We study interand intra-valence-band relaxation of germanium, both experimentally and theoretically, by saturation spectroscopy. Far-infrared laser pulses with intensities between 1 and 10' W/cm are applied to saturate direct heavy-hole-to-light-hole transitions. The characteristic saturation intensity I, is measured for a range of frequencies (28 -174 cm ') and temperatures (20-100 K) and found to vary over two orders of magnitude: l, increases approximately linearly with frequency; a minimum is observed at 30 K. This complex behavior is consistent with a model of inhomogeneously broadened twolevel 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 T& =43 ps and T2 = 1.5 ps, for energy and phase relaxation, respectively, at 31.2 cm ' and 40 K.

We study the response of holes in germanium to intense radiation from far-infrared gas lasers, in the range of 28 to 174cm". Two types of nonlinear response are evaluated: (i) the saturation of inter-valence-band absorption and ( i i ) the saturation of stimulated inter-valence-band emission.The effect (i) yields critical intensites I, between 10 and 1000Wcm-2, depending also on temperature between 20 and 100 K; the saturation behaviour as well as the values of /s follow a model that treats the holes a s inhomogeneously broadened two-level systems. In the presence of electric fields /$ is found to increase drastically. We interpret this as a transit-time effect of rapidly moving carriers. The effect (ii) measures saturation in the presence of strong E I S 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.

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We study the dynamics of an electrically pumped p-Ge laser with frequency fixed to 100 cm−1 by a novel selective cavity. The growing laser power increases the pump current long before it reaches a saturating power level, an effect explained by photoionization. We further find that the laser pulse can be delayed or even quenched by injected off-resonance far-infrared radiation. This effect not only gives a gain coefficient of 0.01 cm−1 and a saturation power of 400 W−which confirms the high-power capability of the laser−but furthermore reveals a novel quasihomogeneous broadening of the gain spectrum caused by transit-time effects of rapidly moving holes.