OPTICAL THIRD-ORDER MIXING IN GAAS GE SI AND INAS

Nonlinear optical difference mixing of CO2 laser radiation is studied in the semiconductors GaAs, Ge, Si, and InAs. The fourth-rank electric susceptibility tensor receivers independent contributions from the bound or valence electrons, χb, and, in n-type material, from the conduction electrons χn. These two contributions are separated and measured in GaAs. χb is found to be anisotropic and χn to be isotropic for carrier concentrations n<~5×1016. The relative signs of the susceptibilities are determined. χb in Ge and GaAs and χn in GaAs and InAs all have the same sign for the particular frequency combination studied. Theoretical and experimental evidence indicate that this sign is positive. At room temperature, χn in GaAs is a liner function of n for n<~1016. The value of the slope <χn <n is a direct measure of the nonparabolicity of the conduction band in GaAs. It is shown to be inconsistent with Kane's model for small direct-band-gap semiconductors, and in agreement with Cardona's indirect measurements of the nonparabolicity in GaAs. © 1969 The American Physical Society.