Local and nonlocal surface nonlinearities for surface optical second-harmonic generation

We investigate the physical origins of the surface nonlinear susceptibility responsible for surface optical second-harmonic generation. Experiments performed on simple covalent systems were designed to distinguish between the nonlocal electric-quadrupole-type nonlinearity induced by field discontinuity and the local intrinsic, electric-dipole-type nonlinearity of a surface or interface. We find that both mechanisms could operate: The latter usually dominates when the surface layer has a strong structural asymmetry with the fundamental or the second-harmonic frequency at resonance with a dipole-allowed transition.