Measurement of free-carrier nonlinearities in ZnSe based on the Z-scan technique with a nanosecond laser

Self-defocusing and nonlinear absorption resulting from two-photon-excited free charge carriers were observed in polycrystalline ZnSe at the low power density of ~ 30 MW/cm(2) by use of the Z-scan technique with nanosecond laser pulses. The total carrier absorption cross section and the variation of refractive index per unit of photoexcited carrier density were estimated to be 0.80 +/- 0.10 x 10(-18) cm(2) and 0.60 +/- 0.15 x 10(-21) cm(3), respectively.