The functioning of electro-optic, magneto-optic, and acousto-optic devices discussed in earlier chapters is based on the fact that the optical properties of a material depend on the strength of an electric, magnetic, or acoustic field that is present in an optical medium. At a sufficiently high optical intensity, the optical properties of a material also become a function of the optical field. Such nonlinear response to the strength of the optical field results in various nonlinear optical effects. Nonlinear optics is an established broad field with applications covering a very wide range. The most important nonlinear optical devices are optical frequency converters. The frequency-converting function of such devices is uniquely nonlinear and is difficult, if not impossible, to accomplish by other means in the absence of optical nonlinearity. Other unique nonlinear optical devices include all-optical switches and modulators. Many interesting nonlinear optical phenomena, such as optical solitons, stimulated Raman scattering, and optical phase conjugation, also find useful applications.
The origin of optical nonlinearity is the nonlinear response of electrons in a material to an optical field as the strength of the field is increased. Macroscopically, the nonlinear optical response of a material is described by a polarization that is a nonlinear function of the optical field. In general, such nonlinear dependence on the optical field can take a variety of forms. In particular, it can be very complicated when the optical field becomes extremely strong.