Instrumentation: Ellipsometry

Under sufficiently intense illumination the optical properties of a material system depend nonlinearly on the strength of the electromagnetic field. Related to this nonlinear optical response is a large number of phenomena and fundamental processes which are discussed in various articles of this encyclopedia. This article will focus on second-harmonic generation (SHG) and sum-frequency generation (SFG) for the investigation of surfaces and interfaces. The intrinsic surface sensitivity of these techniques allows for investigations of surface properties not readily accessible by other spectroscopies. Here, the basic principles of these optical processes as well as their experimental implementation are discussed, and a summary of the applications to different material systems is given.