Dispersion of Nonlinear Optical Susceptibilities in Hexagonal II-VI Semiconductors

We have determined the dispersion of three independent nonlinear optical susceptibilities, $\left|d_{15}\right|$, $\left|d_{31}\right|$, and $\left|d_{33}\right|$, for CdS and CdSe. Tunable dye lasers (0.73 to 1.0 μm) were used. In spite of the large absorption at the second harmonic and/or fundamental wavelengths, Kleinman's relation ($d_{15}=d_{31}$) holds. The ratio $\left|\frac{d_{33}}{d_{31}}\right|$ exhibits dispersion and deviates from Robinson's relation (ratio of 2). The relative phase angles between $d_{15}$ and $d_{31}$ and between $d_{31}$ and $d_{33}$ were measured at two wavelengths by a new reflection interference technique.