Surface nonlinear optics for material studies

Optical second harmonic generation (SHG) and infrared-visible sum-frequency generation (SFG) as surface probes have recently attracted a great deal of attention because of their seemingly unlimited possible applications to all areas of surface science. We review here the recent progress in this field. We first discuss how surface-specific these techniques really are. This is important because the bulk contribution to the signal sets a limit to the sensitivity of such surface probes. We then give a number of examples to illustrate the power of the techniques. Most of them are chosen to show that SHG and SFG are unique in providing specific information about the particular surface or interfacial systems. The following problems will be discussed in some detail. 1. Measurements of molecular nonlinearities and the environmental effect on soluble and insoluble molecular monolayers at air/water interfaces. 2. Studies of molecular orientations of liquid surfaces. 3. Creation and detection of monolayer gratings on crystalline surfaces and measurements of anisotropic surface diffusion. This work was supported by DOE under Contract No. DE-ACO3-76SF00098.