Brillouin and Rayleigh Scatterings

In this case the scattering process will take on a totally different form, and the Stokes wave will grow exponentially. To see this, consider Equation (5.59) for the case n, 1. We have 

In other words, the Stokes wave experiences an exponential gain constant (if jV, N2) that is proportional to the incident laser intensity. If this gain is larger than the loss at owing to absorption, random scatterings, and so on experienced by the Stokes wave, it is possible to have laser oscillations (i.e., the generation of Stokes lasers). Such a process is called stimulated Raman scattering. 

For a more detailed discussion of stimulated scatterings as nonlinear optical wave mixing processes, the reader is referred to Chapter 11. 

Rayleigh wing scattering is due to the orientational fluctuations of the anisotropic molecules. For typical liquids these orientational fluctuations are characteristic of the individual molecules movements and occur in a very short time scale ( 10 s). Consequently, the spectrum is quite broad. In liquid crystals studies of individual molecular orientation dynamics have shown that the relaxation time scale is on the order of picoseconds and thus the Rayleigh wing spectrum for liquid crystals is also quite broad. 

From the discussion given in earlier chapters, we note that in liquid crystals, the main scattering is due to collective or correlated orientational fluctuations. These, of course, are much slower processes than individual molecular motions. The spectrum from these correlated or collective orientational fluctuations is therefore very sharp and is embedded in the central Rayleigh peak region. 

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Figure 4.18 Schematic of Raman, Brillouin and Rayleigh scattering spectra in solids, indicating the characteristic frequency ranges for each type of scattering. (After Cummins, 1971)...

The dynamics of orientational scattering has been discussed on various occasions in the preceding chapter. We shall discuss here the mechanism of Brillouin and Rayleigh scatterings. 

Stimulated effects in nonlinear optics, such as stimulated Brillouin and Raman scattering in liquids and gases and stimulated thermal Rayleigh scattering, have been reviewed by Rank 260a)... 

Figure 1. A schematic diagram showing the relative positions of Rayleigh and Brillouin scattering peaks. The incident light has frequency cOq the Brillouin frequency shift is cOg. The widths of the Brillouin and Rayleigh peaks (Tg and Tp.) are also shown.

There is some arbitrariness in what we call Raman and Rayleigh-Brillouin scattering. It should be... 

A more accurate light-scattering method involves analysing the ratios of the intensities of the three closely spaced peaks (the Brillouin spectra) of the Rayleigh scattering. A theoretical analysis by Fishman and Mountain ... 

The power spectrum of the light, scattered from the ice-water interface at conditions close to equilibrium, contains only Raman and Brillouin contributions. There is no Rayleigh scattering. At a critical growth rate an onset of quasi elastic light scat-... 

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