  Optical pump-THz probe spectroscopy is used to investigate the exciton
formation dynamics and its intensity dependence in bulk Ge. Associated with the
intra-excitonic 1s-2p transition, the gradual build-up of an absorption peak
around 3.1 meV (0.75 THz) signifies the 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.
