Raman-Brillouin-Rayleigh Diffusion

Abstract

11.1. Raman, Brillouin, Rayleigh, and Mie Scattering As early as 1923, Einstein generalized the notion of stimulated and sponta-neous emission to a new kind of light scattering process, in which the fre-quency of the scattered light is shifted with regard to the incident light. It was only after their experimental demonstration that these new effects were given a name, the spontaneous Raman effect in 1928 and the Brillouin effect in 1932. It was not until 1963, and with the availability of powerful and monochromatic laser beams, that the existence of the stimulated Raman effect was experi-mentally demonstrated. In the case of stimulated interactions many photons are emitted in the same mode (see Section 9.2.1) and are coherent. It is possible to obtain a laser action without the necessity of an inversion of population. Raman and Brillouin diffusion of light is based on the electric dipolar inter-action of an incident light beam of frequency w with the molecular electric dipoles of a transparent material. The same model that allowed the calcula-tion of the index of refraction (8.1) can describe the situation. In this model the electric field of the light wave creates oscillating dipoles vibrating at the same frequency w as the incident wave; the light that is transmitted by a piece of material is the result of the interference of the incident beam with the elec-tromagnetic field radiated by the dipoles. In the case of the Raman and Bril-louin interactions, the oscillation frequency w m of the dipoles is different from the incident frequency w. The frequency shift (W m = w -w m), which can be pos-itive or negative, is always small as compared to w and is specific to the mol-ecular dipole. As new frequencies are generated, the interaction between a light wave and a piece of material is a nonlinear interaction. The distinction between the Raman and Brillouin diffusions is not always clear-cut; it is based on the fact that the vibrations of the different dipoles of the transparent material can either be independent or coupled to one another. 479 • The Raman effect corresponds to the case when the vibrations are inde-pendent, which mainly occurs for gases or liquids. • The Brillouin effect corresponds to the case when the molecular dipoles are coupled by mechanical waves (acoustic waves if the frequency is low enough) propagating inside the transparent material.