3 Ultrafast and Nonlinear Phenomena: Condensed Phase

In this research area, studies of ultrafast photo-induced dynamics of elementary excitations, electronic structure and atomic rearrangements in the condensed phase are complemented by research on the spin degree of freedom. The combined view on electrons, spin, and lattice and their coupling allows for new insight, exploiting complementary information available through spectroscopy, scattering and imaging with femtosecond soft and hard x-ray pulses. Moreover, the strength of light-matter interactions is varied from the perturbative to the nonperturbative regime, the latter being particularly interesting for elucudating field-driven processes in the quantum-kinetic limit. MBI experiments include the ability to map the transient redistribution of charge and magnetization in solids, as well as the impact of coherently driven lattice motions on the microscopic and macroscopic electronic properties.  In future work, multi-method investigations will be exploited to study various relevant aspects of prototype systems on the one hand and enable moving to more complex materials on the other hand. Solids with correlated charges and spins, electron transfer systems, extended spin textures as well as materials where local excitations, phase transitions and mesoscale order determine functionality are addressed. In the area of functional materials, this has required to set up instruments for the growth of magnetic materials with a tailored response to light pulses, as well as micro- and nanostructuring capabilities.

The studies on the correlated spatio-temporal response after selective excitation are closely related to work on field-induced processes in solids, where charge transport is driven by strong THz and/or mid-infrared electric fields and where the regimes of quantum-coherent vs. diffusive transport are explored. Understanding the basic processes behind decoherence of charge motions and energy dissipation has direct impact on clarifying the mechanisms governing changes of spatial electron distributions and irreversible charge transfer processes. Such insight may result in new strategies for driving structural dynamics in the electronic ground state of a material by tailored strong fields. Analogously, concepts to drive coherent magnetization dynamics via the magnetic field component of THz pulses and decoherence mechanisms will be investigated in the next years.

Fluctuating structures and interactions in biomolecular systems and photoinduced chemical processes are an essential part of the research in this topical area.  Basic hydration processes of biomolecules, the role of an aqueous environment within functional biomolecular processes and structures are studied with methods of nonlinear multidimensional spectroscopy in the mid- and far-infrared spectral range. The impact of electric interactions, e.g., with solvated ions, on the structure of biomolecules and their role in folding processes represents a major research topic. Femtosecond x-ray spectroscopy is applied in order to follow molecular structure changes in photoinduced chemical processes.