Optical Second Harmonic Generation as a Probe of Properties and Processes at Surfaces and Interfaces

Abstract

The technological significance of surfaces and interfaces hardly can be overstated. Whether the effects of surface processes are primary to the technology, as in heterogeneous catalysis and tribology, or limiting of the application of bulk material properties, such as corrosion of structural metals and ceramic bonding, the chemistry and physics of interfaces are critical to successful long-term operation of systems that are dependent on specific materials. The necessity to understand beneficial and deleterious changes at these interfaces, coupled with their relative inaccessibility, requires the utilization of every analytical technique that is surface specific. An important, new diagnostic tool of this type is surface optical second harmonic generation (SHG).The coherent interaction of intense laser beams to generate new wavelengths is determined by material-specific nonlinear optical susceptibilities. The lowest order nonlinear response, the second order susceptibility, which mediates the bilinear combination of optical frequencies, is nonzero only in those regions of space that lack a center of symmetry. At the interface of two bulk centrosymmetric media (all gases and liquids, nearly all metals, many oxides, and chalcogenides), the second order susceptibility, therefore, endures only at the discontinuously thin surface separating the materials. The pairwise coupling to form sum and difference frequencies is dependent on this local second order susceptibility, which is a function of the electronic and vibrational structures of the interface. As a chemical reaction or other process changes the electronic states of the interface, or a change in bonding alters the vibrational frequencies of interface species, characteristic resonances between photon energies of the incident or generated frequencies, and transitions connecting electronic and vibrational states, will evolve and change the second order susceptibility. Thus, the intensities of these new coherent beams can be used in situand in real time to follow the evolution of these interface properties.The practical application of these techniques, with special attention to second harmonic generation and sum frequency generation, will be discussed in detail. Specific examples will be given for adsorbed gases on metals, high temperature segregation of bulk impurities to the metal surface, surface states of dielectrics and ceramics, ceramic—metal interactions, and other topics.Index EntriesInterfacesSurfacesoptical susceptibilities