  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.
