Self-induced optical activity in crystals

We have observed non-linear rotation of the plane of polarization of a linearly polarized light beam in GaAs crystals ($\text{4}$3m). The effect has been investigated by means of YAG : Nd^3- laser and pulse polarimeter with accuracy to non-linear rotation angle 0.02°. Two mechanisms caused by the non-linearity of the spatial dispersion and the anisotropy of two-photon's absorption have been distinguished and corresponding susceptibilities have been calculated.