Abstract

We report intensity-dependent self-defocusing in polycrystalline ZnSe via beam distortion measurements using 30-ps frequency-doubled Nd:YAG laser pulses. The dominant mechanism is nonlinear refraction due to carriers created by two-photon absorption (2PA). This sequential X⁽³⁾:X⁽¹⁾ process is equivalent to a fifth-order process. The numerical fitting to the experimental results gives the refractive index change per carrier of 3.3 × 10⁻²¹ cm³ using a 2PA coefficient of 5.5 cm/GW.¹ We interpret these measurements in combination with DFWM and Z-scan (a method for n₂ measurements) experimental results,² where a fast third-order followed by a slower fifth-order nonlinearity are detected. The magnitude of the third-order nonlinearity given by the Z-scan experiment was n₂ ⁼ −3.5 × 10⁻¹¹ esu. At low irradiance levels where the third-order nonlinearity governs the nonlinear process, this value is too small to cause any observable change in the beam shape. For this reason, we can conclude that only the carrier induced nonlinearity is important in passive semiconductor optical limiters.¹ While this highlights the insensitivity of beam distortion measurements to phase distortions of less than λ/4, the technique remains useful for quantifying large refractive nonlinearities, such as those required for optical limiting.

© 1989 Optical Society of America