Lasers in Physics: Sum-frequency generation at surfaces
By: M. B. Raschke and Y. R. Shen
in Encyclopedia of Modern Optics, eds. B. Guenther et al., Academic Press, London, 2004
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. Among the various techniques employed for the characterization of surfaces and interfaces, those using light are particularly attractive. They are applicable in situ to all interfaces accessible by light, are nondestructive, and offer unprecedented time resolution. However, the penetration depth of optical radiation in matter is generally of the order of a wavelength, which makes isolation of the surface or interface contribution to the optical response from the bulk contribution difficult. In contrast, for reasons of symmetry, SHG and SFG are intrinsically surface sensitive in media with inversion symmetry, and hence the signal generated mainly originates from the topmost surface layer where the inversion symmetry is broken. By means of electronic or vibrational SHG or SFG spectroscopy, information on surface structure, chemical composition and bond or molecular orientation at solid and liquid interfaces can be deduced. To date, SFG and SHG have been well established as important tools for the investigation of surfaces and interfaces of solids ranging from metals and semiconductors to insulators and magnetic materials, liquids, polymers, biological membranes, and other systems. The studies are motivated by fundamental interests as well as applications in many areas such as heterogeneous catalysis, electrochemistry, device fabrication, epitaxial growth, and environmental science.