Nonlinear transmission, degenerate four-wave mixing, photodarkening, and the effects of carrier-density-dependent nonlinearities in semiconductor-doped glasses

A model for nonlinear transmission and degenerate four-wave mixing in semiconductor-doped glasses was developed and compared with experiment. The results of a plasma screening theory were used to model the nonlinear response of the semiconductor microcrystallites. The effects of carrier-density-dependent relaxation times, two-photon absorption, free-carrier absorption, and background absorption were included in the analysis. Good agreement between theoretical predictions and both nonlinear transmission and degenerate four-wave mixing experiments at λ = 532 nm was obtained for 15-nsec laser pulses. For 30-psec pulses, with which higher intensities were available, features suggestive of two-photon absorption and background absorption were observed. We conclude that free-carrier absorption is not significant in these systems. Finally, it was shown that photodarkening is associated with the semiconductor crystallites in the glass matrix and that the host glass itself and unstruck samples exhibit little or no photodarkening.