Infrared saturation spectroscopy in p-type germanium

We investigate spectral changes induced by high-power CO
2
laser radiation in the direct intervalence band absorption of p-type germanium. Using a model of inhomogeneously broadened two-level systems, we deduce a room-temperature saturation intensity of 4 MW/ cm
²
from published observations of saturation of this absorption. The applicability of the two-level model to transitions between two electronic bands is discussed. We relate the linear absorption coefficient to the saturation intensity and the effective population difference, where the latter is obtained in a spherical band approximation. The result yields a value of 0.55 ps for the geometric mean of the phase and energy relaxation times. The meaning of these times is explained for our system, and quantitative agreement is found with published data on optical and acoustical lattice scattering probabilities. Specifically, we find a very short phase relaxation time of 0.09 ps. This allows the prediction of a burnt-hole spectral width of 57 cm
^-1
(HWHM) at small saturation. In a preliminary experiment, we have observed the modulation of a CW CO
²
laser beam by intense nanosecond pulses of a second CO
²
laser offset in frequency by up to 140 cm
^-1
.