Ultrafast spectroscopy under high pressure
The development of femtosecond lasers has enabled direct insight into the dynamics of photoinduced phase transitions. Experiments of this type exploit so-called pump-probe schemes. The system under investigation is excited by an optical pump pulse which induces a transient non-equilibrium phase. A probe pulse delayed with respect to the pump pulse traces the evolution of the system during the ultrafast phase transition. Optical probing is extremely sensitive to the changes in the spectrum of the elementary excitations. Therefore, time-resolved optical spectroscopy has become a very efficient tool for the study of insulator-to-metal transitions, high-temperature superconductivity, and charge/spin-density-wave ordering where the optical response of the material is drastically modified in the photoexcited state.
Until now the absolute majority of time-resolved experiments were performed at ambient pressure. Mostly temperature and sometimes gating voltage is used to tune a system across a phase transition. However, variation of pressure offers an additional degree of freedom in the thermodynamic phase space of the system. The delicate balance of interactions in compounds with strong electronic correlations makes them extremely susceptible to lattice compression driven by external pressure.
In our group we focus on time-resolved spectroscopy under pressure of strongly correlated systems undergoing insulator-metals transitions such as VO2  and hematite (a-Fe2O3) as well as iron-based high-temperature superconductors (BaFe2As2, SrFe2As2, FeSe etc.) .
This research is supported by the German Research Foundation (DFG) via the projects PA 2113/1-1 and PA 2113/1-2.
 J. M. Braun, H. Schneider, M. Helm, R. Mirek, L. A. Boatner, R. E. Marvel, R. F. Haglund Jr, and A. Pashkin, Ultrafast response of photoexcited carriers in VO2 at high-pressure, New J. Phys. 20, 083003 (2018).
 I. Fotev, S. Winnerl, S. Aswartham, S. Wurmehl, B. Büchner, H. Schneider, M. Helm, and A. Pashkin, Ultrafast relaxation dynamics of spin density wave order in BaFe2As2 under high pressures, Phys. Rev. B 108, 035101 (2023).