Specular nonlinear anisotropic polarization effect along fourfold crystal symmetry axes

We present what is to our knowledge the first experimental observation of the specular nonlinear anisotropic polarization effect of a pump-induced polarization-plane rotation for normal-incidence reflection from the (001) surface of a cubic crystal. In GaAs, azimuth rotation of the order of 9 × 10−6 rad is seen for a pump intensity of 75 MW/cm−2 at 750 nm, from which the anisotropic component of the cubic nonlinearity |Re(χxxxx − 2χxxyy − χxyyx )| = 5 × 10−9 esu is found.