Unraveling energy flow mechanisms in semiconductors by ultrafast spectroscopy: Germanium as a case study

TL;DR

This study uses ultrafast spectroscopy to investigate energy transfer and dissipation mechanisms in germanium, revealing how optical phonons, acoustic phonons, and strain dynamics interact on sub-picosecond timescales.

Contribution

It provides new insights into ultrafast energy flow in semiconductors, combining experimental spectroscopy with theoretical simulations to elucidate phonon and strain dynamics.

Findings

Optical phonon temperature increases within a few picoseconds due to hole energy transfer.

Decay of optical phonons into acoustic phonons via anharmonic coupling.

Observation of Brillouin oscillations linked to strain pulse propagation.

Abstract

Semiconductor materials are the foundation of modern electronics, and their functionality is dictated by the interactions between fundamental excitations occurring on (sub-)picosecond timescales. Using time-resolved Raman spectroscopy and transient reflectivity measurements, we shed light on the ultrafast dynamics in germanium. We observe an increase in the optical phonon temperature in the first few picoseconds, driven by the energy transfer from photoexcited holes, and the subsequent decay into acoustic phonons through anharmonic coupling. Moreover, the temperature, Raman frequency, and linewidth of this phonon mode show strikingly different decay dynamics. This difference was ascribed to the local thermal strain generated by the ultrafast excitation. We also observe Brillouin oscillations, given by a strain pulse traveling through germanium, whose damping is correlated to the optical phonon mode. These findings, supported by density functional theory and molecular dynamics simulations, provide a better understanding of the energy dissipation mechanisms in semiconductors.