Homeotropic polar anchoring energy of a nematic liquid crystal using the fully leaky waveguide technique

Optical excitation of a series of fully leaky guided modes has been used to explore the director distortion of a hybrid-aligned nematic liquid crystal with a negative dielectric anisotropy (Merck-BDH, MLC 6608) under application of an ac electric field. Hybrid alignment is realized by the use of a lecithin film on the top glass plate and rubbed polyimide on the bottom plate, both plates having indium–tin–oxide coatings. Continuum theory is used to model the director profile through the cell. Fitting the fully leaky guided mode data both in transmission and reflection using optical multilayer theory together with continuum modeling gives the director profile both with and without applied fields. From this fitting the changes in the surface tilt angle and its gradient are obtained. Using this information the polar anchoring coefficient of the homeotropic surface $W_{\theta }$ is deduced. At 23.1 °C we find $W_{\theta }{\text{=(4.6±0.2)×10}}^{\text{−4}}\hspace{0.5em}{\mathrm{J\hspace{0.17em}m}}^{\mathrm{-2}}.$