Non-saturation intensity dependence of anisotropic third-order optical nonlinearity approaching the damage threshold in ZnSe and GaP

The intensity dependence of anisotropic third-order optical nonlinearity approaching the damage threshold in ZnSe and GaP crystals is studied by the femtosecond laser pump–probe measurements, which can greatly reduce the laser-matter interaction length and thus realize the probing of orientation-dependent characteristics of nonlinear optical phenomena in the near-damage-threshold intensity regime without significant photon depletion. In the measured transient 3D map, the typical third-order nonlinear optical signals of two-beam coupling (TBC) and two-photon absorption (TPA) can be clearly found out, which both exhibit the pronounced orientation-dependent periodic modulation corresponding to a specific lattice symmetry. Interestingly, the further fixed-delay-time measurements focusing on TBC and TPA confirm that the modulation amplitude of the orientation-dependent curves always increases with the increase of pump intensity towards the damage threshold, which has not been observed in previous studies. Such a definite upward trend of orientation-dependent third-order nonlinear optical effects in the near-damage-threshold regime indicate that, as long as the laser-matter interaction length is small enough, the third-order nonlinear optical phenomena can still be in a non-saturation physical regime till the damage threshold, and thus exhibit significant crystallographic dependence as that of laser-induced damage at the similar intensity ranges.