Quantification of the surface- and bulk-order parameters of a homogeneously aligned nematic liquid crystal using fully leaky guided modes

An optical waveguide technique, involving the excitation of fully leaky guided modes, has been used to quantify the director distortions induced by the application of weak in-plane electric fields to a homogeneously aligned nematic liquid crystal, E7. The twist profile of the nematic is determined by fitting angle-dependent reflectivity and transmission data to multilayer optical theory. By considering the Frank–Oseen free energy in the bulk, with Rapini–Papoular-type anchoring at the surfaces, we have determined accurate values for both the azimuthal anchoring strength Wa and the twist elastic constant k22 as a function of temperature. Values for the temperature-dependent surface and bulk average-order parameters have also been deduced by allowing for a spatially varying nematic-order parameter.