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Ultrafast lattice dynamics

The group has substantial experience in analyzing the ultrafast response of crystalline thin films and nanostructures to laser excitation. Although the ultrafast motion associated with optical phonons is encoded in the intensity oscillations of x-ray diffraction signals, we emphasize the analysis of the transient strain via shifts and shape variations of the Bragg reflections. As a related technique, we have developed a broadband time-domain Brillouin scattering approach. This research direction is embedded in the community of scientists studying picosecond ultrasonics and surface acoustic waves. The control of ultrafast acoustics is highly relevant for, e.g., controling ultrafast magnetism. A special focus of the research on lattice dynamics lies on the propagation of strain wave packets, which may exhibit nonlinear phenomena such as second harmonic generation of phonons as analogs of nonlinear optics for light fields.  The anharmonicity  that governs nonlinear propagation is also relevant for thermal expansion and thermal transport. A very recent direction is the inversion of strain waves via ultrafast contractive stresses, which are typically induced by entropy changes of the spins or a subset of phonons, as predicted by negative thermal expansion (NTE) in equilibrium. We have studied various metallic and semiconducting systems with an emphasis on oxides.

Collage showing the shaping and x-ray/optical detection of hypersound wavepackets using laser pulse sequences (left) and a sketch of the relevance of electronic, phononic and spin degrees of freedom for the total laser-induced stress in a material (right)
Photo: Marc Herzog