DEGENERATE 4-WAVE-MIXING MEASUREMENTS OF HIGH-ORDER NONLINEARITIES IN SEMICONDUCTORS

We describe degenerate four-wave mixing experiments on ZnSe and CdTe semiconductor samples with pico-second laser pulses at wavelengths below the bandgap. Nonlinearities of third, fifth, and seventh order are observed and the mechanisms for each are identified. In all of our measurements, we observe a fast third order nonlinearity. For two-photon absorbers, this is attributed to contributions from both the real (refractive) and imaginary (absorptive) parts of the third-order susceptibility. Below the two-photon absorption edge, the nonlinearity is purely refractive. The higher order effects are due to carriers generated by multiphoton excitation. In ZnSe at 0.532-mu-m, carriers are generated by two-photon absorption such that a fifth order nonlinearity arises from the change in index due to these carriers, a sequential chi(3):chi(1) nonlinearity. From such measurements we determine the refractive index change per photoexcited carrier pair and the density dependence of the carrier diffusion coefficient. Analogous signals are observed in CdTe at 1.064-mu-m. The seventh order nonlinearity observed in ZnSe at 1.064-mu-m results from the refractive index contribution of carriers generated by three-photon absorption.