Density-dependent exciton dynamics and L-valley anisotropy in germanium

Optical pump-THz probe spectroscopy is used to investigate the exciton formation dynamics and its intensity dependence in bulk Ge. Associated with the intraexcitonic 1s-2p transition, the gradual buildup of an absorption peak around 3.1 meV (0.75 THz) signifies a delayed exciton formation after optical pump which is accelerated for higher excitation densities. Analyzing the spectral shape of this THz absorption resonance, two distinct resonances are found which are attributed to the mass anisotropy of L-valley electrons via a microscopic theory.