Nonlinear far-infrared response of passive and active hole systems in p-Ge

The authors study the response of holes in germanium to intense radiation from far-infrared gas lasers in the range of 28 to 174 cm -1 . Two types of nonlinear response are evaluated: (i) the saturation of inter-valence-band absorption and (ii) the saturation of stimulated inter-valence-band emission. The effect (i) yields critical intensities I S between 10 and 1000 W cm -2 , depending also on temperature between 20 and 100 K; the saturation behaviour as well as the values of I S follow a model that treats the holes as inhomogeneously broadened two-level systems. In the presence of electric fields I S is found to increase drastically. They interpret this as a transit-time effect of rapidly moving carriers. The effect (ii) measures saturation in the presence of strong E perpendicular to B fields in an active p-Ge laser. In this case the transit-time broadening is extreme and imposes a fully homogeneous character to the transition. Conclusions are drawn on the relaxation mechanisms of low-energy holes as well as on the functioning of hot-hole lasers.