Mattijs Koeberg

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.

Liquid Crystal Colloids Studied by THz Time-Domain Spectroscopy

THz time-domain spectroscopy is used to measure the frequency dependent (0.2–2.0 THz) complex refractive index of a pure liquid crystal (LC), 4′-n-pentyl-4-cyanobiphenyl (5CB), and its LC colloids with SiO2 particles. While the refractive index of the pure LC is found to vary markedly due to distinct orientated domains within the sample, the LC colloids provide us with very stable and reproducible spectra, from which we can reliably deduce the optical constants of pure 5CB using effective medium theory. While the absorption coefficient is found to be very small, the refractive index of 5CB decreases considerably over our probe frequency range.