An AC electro-optic technique of comparing the two anchoring energies of a hybrid aligned nematic cell

Abstract

We have found that the AC electro-optic response of a hybrid aligned nematic cell at the frequency (f) of an electric field applied normal to the plates can be conveniently used to compare the anchoring energies at the two surfaces. The f signal arises from the flexoelectric contribution to the surface torques when the anchoring energies are weak. For CCH-7, we have found by measurement of the optical path difference that the homeotropic alignment produced by a silane treatment has a weak anchoring energy. In this case, the f signal produced by an applied AC field exhibits a maximum and then a minimum as a function of the field. The reduction in the signal level after the maximum is caused by a lowering of the tilt angle at the surface treated for homeotropic alignment at high fields. The subsequent increase is caused by an effective weakening of the anchoring energy at the homogeneously aligned surface because of a sharp curvature distortion close to that surface. We have also found that the anchoring energy for homogeneous alignment of PCH-7 on an obliquely coated SiO plate is comparable to that at the homeotropically aligned surface.