Difference-frequency variation of the free-carrier-induced, third-order nonlinear susceptibility in n-InSb

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Abstract
Free‐carrier‐induced optical nonlinearities were studied, via four‐wave mixing, in n‐InSb as a function of the frequency difference Δω, between two CO2 laser beams. The third‐order nonlinear susceptibility increases by more than an order of magnitude as Δω→0. This effect is ascribed to electron temperature fluctuations produced by the nonlinear optical interactions. An energy relaxation time of 3–4 ps is inferred for the highly excited electron gas. Studies of the frequency dependence of the nonlinear susceptibility may provide a new technique for measuring fast relaxation times in semiconductors.