BISTABLE NEMATIC LIQUID CRYSTAL DEVICE

摘要

A bistable nematic liquid crystal device cell is provided with a surface alignment treatment on one cell wall which is capable of sustaining two stable states in which the liquid crystal director is in the same azimuthal plane but at two different tilt angles out of the plane of the cell. This surface may be provided using a symmetric or asymmetric grating structure formed by photolithographic or embossing techniques in which the local orientation of the liquid crystal molecules at the surface is normal to the local direction of the surface. Correct choice of the grating period, depth, shape and anchoring properties leads to two stable director configurations: one with the director normal to the plane of the cell at the grating surface, and the other with the director at a pretilt usually in the range 0 DEG to 60 DEG from the cell plane. This surface is combined with a monostable surface which leads to substantially planar homogeneous alignment formed e.g. by a rubbed polymer, or a grating surface. The preferred direction of the monostable surface may be arranged to be at any angle with respect to the grating grooves on the opposite surface. In one embodiment, the monostable surface is aligned parallel to the grooves so that in one state the director twists through 90 DEG from one surface to the other, whereas the other state has little or no twist. The cell may be switched by dc pulses which couple to the flexoelectric coefficients of the material. The material has a negative dielectric anisotropy and may include a chiral dopant to impart a preferred twist direction. The cell walls may be formed from rigid or flexible transparent material coated with electrode structures which form a matrix of pixels. Means for optical discrimination for the different states is provided by one or more polarisers, or a dye with or without a polariser, and/or other optic elements.