By use of an expansion in Gaussian functions we solve the problem of the propagation of a laser beam that has been passed through a thin slab of nonlinearly refracting material. We compare the theoretical results for beam profile with experimental measurements for the semiconductor InSb and confirm that self-defocusing has been observed by D. A. B. Miller, M. H. Mozolowski, A. Miller, and S. D. Smith [Opt. Commun. 27, 133 (1978)]. The deduced value for the nonlinear refractive index at ~5 K is -6 x 10(-5) cm(2) W(-1) at 1886 cm(-1). This implies a third-order optical nonlinearity chi((3)) ~ 10(-2) esu, much larger than any previously reported for a solid.