Georges Durand

Fast bistable nematic display from coupled surface anchoring breaking

We present new experimental results on bistable nematic display, controlled from surface anchoring breaking. Using simple planar anchorings on both plates and chiralized 5CB material, we prepare thin cells with two equal energy states: one uniform planar and the other one twisted by 180 degrees. Applying short electric pulses we break transiently both surface anchorings (to write) or only one of them (to erase). After these short pulses, in absence of any field, we obtain at will the uniform state or the twisted one, with infinite lifetime and full bistability. Using polarizers an optical contrast of 50 is measured between the two states. The write/erase pulse duration is short, comparable with the surface anchoring response time approximately 10 microseconds. The optical response time, classically related to the cell thickness, is here in the ms range. The temperature dependence of the threshold is also discussed. The existence of the two relatively close writing and erasing thresholds allows in principle simple multiplexability.

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 tow 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 deice 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) equals 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 a usual monostable substrate, due to topological constraints. Particular surface conditions are required to stabilize them. The surface defects depolarize 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 microsecond(s) ec on a sample of 5 micrometers thickness.

Charged Twist Walls in Nematic Liquid Crystals

The influence of electric fields on nematic liquid crystals has been extensively studied in the past.1 The field can reorient the nematic texture by coupling with the dielectric anisotropy. A curvature distortion can create space charges, through the anisotropy of conductivity, and give rise to electrohydrodynamic instabilities.2 The same curvature also creates a flexoelectric polarization which can be acted upon by an external field in suitable geometries.3 In d.c. or at low frequencies, the applied field is usually decreased by nonohmic electrodes, which become polarized by the ions available in the medium which are responsible for its conductivity. When reversing the field one can observe4 in the transient current a signal which corresponds to the transport of these polarizing ions from one electrode to the other. We describe in this paper an experiment which shows the propagation of these ions, dynamically trapped into a twist wall of the nematic texture. The point is that this charged twist wall cannot exist for small distortions and must be considered as a singular wave