Brillouin Scattering from Smectic Liquid Crystals

Brillouin-scattering results are reported for monodomain samples of $\beta$-methyl butyl $p$ [($p$-methoxy benzylidine)amino] cinnamate in both the smectic-$A$ and -$B$ phases. For both phases theoretical predictions are confirmed concerning the number and polarizations of the propagating hypersonic waves. We report quantitative measurements of various thermodynamic derivatives, including the coupling between density and smectic-layer spacing. We discuss preliminary evidence concerning one model of the smectic-$B$ phase.