We present what is to our knowledge the first experimental observation
of the specular nonlinear anisotropic polarization effect of a pump-i
nduced polarization-plane rotation for normal-incidence reflection fro
m the (001) surface of a cubic crystal. In GaAs, azimuth rotation of t
he order of 9 x 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 nonline
arity \Re((chi xxxx) - 2(chi xxyy) - (chi xyyx))\ = 5 x 10(-9) esu is
found.