INTRINSIC AND INDUCED ANISOTROPY OF NONLINEAR ABSORPTION AND REFRACTION IN ZINC BLENDE SEMICONDUCTORS

An ellipsometric picosecond pump-probe technique is used to measure the birefringence and dichroism induced in the zinc blende semiconductors ZnSe, GaAs, and CdTe by an intense linearly polarized pump pulse at 950 nm. We show that the induced birefringence and dichroism depend strongly on sample orientation (i.e., they are anisotropic). Furthermore, we demonstrate that by measuring the induced birefringence and dichroism for the pump polarized along the [100] and [110] crystal axes we can determine the sign and the magnitude of the intrinsic anisotropy both in the bound-electronic nonlinear refractive index and in the two-photon absorption coefficient; and, finally, we can extract the anisotropy in both the real and the imaginary parts of the product of the anisotropy parameter sigma and the diagonal element of chi((3)), the third-order susceptibility tenser. The latter product is a measure of the intrinsic anisotropy of the material. We find that the induced birefringence varies by roughly a factor of 2 with sample orientation in all three materials. The induced birefringence is, however, roughly an order of magnitude larger in size and opposite in sign for excitation above half the band gap (in GaAs and CdTe) than it is for excitation below half the band gap (in ZnSe). As expected, no dichroism is observed in ZnSe, but it is large in GaAs and CdTe, and, as with the birefringence, the dichroism varies by roughly a factor of 2 with crystal orientation. In order to test the effect of the measured anisotropy on device performance, we construct a simple on-off optical switch that exploits the measured birefringence and dichroism. Figures of merit are defined, and switch performance is investigated as a function of crystal orientation for excitation above and below the two-photon resonance (i.e., half the band gap). The figures of merit are shown to be extremely sensitive to crystal anisotropies for excitation below half the band gap and less so for excitation above half the band gap.