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surface second harmonic generation
Surface second harmonic generation is a method for probing interfaces in atomic and molecular systems. In second harmonic generation (SHG), the light frequency is doubled, essentially converting two photons of the original beam of energy ''E'' into a single photon of energy 2''E'' as it interacts with noncentrosymmetric media. Surface second harmonic generation is a special case of SHG where the second beam is generated because of a break of symmetry caused by an interface. Since centrosymmetric symmetry in centrosymmetric media is only disrupted in the first (occasionally second and third) atomic or molecular layer of a system, properties of the second harmonic signal then give us information about the surface atomic or molecular layers only. Surface SHG is possible even for materials which do not exhibit SHG in the bulk.
== History ==
Second harmonic generation from a surface was first observed by R. W. Terhune, P. D. Maker, and C. M. Savage working for the Ford Motor Company in 1962,〔Terhune, R.W., Maker, P. D., and Savage, C. M.. ''Phys. Rev. Letters''. 8:404 (1962)〕 one year after Franken et al. first discovered second harmonic generation in bulk crystals. Prior to Terhune’s discovery, it was believed that crystals could only exhibit second harmonic generation if the crystal was noncentrosymmetric. Terhune observed that calcite, a centrosymetric crystal which is only capable of SHG in the bulk in the presence of an applied electric field which would break the symmetry of the electronic structure, surprisingly also produced a second harmonic signal in the absence of an external electric field. During the 1960s, SHG was observed for many other centrosymmetric media including metals, semiconductors, oxides, and liquids. In 1968, Bloembergen et al.〔Bloembergen N, Chang R K, Jha S S and Lee C H ''Phys. Rev.'' 174:813 (1968)〕 showed that the second harmonic signal was generated from the surface.
Interest in this field waned during the 1970s and only a handful of research groups investigated surface SHG, most notably Y. R. Shen’s group at University of California at Berkeley who has written two reviews on surface SHG.〔Shen, Y. R.. A''nn. Rev. Mater. Sci'' 16:69-86 (1986)〕〔Shen, Y. R. ''Annu. Rev. Phys. Chem'' 40:327-350 (1989)〕 During the 70s and 80s, most of the research in this field focused on understanding the electron response, particularly in metals. In 1981, Chen et al. showed that SHG could be used to detect individual monolayers,〔Chen, C. K., Heinz, T. F., Ricard, D., Shen, Y. R. ''Phys. Rev. Letters'' 46:1010-1012 (1981)〕 and since then, much research has gone into using and understanding SHG as surface probe of molecular adsorption and orientation.〔Heinz, T. F. ''Nonlinear Surface Electromagnetic Phenomena''; North-Holland: New York, 1991; Chapter 5〕
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