Optical Second-Harmonic Generation in ZnS—CdS and CdS—CdSe Alloys

Optical second-harmonic generation has been measured as a function of alloy composition in wurtzite ZnS—CdS and CdS—CdSe monocrystals. The relative values of the nonlinear susceptibilities were determined; χ33 increased by nearly one order of magnitude as the band gap was decreased from 1.52(2h/ω) to 0.73 (2h/ω). (Because of this strong increase in χ33, appreciable second-harmonic signals were detected in the crystals of the series which were highly absorbing at the harmonic frequency.) This agreed with the predictions of a quantum-mechanical analysis. Studies of other II-VI and III-V compounds indicated a strong dependence of the second-order susceptibilities upon the band gap.