High-frequency dielectric relaxation of liquid crystals: THz time-domain spectroscopy of liquid crystal colloids

Terahertz time-domain spectroscopy has been used to study the dielectric relaxation of pure 4'-n-pentyl-4-cyanobiphenyl (5CB) liquid crystal (LC) and its mixtures with 10 µm SiO₂ particles in the frequency range 0.2-2 THz. For the pure sample, we find that spatial inhomogeneities consisting of oriented domains, comparable in size to our probe area (~1 mm²), cause a large scatter in the measured dielectric function, due to varying contributions from the ordinary and extraordinary components. In the LC/particle mixtures, ordering of the LC at the surface of the SiO₂ particles results in a break-up of these domains, giving rise to a spatially much more homogeneous dielectric response. The inferred dielectric function can be interpreted using effective medium theory and the Debye relaxation model. We observe this stabilizing effect for interparticle distances < ~30 µm, setting a lower limit for the size of oriented domains in the bulk LC.