Anomalous field-induced director deformation in a homogeneous thin nematic liquid crystal cell

We have investigated the voltage dependence of the light transmittance for homogeneous thin nematic liquid crystal (NLC) cells composed of 4-pentyl-4'-cyanobiphenyl (5CB), which were confined between two glass plates. The substrate surfaces with polyimide layers were rubbed unidirectionally to make a uniform planar director orientation with a strong surface anchoring condition. An NLC cell was set between polarizer and analyzer. The He-Ne laser beam was passed through the NLC slab oriented with its optic axis parallel to the linearly polarized direction of the light beam. The light transmitted through the analyzer, which was crossed with the polarizer, was detected as a function of the applied voltage. In zero electric field it is expected that the director will have a uniform planar alignment and this orientation gives a minimum intensity of the light transmitted. When the voltage is applied to the NLC cell, the director for 5CB with its positive dielectric anisotropy rotates to orient parallel to the electric field. The voltage dependence of the light transmittance showed an anomalous result; that is, light leakage was observed for a certain voltage. This was interpreted by considering the director deformation at the boundaries between rubbed and non-rubbed regions of the polyimide surface.