6A2 - The influence of self-focusing on the stimulated Brillouin, Raman, and Rayleigh effects

In liquids with optically anisotropic molecules the largest nonlinear polarization, cubic in the field amplitudes, corresponds to the intensity dependent index of refraction. The resulting self-focusing of the laser beam is in turn responsible for the anomalous gain of the stimulated Raman emission. The stimulated Brillouin effect in these self-focusing fluids has the same threshold condition as the stimulated Raman effect. In both cases the condition is that self-focusing occurs within the cell length. The anomalous frequency broadening in these same liquids depends on the parameter
(\omega_{L} - \omega'_{L})\tau_{c}
, where ω
L
and
\omega'_{L}
are two frequencies occurring in the laser beam and τ
c
is a correlation time for molecular reorientation. In agreement with this theory, more broadening was observed at higher temperatures, although the Raman gain has the opposite trend.