Anisotropy in second-harmonic generation from reconstructed surfaces of GaAs

We report on the first observation of the twofold rotational anisotropy of second-harmonic generation (SHG) in the reflected light from a noncentrosymmetric crystal surface of GaAs(001). We interpret this as meaning that the interference of surface SHG and dipole-allowed bulk SHG having fourfold symmetry results in twofold anisotropy. The degree of anisotropy changes according to the surface reconstruction.