Fast bistable nematic display using surface defects

We present a novel fast bistable nematic cell with intrinsic grey scale. The geometry of a single pixel is the usual sandwich one, with two conductive flat plates that contain a film of nematic material with positive dielectric anisotropy. An electric field is applied perpendicularly to the boundary plates. This novel device works by controlling the creation and the erasure of surface defects on a suitable surface when the applied electric field is strong enough to achieve the anchoring breaking condition: xi=L, where xi is the coherence length of the electric field and L is the anchoring extrapolation length. Note that these surface defects should disappear onto an usual monostable substrate, due to topological constraints. Particular surface conditions are required to stabilise them. The surface defects depolarise the incident light and, as their density can be modulated, a grey scale can be achieved. Up to now, the maximum measured optical contrast is 200:1. The typical writing and erasing voltages are in the order of 50 Volts for a pulse length of 50 mu sec on a sample of 5 mu m thickness.