Chris Desimpel

Lateral ion transport in nematic liquid-crystal devices

Liquid crystals used in electronic displays usually contain small amounts of ions that move under the influence of the varying applied electric field. It is well known that the motion of ions perpendicular to the substrates may lead to modified electric fields resulting in image sticking effects. During operation, the modulation in the director tilt angle can also lead to a net residual lateral component of the ion motion, parallel with the glass plates. A sustained ac driving voltage will accumulate the lateral ion motion and may result in image sticking effects near the pixel edge. Such effects have indeed been observed in supertwisted nematic cells.

Measurements of lateral ion transport in LCD cells

The ions present in liquid crystal devices modulate the applied electric field and lead to deterioration of the expected good optical response. In addition to the flicker and ghost images, a boundary image-retention effect is also possible. It occurs near the edges of a stressed pixel. We have attributed this effect to ions moving in the plane perpendicular to the applied electric field. This hypothesis has been proven using a combination of electrical and optical measurements. The observed optical non-homogeneity and its evolution with stress time were explained using the new model of lateral ion transport. The physical cause of this phenomenon is subject to further study.

A four-electrode liquid crystal device for 2 pi in-plane director rotation

A novel type of liquid crystal device is described, based on a four-electrode unit, arranged in a hexagonal array. Full three-dimensional simulations were performed using a finite elements algorithm demonstrating a 2 pi rotation of the director in the plane parallel to the substrate surface. Applications for the device are situated in the field of multistable wave plates, spatial light modulators and electrically controllable anchoring.

Realization of a Four-Electrode Liquid Crystal Device With Full In-Plane Director Rotation

A liquid crystal device with micrometer-scale hexagonal electrodes has been fabricated and characterized. By using weak anchoring at the liquid crystal interfaces, the orientation of the director is completely governed by the applied electric fields. The appropriate voltage waveforms applied to electrodes allow the director in the liquid crystal layer to be rotated in the plane parallel to the substrates over large angles, exceeding 180 deg. This paper is a technological and experimental verification of an earlier proposed device concept

Simulation of V-shaped stability in AFLC.

The V-shaped transmission-voltage characteristics in FLC have been explained by the existence of a splayed state, caused by strong polar interaction with the alignment layers. Both simulation results and analytical calculations have been used to confirm this statement. The conditions that guarantee V-shaped characteristics have been described. There is no consensus on whether V-shaped characteristics can exist in AFLC. In tri-state switching the AFLC will be in the so-called ferroelectric up- (or down-) state FU (FD) for sufficiently high applied positive (negative) voltage. By means of the uniform-(phi) theory it has been shown that if V decreases to zero, one first follows the symmetrical up- (down-) state SU (SD), and then switches back to the normal alternating state so called antiferroelectric state AF. In this article we investigate switching from a strong positive voltage to zero and check under which conditions one ends up in a special alternating state SA, with both polarizations parallel to the glass surfaces, instead of in the normal alternating state, with both polarizations perpendicular to the glass surfaces. The first case guarantees V-shaped switching in AFLC, the second case leads to tri-stable switching. The simulation program is based on implicit iteration and on the Newton-Raphson linearization method. Several simulation results will be shown and discussed. They confirm that V-shaped switching occurs in AFLC under approximately the same conditions as for FLC, i.e. with strong interactions with the alignment layers.