overview of Nonlinear Optics

The invention of the laser provided enough light intensity that nonlinear optics (NLO) could be observed for the first time, almost exactly one year after the first ruby laser – second harmonic generation, observed fifty years ago (Franken, 1961), with a theoretical examination of interactions between light waves in a nonlinear dielectric following very soon thereafter (Armstrong, 1962). The field has grown so enormously that it is impossible to review all topics. Theoretical approaches have bifurcated into macroscopic and microscopic viewpoints. Both have validity and usefulness, but this review will focus primarily on experimental results and applications, as well as models for optical nonlinearities that stress the macroscopic character of materials, with parameters determined experimentally. At times the wavelike nature of light is sufficient to explain concepts and at other times it is easier to refer to the photon-like nature of light. In nonlinear optics we become fluent in both concepts.