Ultrafast measurements and modeling of electron relaxation in germanium nanoparticles

The ultrafast electron-relaxation dynamics in germanium nanoparticles is investigated by pump-probe measurements using different excitation energies, in order to characterize the electron-scattering processes. We demonstrate that these processes can be described using a model based on Ge bulk band structure. It is shown that the intervalley relaxation occurs on a picosecond time scale, while interband relaxation towards the L and X valleys occurs on a sub-150-fs time scale. Band-gap renormalization effects on the electron-relaxation dynamics have been carefully investigated, both experimentally and theoretically, in the framework of a simple effective-medium model.