Surface contribution versus bulk contribution in surface nonlinear optical spectroscopy

Surface versus bulk contributions to second-harmonic generation and sum-frequency generation is reviewed. For use as effective surface probes, these processes must have a surface contribution dominating over, or separable from, the bulk contribution. The few experimental methods that have been employed to look into the problem are discussed. It is also noted that the interfacial layer defined in an experiment often depends on the property measured. Our surface nonlinear optical spectroscopy probes mainly an interfacial layer whose structure lacks inversion symmetry. In the cases of surface-induced or surface-field-induced polar orientation, the interfacial layer thickness can extend to several tens of monolayers.