Smectic Brillouin scattering

A direct confirmation of the existence of these two branches has been found by Liao, Clark and Pershan from their Brillouin scattering experiments on a monodomain sample of jff-methyl butyl /K(p-methoxy-benzylidene)amino) cinnamate. This compound shows the nematic, smectic A and smectic B phases. Choosing both the incident and the scattered light to be polarized either as ordinary or extraordinary waves, they observed two peaks corresponding to the two modes, the angular dependence of which is in excellent agreement with the theory (fig. 5.3.11). 

Fig. 5.3.11. Dependence of the sound velocities on the polar angle from Brillouin scattering experiments on (J-methyl butyl p((p-methoxy-benzylidene)amino) cin-namate. (a) Smectic A(T = 60.7 °C), (b) smectic B (F = 48.1 C). The dashed lines are calculated from theory. The presence of a third component in (b) indicates that the shear modulus does not vanish in smectic B at these very hi frequencies. Circles, triangles and squares represent measurements at different scattering angles. (After Liao, Clark and Pershan. )...

Fig. 6.3.1 depicts the variation of the velocities of the three waves with polar angle. This angular dependence can be studied by Brillouin scattering, as has been demonstrated for smectic A (see fig. 5.3.11). In the present case, there should be one, two or three pairs of Brillouin components depending on the orientation, but no experiments have yet been reported. 

The first Brillouin scattering measurements on the smectic A phase were made by Liao et al. [33] who confirmed the predictions of the propagating mode structure in both smectic A and smectic B phases. The first and second sound velocities were measured for racemic P-methylbutyl p-[(p -methoxy-benzylidene)amino]cinnamate, and they showed that the first sound speed is almost isotropic with a minimum in the off-symme-try direction, with the speed of second sound being extremely anisotropic. The data are well described by theory. No 2nd sound measurements were made for (])>45°, indicating very anisotropic damping that was at-... 

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