11.1 Raman, Brillouin, Rayleigh, and Mie Scattering

As early as 1923, Einstein generalized the notion of stimulated and spontaneous emission to a new kind of light scattering process, in which the frequency 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 experimentally 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 interaction of an incident light beam of frequency ? with the molecular electric dipoles of a transparent material. The same model that allowed the calculation 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 ? 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 electromagnetic field radiated by the dipoles. In the case of the Raman and Brillouin interactions, the oscillation frequency ? ?, of the...