Stryland, Determination of bound-electronic and free-carrier nonlinearities in

We extend the application of the Z-scan experimental technique to determine free-carrier nonlinearities in the presence of bound electronic refraction and two-photon absorption. We employ this method, using picosecond pulses in CdTe, GaAs, and ZnTe at 1.06 m and in ZnSe at 1.06 and 0.53 Am, to measure the refractive-index change induced by two-photon-excited free carriers (coefficient ar,), the two-photon absorption coefficient 1, and the bound electronic nonlinear refractive index n2. The real and imaginary parts of the third-order susceptibility (i.e., n2 and 13, respectively) are determined by Z scans with low inputs, and the refraction from carriers generated by two-photon absorption (an effecitve fifth-order nonlinearity) is determined from Z scans with higher input energies. We compare our experimental results with theoretical models and deduce that the three measured parameters are well predicted by simple two-band models. n2 changes from positive to nega-tive as the photon energy approaches the band edge, in accordance with a recent theory of the dispersion of n2 in solids based on Kramers-Kronig transformations [Phys. Rev. Lett. 65, 96 (1990); IEEE J. Quantum Elec-tron. 27, 1296 (1991)]. We find that the values of 0-r are in agreement with simple band-filling models. I