A review of optical second-harmonic and sum-frequency generation at surfaces and interfaces

Optical second-harmonic and sum-frequency generation at surfaces and interfaces is discussed and recent work reviewed. Examples from well-characterized interfaces are emphasized and particular attention is paid to spectroscopic studies which have recently become simpler due to the availability of tuneable broad band laser sources. It is shown that the techniques are now well understood at the phenomenological level and, although theoretical calculations are proving to be difficult, interfaces not accessible to conventional surface techniques can now be studied with some confidence.