Advancements of lasers and optical fibers contributed to optical communications and optical disks for practical use. Further development of optical materials is urgently required. Nonlinear optical materials efficiently exhibit nonlinear optical phenomena, which are conversion of light wavelength, amplification of light, and conversion of the refractive index depending on optical intensity. Since a laser oscillation was reported from an inorganic compound in the 1960s, inorganic nonlinear optical materials have been well developed. In addition to these inorganic materials, organic nonlinear optical materials have also been studied. This is because organic materials show efficient nonlinear optical properties and rapid responsiveness based on highly movable -electrons, thanks to delocalized electrons in organic materials. Nonlinear optical materials may be important for large-capacity communications, because further application of this material may provide a device in an all optical system. As one of the basic technologies of photonics, lightwave technology using organic nonlinear optical effects are important to develop. Further research on organic materials with excellent nonlinear optical properties and vigorous applied studies has been carried out.
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2) D. S. Chemla, J. Zyss, in Nonlinear Optical Properties of Organic Molecules and Crystals, Academic Press, New York, 1987.