Picosecond infrared studies of hot holes in p-type Ge
Hot holes in p-type germanium are investigated via picosecond changes of the inter-valence band absorption in the mid-infrared. The transient absorption from the heavy to the light hole band monitored in spectrally and time resolved pump-probe experiments gives evidence of the subpicosecond inter-valence band scattering of photoexcited holes. This process is followed by the cooling of heavy holes on a time scale of several tens of picoseconds. Nonequilibrium split-off holes created by intense picosecond excitation give rise to a short-lived population inversion between optically coupled states in the split-off and the light hole band. A transient gain of 5 cm-1 is observed with probe pulses of a photon energy around 260 meV. The amplification is due to the very high generation rate of split-off holes, the different transient temperatures of split-off and light holes and the exceptionally strong radiative transition probability.