Direct free-hole absorption induced in germanium by 1.06 µm picosecond pulses

The intervalence-band absorption coefficient in germanium is calculated as a function of the hole density and temperature for light with a wavelength of 1.06 μm. At this wavelength, the direct free-hole transitions in germanium occur between the heavy-and split-off hole bands, and between the light- and split-off hole bands. This absorption mechanism is normally negligible compared to the interband absorption; however, for a sufficiently dense electron-hole plasma, the free-hole intervalence-band absorption can become comparable to the inter-band absorption. Furthermore, the interaction of the intense 1.06 μm light alters the distribution of resonantly coupled hole states, and leads to an intensity dependence in the intervalence-band absorption cross section. For a fixed hole density, the intervalence-band cross sections due to free-hole transitions between the heavy- and split-off hole bands and between the light- and split-off hole bands are found to decrease with increasing intensity in a manner closely approximated by an inhomogeneously broadened two-level model. Values for the saturation intensity of each resonant transition are calculated as a function of the hole density and temperature. The intensities required to begin to saturate the direct free-hole transitions are easily attainable using picosecond pulses.