Nonlinear optical coefficients of narrow-gap semiconductors

Nondegenerate four-wave mixing experiments have been performed at CO2 laser wavelengths on HgTe–CdTe superlattices and α-Sn1−xGex films grown by molecular beam epitaxy. In HgTe–CdTe, the magnitude, temperature dependence, and laser-intensity dependences of χ(3) are found to vary systematically with superlattice energy gap. A theoretical model for nonlinear optical coefficients in narrow-gap semiconductors gives excellent agreement with the experimental results when the nonlinearity is assumed to be due to optical modulation of the free carrier temperature. In the first investigation of α-Sn1−xGex as a nonlinear optical material, we find that the measured χ(3)’s are quite large (up to 8×10−5 esu), particularly in heavily strained films with compositions up to x=0.08. The mechanism responsible for the nonlinearity in that system is unclear at present.