Refractive-index measurements and nonlinear moment parameters for polycrystalline ZnSe

Abstract
We have measured the refractive index and its temperature derivative, of polycrystalline ZnSe at seven wavelengths between 3.5 and 15 μm (3 to 14 times the reststrahl frequency). We have used these data to determine relations among the effective ionic charge and the coefficients of the second- and third-order terms in an expansion of the crystal electric-dipole moment in powers of the lattice displacements. Comparison of ours with other data suggests that two-phonon absorption in ZnSe arises mainly from this nonlinear behavior of its electric-dipole moment.