Time-Resolved Probing of Coherent Acoustic Phonons and Surface Waves

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Contents of the Talk

This presentation explores the generation, detection, and coherent control of Coherent Acoustic Phonons (CAPs) and Surface Acoustic Waves (SAWs) in functional oxide heterostructures using ultrafast X-ray and optical techniques.

We begin by introducing the fundamental mechanism of photoacoustic excitation, where femtosecond laser pulses are absorbed in a thin transducer layer, leading to rapid thermal expansion and the launch of coherent strain waves into the material. These strain pulses—CAPs—propagate through the lattice at the speed of sound and can be used as dynamic probes of structural and electronic properties on ultrafast timescales.

To investigate these phenomena, we employ time-resolved X-ray diffraction (TR-XRD) and laser-generated transient gratings (TGs). The TG technique enables spatially periodic excitation, allowing for the selective generation of acoustic modes such as SAWs and Skimming Surface Longitudinal Waves (SSLWs). By tuning the spatial phase and temporal delay between excitation pulses, we demonstrate spatiotemporal coherent control of surface distortions, enabling precise manipulation of acoustic waveforms and frequencies.

Our experiments focus on oxide systems including SrTiO₃, LaSrMnO₃, and KaNaNbO₃, where the interplay between lattice dynamics and functional properties is particularly rich. We show how CAPs and SAWs can be tailored to probe and influence phase transitions, strain coupling, and interfacial effects.

Together, these studies demonstrate the power of ultrafast X-ray and optical methods to probe and control lattice dynamics, opening new avenues for phononic devices, dynamic material design, and strain-based functionality in complex materials.

Short CV

Daniel Schmidt was born in 1997 in Hamburg, Germany. He received a bachelor's degree in Nanoscience from the University of Hamburg in 2018 with the topic of "Optical generation of surface acoustic waves". In 2021 he earned his master's degree in Nanoscience from the University of Hamburg in the group of Peter Gaal. In the same year he founded a start-up company named TXproducts and started working on his Ph.D. degree in the group of Robert Blick (University Hamburg) in cooperation with the Leibniz-Institut für Kristallzüchtung – DESY Joint-Lab with the topic of ultrafast active X-ray optics.

© Photos: Daniel Schmidt