Using a pulsed tunable dye laser pumped by a $Q$-switched ruby laser, the frequency dependence of second-harmonic generation has been measured in the region of the first and second $1s$ excitons of CuCl from about 3.18 to 3.34 eV and the region of the $C$ exciton of ZnO from about 3.38 to 3.48 eV. From the data, the frequency dependence of the optical nonlinear susceptibility has been determined using linear optical constants calculated by a Kramers-Krönig analysis of reflectivity. The results for the single nonzero term of the nonlinear susceptibility of CuCl and the two terms $d_{\mathrm{zzz}}$ and $d_{\mathrm{zxx}}$ of ZnO have been fit theoretically by an anharmonic-oscillator model employing two oscillators.

• Received 10 January 1973

DOI:https://doi.org/10.1103/PhysRevB.8.734