Nonlinear Optical Phenomena in Active Media

Abstract

Chapter 4 described phenomena in which photons are exchanged among two or more light beams having simply-related phases and frequencies. The harmonic, subharmonic, or mixture frequencies which are generated in those processes are determined only by the frequency composition of the applied light beam. Although indispensible to these processes, a nonlinear optical medium plays only a catalytic role, even when by its crystal symmetry and by fortuitous phase matching, it singles out one of several possible processes. As in ordinary linear dispersion, the medium responds coherently, but in a passive manner, to the electromagnetic disturbance. Photons collide elastically with the atomic systems; i.e., no detectable exchange of quanta occurs between the light waves and the dispersion systems, and the new frequencies generated are not characteristic of the medium. When sufficient care is taken to satisfy the rather stringent demands of coherence, a passive optical medium may act as an amplifier, facilitating the sustained exchange of energy between light beams. Such energy exchange, accomplished with coherently driven scattering systems, is more nearly akin to stimulated than to spontaneous emission.