NONLINEAR OPTICAL COEFFICIENTS OF NARROW-GAP SEMICONDUCTORS

Nondegenerate four-wave mixing experiments have been performed at CO2laser wavelengths on HgTe-CdTe superlattices and a-Sn, xGexfilms grown by molecular beam epitaxy. In HgTe-CdTe, the magnitude, temperature dependence, and laser-intensity dependences 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 a-Sn, xGexas a nonlinear optical material, we find that the measured χ (3)s are quite large (up to 8x 10-5esu), 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. © 1990, American Vacuum Society. All rights reserved.