Liquid Crystal Director Distribution Research Articles

Electrically and Optically Controllable Liquid Crystal Grating with a Patterned Surface-Command Layer

We propose a liquid crystal (LC) binary grating that can be controlled both electrically and optically in a simple scheme. The optical control scheme is obtained through the surface-induced reorientation of the LC molecules on a patterned surface-command layer by a single pump irradiation. The electrical control of the diffraction efficiency is achieved through the change in the phase retardation by the application of a bias voltage for given optically induced LC director distribution. In a coupled scheme of the optical and electrical control, the diffraction efficiency can be easily optimized.

프린지 필드에 의해 구동되는 하이브리드형 네마틱 액정 셀에서 러빙방향에 따른 전기광학 특성

We have studied the influence of rubbing directions on electro-optic characteristics of the fringe field driven hybrid aligned nematic liquid crystal (LC) cell by computer simulation. When a LC with positive dielectric anisotropy is used, the results show that the driving voltage and transmittance decreases as the rubbing direction with respect to horizontal electric field direction increases. The reason for decrease in light transmission is the increase of tilt angle on the center of common electrode. In this paper, we have studied how the rubbing angle effects on the transmittance of the cell by investigating a distribution of electric field and LC director.

Fringing fields in a liquid crystal spatial light modulator for beam steering

Phase modulating spatial light modulators (SLMs) can be used to alter the shape of a laser wavefront to achieve a deflection or change in the shape of a laser beam. This paper reports the results of characterization, simulation and optimization of a one-dimensional liquid crystal (LC) SLM. The device has a large ratio between LC layer thickness and pixel pitch that results in a fringing field between pixels. In effect, the applied phase patterns will be lowpass filtered and the loss of high frequency components limits, for instance, the usable steering range. A method is presented where intensity measurements in the far field are used to determine how the phase modulation at the SLM is distorted. The inhomogeneous optical anisotropy of the device was determined by modelling the liquid crystal director distribution within the electrode-pixel structure. Finite-difference time-domain (FDTD) simulations were used to calculate the light propagation through the LC. The simulated phase distortion was compared with the experimental results. A voltage compensation scheme to improve the diffraction efficiency was developed utilizing the measured and simulated results. It is demonstrated that a modification of the voltage patterns can give a better realization of high frequency components in the phase distribution and an increase in maximum steering angle by a factor two.

On mechanisms of the helix pitch variation in a thin cholesteric layer confined between two surfaces

Two possible mechanisms of the temperature-induced variation and jump of the helix pitch in a spatially bounded planar layer of a cholesteric liquid crystal (LC) was considered within the framework of the continuum theory of elasticity. These mechanisms are related to the existence of two configuration curves of the system free energy. The states with local free energy minima on each of the configuration curves correspond to topologically equivalent configurations of the LC director distribution and are quasi-equivalent in this sense. The transitions between such quasi-equivalent states are especially important in the first mechanism of the helix pitch jump proceeding without participation of defects. The second mechanism is related to transitions between the ground states of different configuration curves corresponding to topologically nonequivalent configurations. This mechanism requires either participation of disclination lines or the formation of defects.

Switching of negative and positive dielectro-anisotropic liquid crystals by in-plane electric fields

This article compares switching behaviors between negative (Nn) and positive (Np) dielectro-anisotropic nematic liquid crystals driven by an in-plane electric field which is generated with interdigital electrodes. Even for Np type liquid crystals, excellent viewing angle characteristics were obtained as expected. Theoretical descriptions of the switching principle, i.e., threshold behavior and response mechanism, could be applied to both the Nn and Np type liquid crystals. However, the orientational deformation of the Np type liquid crystals caused by a distorted electric field which occurred near the edges of electrodes was not the same as that of the Nn type liquid crystals. The switching of the Np type liquid crystals near the edges of the electrodes by this field was followed by the in-plane switching of the liquid crystals between the electrodes. This remarkably distinctive dynamical behavior implied a difference in the response of the longitudinal axes for Nn and Np type liquid crystal molecules. The longitudinal axes of the latter seemed to be sensitive to the electric field component perpendicular to the substrates when applying the in-plane electric field. Furthermore, these experimentally obtained results were supported by computer simulations which analyzed the liquid crystal director distribution and transmittance pattern from edge-to-edge of a pair of electrodes when applying the in-plane electric field.

Simple method for the calculation of the deformation profiles in chiral nematic liquid crystal cells with asymmetric pretilt

We present a one-dimensional simulation methodology for determining the liquid crystal director distribution in a cell for arbitrary pretilt angles on two glass substrate surfaces. The method is illustrated for cases of an asymmetric twisted nematic cell, a hybrid alignment nematic cell, and a bend alignment nematic cell.