Novel, free-carrier-induced optical nonlinearities of narrow-gap semiconductors

The theory of free-carrier-induced optical non-linearity is reviewed. Examples of such mechanisms in narrow gap semiconductors are discussed. HgTe, HgMnTe and zero-gap HgCdTe are shown to have record, picosecond speed, optical non-linearities at 10.6 mu m. The largest occurs in Hg0.84Cd0.16Te, whose chi (3)=2*10-3 ESU at 80 K. The theoretical model suggests that these non-linearities are caused by laser-induced carrier temperature modulation, which produces large carrier density variations in zero-gap materials. The thermal processes have saturation power densities in the 100 kW cm-2-1 MW cm-2 range. At such intensities, the dielectric constant of HgTe is modulated by about 10%.