ABSTRACT
The penetration depth of optical radiation into condensed matter is large, in general, which makes the isolation of an interface contribution difficult. However, deeper understanding of the underlying physics of the optical response, combined with advances in instrumentation, has allowed the contribution from the interface to be identified. Particularly important has been the recognition that symmetry differences between the bulk and interface can be exploited, as can interface electronic and vibrational resonances. The potential of optical techniques for interfaces, surfaces, and thin films characterization, with submonolayer resolution, began to be recognized in the 1980s. Thus, in the field of surface science, linear optical techniques are usually flexible, but it has been demonstrated among the literature that they sometimes suffer from the lack of surface specificity and sensitivity. Besides, in the field of nonlinear optics (NLO), secondharmonic generation (SHG) and sum-frequency generation (SFG) have developed into powerful tools for characterizing thin films and surfaces. SHG experiments are generally simple to perform, typically yield submonolayer interface-specific sensitivity with minimal optimization, and can be done with fairly inexpensive equipment. Coherent SHG and SFG are dipole forbidden in randomly oriented media, but allowed at the interface between two such media where inversion symmetry is broken. As a result, SHG can often be used to probe surfaces and interfaces in situ under normal conditions with negligible contributions from the bulk. Thus, surface-specific studies have been carried at several interfaces (liquid-liquid, liquid-solid, liquid-air, air-solid). Also, measurements of molecular order and orientation at surfaces and interfaces can yield a unique and powerful description of surface systems down to a monolayer.
