Saturation of Band-Tail Optical Absorption in InSb

The optical absorption in InSb at 5 K, in the region just below the band-gap energy, is investigated as a function of the incident intensity by using a continuous wave CO laser. The linear spectrum agrees with previous measurements but saturation is seen at intensities more than about 1 W/cm^2. In the region where the linear absorption has been ascribed to acceptor-to-conduction-band transitions, the detailed variation of absorption with intensity agrees well with a saturation model developed in this paper. Nearer to the band-gap, the absorption shows some saturation at much lower intensities, and a non-saturating absorption tail is also resolved; the implications for possible absorption mechanisms are discussed.