Ultrafast Optical Probing of Hypersonic Phonons in Nanoscale GeTe Films

Using time-resolved optical spectroscopy, we investigate the acoustic dynamics of polycrystalline germanium telluride (GeTe) nanoscale thin films, a multifunctional material combining phase-change behavior for nonvolatile memory and reconfigurable photonics with promising thermoelectric performance. We probe the generation and propagation of longitudinal acoustic waves with submicrometer wavelengths and frequencies in the range of tens of gigahertz. Our experimental approach integrates phonon echo measurements in thicker layers (∼360 nm) with the analysis of fundamental thickness-breathing modes in thinner layers (∼80 nm), enabling separation of intrinsic and interface contributions to attenuation. This allows the direct extraction of both intrinsic acoustic attenuation and longitudinal sound velocity in polycrystalline GeTe thin films. We determine mean free paths of approximately 0.7-7 μm for 10-20 GHz acoustic waves. These findings clarify hypersonic phonon dynamics in GeTe, establish a benchmark for high-frequency acoustic dissipation in chalcogenide thin films, and support the development of devices ranging from phase-change memory to thermoelectrics.