DYNAMICS OF LOW-ENERGY HOLES IN GERMANIUM

We study inter- and intra-valence-band relaxation of germanium, both experimentally and theoretically, by saturation spectroscopy. Far-infrared laser pulses with intensities between 1 and 10(5) W/cm2 are applied to saturate direct heavy-hole-to-light-hole transitions. The characteristic saturation intensity I(s) is measured for a range of frequencies (28-174 cm-1) and temperatures (20-100 K) and found to vary over two orders of magnitude: I(s) increases approximately linearly with frequency; a minimum is observed at 30 K. This complex behavior is consistent with a model of inhomogeneously broadened two-level systems that takes explicit account of, and thus quantifies, the various scattering contributions from phonons, impurities, and holes. The theory predicts a saturation-induced dip in the absorption spectrum, which is also experimentally observed and yields the dynamical time constants T1 =43 ps and T2 = 1.5 ps, for energy and phase relaxation, respectively, at 31.2 cm-1 and 40 K.