Chapter 1 - The Nonlinear Optical Susceptibility

Abstract

This chapter elaborates the basic concepts of nonlinear optical susceptibility. Nonlinear optical phenomena are “nonlinear” in the sense that they occur when the response of a material system to an applied optical field depends in a nonlinear manner on the strength of the optical field. In nonlinear optics, the optical response can be described by expressing the polarization as a power series in the field strength. The nonlinear susceptibilities depend on the frequencies of the applied fields. The chapter shows that second-order nonlinear optical interactions can occur only in noncentrosymmetric crystals, that is, in crystals that do not display inversion symmetry, while the third-order nonlinear optical interactions can occur for both centrosymmetric and noncentrosymmetric media. The chapter presents brief qualitative descriptions of a number of nonlinear optical processes. For the processes that can occur in a lossless medium, the chapter indicates how they can be described in terms of the nonlinear contributions to the polarization. The chapter also introduces some notational conventions and the basic concepts of nonlinear optics.