BISTABLE NEMATIC LIQUID CRYSTAL DEVICES

摘要

The invention provides a nematic liquid crystal device with two bistable switched states. The device comprises two cell walls carrying electrodes structures and enclosed a layer of a nematic liquid crystal material. The cell walls are treated to provide a bigrating on one or both walls. The bigrating is a shaped surface that permits liquid crystal molecules at a surface to lie in two different angular directions, e.g. 45.degree. or 90.degree. apart, in contrast with conventional surface alignments which align liquid crystal molecules in a single direction at a surface. For a cell with bigratings on both walls the two alignment direction on both walls are in register. Polarisers are arranged in front and behind the cell with their optical axes orthogonal and with the optical axis of one polariser parallel to one of the switched alignments. In this example, as the material is switched between the two alignment states the polariser transmits or blocks light. The bigratings may be symmetric or asymmetric, in which later case an amount of surface tilt is obtained. Appropriate surface tilt on both cell walls may give a splayed molecular configuration in both switched states. For splayed configurations, the device may be electrically switched using flexoelectric coupling. For other configurations the device may be switched by use of dc pulses in combination with chiral ions in the liquid crystal material. The bigratings may be produced by interferography, photolithography, embossing, ruling, or transferred from a carrier layer. A bigrating on one cell wall may be combined with a conventional monograting or e.g. rubbing alignment treatment on the other cell wall. In this case the material is twisted in both its switched states.