as an experimental perturbative approach where correlations governing ground state properties are revealed by the system’s response to modest, but specific excitations. Employing intense ultrashort laser pulses, on the other hand, allows for the preparation of transient states of matter exhibiting strong non-equilibrium, in particular between electrons and lattice. The coupling of the subsystems of such states can be significantly different compared to the ground state and the induced photo-physical or photo-chemical reaction may follow a non-thermal reaction pathway not available in thermal equilibrium.
These concepts require the application of different, complementary probes revealing the temporal evolution of the different subsystems: femtosecond electron diffraction (FED) unfolds the ultrafast dynamics of the atomic structure and phonon populations, femtosecond time- and angle-resolved photoelectron spectroscopy (trARPES) reveals the evolution of the electronic structure, and femtosecond optical spectroscopy provides the optical response. We investigate correlations and photo-induced reactions by employing, and partially developing, these ultrafast techniques. As these phenomena inherently depend on the symmetry of the system, specifically on the dimensionality of the translational symmetry, we put an emphasis on the development of ultrafast probes highly sensitive to structural and electronic dynamics in two- and one-dimensional systems. In particular, we develop point-sources of femtosecond low-energy electron wave packets and use them for diffraction and point-projection microscopy.
Eine Übersicht unserer Forschungsaktivitäten in deutscher Sprache findet sich im MPG Jahrbuch 2015: Im Fokus ultrakurzer Laser: das Wechselspiel von Elektronen und Kristallstruktur.
- Structural dynamics of semiconducting transition metal dichalcogenides
- Electron-phonon interaction in quantum-confined systems
We study the microscopic energy flow within size-selected gold clusters and their coupling to carbon-based substrates.
- Phase change materials
Ultrafast electron dynamics in nanoscale materials
- Excited state dynamics and conduction band mapping of layered semiconductors
- Observation and control of electron motion in solids on the attosecond time scale
Attosecond photoelectron streaking: a time-resolved view on the photoelectric effect.