Acoustic anisotropy in 5CB liquid crystal cells as determined by using Brillouin light scattering

The angular dependence of Brillouin spectra of 4-n-pentyl-4-cyano-biphenyl (5CB) liquid crystals homogeneously and homeotropically confined in a planar liquid-crystal (LC) cell were measured using a 6-pass tandem Fabry-Perot interferometer. The sound velocity and the elastic constant of 5CB were obtained from the Brillouin shift without knowing the refractive index thanks to a special right-angle scattering geometry. The sound velocity and the attenuation coefficient of the longitudinal acoustic waves propagating along the direction of the director were larger than those of the longitudinal sound waves propagating along the direction perpendicular to the director. Based on the observed acoustic anisotropy, the three elastic constants C11, C33, and C13 could be obtained.