Georges E. Durand

Fast bistable nematic display using monostable surface switching

We present a novel surface-controlled bistable nematic liquid crystal display. This black and white device uses simple monostable planar anchorings and a dielectrically positive nematic. The two bistable textures are, respectively, quasiuniform U and almost half-turn twisted T. Short electric pulses break the surface anchorings. Hydrodynamically coupled breaking of both anchorings creates T. U is obtained by breaking only one anchoring. Write and erase times for high resolution video are achieved at voltages compatible with the usual drivers.

Flexoelectrically controlled surface bistable switching in nematic liquid crystals

A new bistable surface switching device using nematic liquid crystals in bookshelflike geometry is presented. The surface bistability is controlled by an external pulsed electric field of defined polarity, which breaks the surface anchoring and selects the final state from dielectric and flexoelectric couplings. A writing time of 32 μs and an optical response time of 300 μs are achieved, for a field of 18 V across a 1 μm thick cell. This simple nematic optical device presents properties directly comparable with those of ferroelectric liquid crystals displays: bistability, sharp threshold, fast address time.

ELECTROCHIRALLY CONTROLLED BISTABLE SURFACE SWITCHING IN NEMATIC LIQUID CRYSTALS

We present an electrically controlled bistable surface switching for nematic liquid crystals, in cells with the usual display geometry. The surface bistability is controlled by means of an electrochiral effect, by doping a nematic with chiral ions. The alternate transient commutation between the two surface states is related to the sign of an applied pulsed electric field. The threshold behavior allows a writing surface time of 32 μs for a field of 21 V/1.3 μm. In the birefringence mode, the optical texture response time is 0.3 ms. The effect, comparable with the one used with ferroelectric smectic liquid crystals, could be used to realize high definition video passive matrix displays.

Fast bistable nematic display with grey scale

We present a novel principle for a fast bistable nematic display with intrinsic grey scale. The geometry of a single pixel is the usual hybrid texture in between two conductive flat plates. The device is written by creating surface walls when an electric field is applied, above the thresholds to achieve both the planar anchoring breaking and an electro-hydrodynamic flow. Erasing is obtained by simply breaking the anchoring in absence of vortices. The distorted regions around surface walls depolarize the incident light. As the surface defect density can be modulated, a grey scale is achieved.

Control of transient biaxial order in calamitic nematics

The biaxial order reconstruction in nematics (BORN) is a dynamical effect which can change the topology of nematic textures. The BORN can be induced in a nematic material submitted to strong external constraints, as in the case of bulk textural distortions due to an electric field in a π cell. In this work, we show how suitable dopants for a thermotropic calamitic nematic can influence its transient biaxial order and, hence, the electric BORN threshold.

Surface order reconstruction in nematics

Investigations on π-cell, a sandwich cell with the director rotating by 180°, demonstrate the possibility to obtain nematic transitions between two textures with different topologies, for instance between an untwisted state and a π-twisted one. These fast textural changes can be obtained by bulk order reconstruction, which allows the director reorientation between two perpendicular directions without macroscopic rotations of the director itself, or by anchoring breaking, which transforms a weak planar anchoring in a homeotropic surface state. Now, we demonstrate that order reconstruction close to a boundary surface with strong or infinite anchoring conditions provides transitions equivalent to anchoring breaking.

Intrinsic multiplexability of surface bistable nematic displays

We present a simple multiplexing procedure for the flexoelectrically controlled surface bistable nematic display. When the surface anchoring is broken, the surface bifurcation between the two stable surface orientations has a very large sensitivity to external fields. This intrinsic property allows an easy multiplexing of the display. We excite each pixel with a sequence of two signals: a large driving square pulse of arbitrary polarity (or at high frequency), which breaks the surface anchoring, and a weak control square pulse, whose polarity determines the final texture of the pixel and hence its optical state. In a matrix geometry, the rows are sequentially addressed with the driving pulse. The columns are excited in parallel with the control pulse to write or erase the corresponding pixel.