SOME FUNDAMENTAL ASPECTS OF NONLINEAR OPTICS
Y. R. Shen
Abstract
Selected topics related to the fundamentals of nonlinear optics are discussed. First, a brief description on the theory of nonlinear optical susceptibilities is given. Emphasis is on the basic properties, on the recently developed bond theory, and on the dispersion of nonlinear susceptibilities. Then the coupled wave approach is discussed. It is shown that the transverse resonant excitation in material can be treated as a material excitational wave, and most nonlinear optics problems can be understood as results of coupling between em waves or em and material excitational waves. A number of examples of current interest are presented. These include sum- and difference-frequency generation, Raman scattering by coherent excitation, stimulated Raman and stimulated polariton scattering, four-wave mixing, and vibrational relaxation measurements. Finally, the physical distinction between laser emission and resonant stimulated Raman scattering in an optically pumped three-level system are considered. A similar problem on the distinction of hot luminescence and resonant Raman scattering has recently attracted much attention. It is shown that the hot laser action is connected directly with the population inversion, while the resonant stimulated Raman scattering is connected with transverse excitation. The problem is, however, more complicated for a multi-level system. (auth)