The refractive indices of the ternary A^{I}B^{III}C_{2}^{VI} semiconductors AgGaS 2 , CuGaS 2 , and CuInS 2 have been measured over the entire range of transparency of these crystals. The optical nonlinear coefficients for second-harmonic generation have also been determined. Three-frequency collinear phase matching is analyzed in detail for AgGaS 2 . The birefringences of CuGaS 2 and CuInS 2 are not large enough to permit three-frequency phase matching within the transparent regions. A parametric oscillator threshold calculation for a pump wavelength 0.89 μ, which is within the range of the GaAs injection laser, indicates that AgGaS 2 is promising for this application. The upconversion efficiency in AgGaS 2 for sum mixing of the CO 2 laser ( \lambda = 10.5 \mu ) with the xenon ion laser ( \lambda = 0.597 \mu ) is also calculated. The result indicates that, depending upon system requirements and the availability of high optical quality material, AgGaS 2 can be comparable to ZnGeP 2 for upconversion. In Appendix II, we present a theory of the wedge technique for the measurement of nonlinear coefficients. This theory takes into account losses and assumes a Gaussian beam geometry. Furthermore, a discussion of units in nonlinear optics is given.

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