General considerations on optical second-harmonic generation from surfaces and interfaces

Second-harmonic generation from an interface, in relation to the second-order nonlinearities of the interface layer and the adjacent bulk, is considered. It is shown that both structural asymmetry and field discontinuity contribute to the interface nonlinearity, which, as far as second-harmonic generation is concerned, can be characterized by a local surface nonlinear susceptibility tensor. The bulk nonlinearity may also contribute to the second-harmonic signal, but is an order of magnitude weaker than the surface nonlinearity in centrosymmetric media with a large optical dielectric constant. The possibility of detecting submonolayers of adsorbates on various substrates is discussed qualitatively.