LARGE, WEAKLY SATURATING THIRD-ORDER NONLINEAR SUSCEPTIBILITIES IN SEMIMETALS AND NARROW-GAP SEMICONDUCTORS

We review recent experimental and theoretical work on optical nonlinearities in semimetals and narrow-gap semiconductors. A strong nonlinear response due to optical modulation of the free carrier susceptibility is seen to be a common feature of these materials, with large third-order nonlinear susceptibilities being reported for Hg 1−x Cd x Te , Hg 1−x Mn x Te , Hg-based superlattices, α- Sn 1−x Ge x, Pb 1−x Sn x Se , and Bi 1−x Sb x. Effects of differences in the various band structures are discussed for nonlinearities due to nonparabolicity, thermal carrier generation, and nonequilibrium carrier generation. Mechanisms leading to either partial or severe saturation are also considered. Optimized nonlinearities for the various mechanisms and materials are critically compared, and promising future directions for the field are suggested.