Theory of resonance saturation of IR absorption in semiconductors with degenerate resonance bands in electric and magnetic fields

The influence of a weak electric and/or magnetic field on the coefficient of nonlinear IR absorption in semiconductors with degenerate valence bands (such as p-Ge) is considered. The absorption is due to vertical transitions of holes between valence-band subbands. At sufficiently high intensities this absorption saturates, decreasing with increase of intensity, owing to equalization of the distribution functions of the heavy and light holes at the vertical-transition point. It is shown that in the nonlinear region even a weak electric or magnetic field applied to the semiconductor increases the absorption coefficient (in proportion to the applied field) up to its linear value. There are two modes of resonance saturation of the absorption in an external field. The first is coherent, takes place at high IR intensities, and is accompanied by inversion of the distribution function in the resonance region. The second is incoherent, occurs in weaker fields and accordingly at lower radiation intensities