Unraveling Energy Flow Mechanisms in Semiconductors by Ultrafast Spectroscopy: Germanium as a Case Study

A multi‐technique pump‐probe setup capable of performing both transient reflectivity and time‐resolved Raman spectroscopy after ultrafast excitation, provides a comprehensive understanding of ultrafast electronic and phononic processes. Distinct decay rates for the temperature, frequency, and linewidth of the phonon mode in germanium are observed, along with the propagation of coherent acoustic phonon oscillations. 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, the ultrafast dynamics in germanium are elucidated. An increase in the optical phonon temperature is observed 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 is ascribed to the local thermal strain generated by the ultrafast excitation. Brillouin oscillations are also observed, 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. A multi-technique pump-probe setup capable of performing both transient reflectivity and time-resolved Raman spectroscopy after ultrafast excitation, provides a comprehensive understanding of ultrafast electronic and phononic processes. Distinct decay rates for the temperature, frequency, and linewidth of the phonon mode in germanium are observed, along with the propagation of coherent acoustic phonon oscillations. 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, the ultrafast dynamics in germanium are elucidated. An increase in the optical phonon temperature is observed 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 is ascribed to the local thermal strain generated by the ultrafast excitation. Brillouin oscillations are also observed, 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. Advanced Science, EarlyView.