Aligned Liquid Crystal Cell Research Articles

Ac and dc electro-optical response of planar aligned liquid crystal cells

Interaction of static electric fields with a nematic is complicated because of the presence of charges, which originates from aligning layers or from molecular breakdown, affecting the electro-optic response of a liquid crystal cell. We analyze the aligning layers effect on the electro-optical response and develop a model based on a description of the ion balance on the conducting plates taking into account both drift current and saturation effects due to the ion migration inside the cell, neglecting diffusion current. The model well agrees with data for low voltages, while for higher ones, diffusion current should be taken into account.

Pretilt Angle Effects on Liquid Crystal Response Time

Pretilt angle effect on liquid crystal dynamics is analyzed theoretically. Analytical expressions are derived to describe liquid crystal response time under nonzero pretilt angle conditions. The theoretical analysis is confirmed experimentally using vertically aligned liquid crystal cells. This finding quantitatively correlates pretilt angles with liquid crystal response time. This study improves the understanding of the liquid crystal dynamic process which is helpful for optimizing liquid crystal response time.

Analysis of the surface order parameter of liquid crystal on a polymer surface using the phase transition droplet method

The essence of the anisotropic alignment of liquid crystal (LC) molecules at the substrate surface is derived from the distribution of the molecular direction of the LC, which is best described in terms of the order parameter. The order parameter of LC molecules at a polymer surface can be determined by measuring the contact angle of a nematic LC droplet at the nematic-isotropic phase transition temperature. In this study, we quantified the relationship between the surface order parameter and contact angle of the droplet for a homogeneous alignment LC cell. Based on the results, we propose a model for a droplet on a polymer surface. Its validity was confirmed by measuring the polarized infrared absorption.

Control of the defect in the liquid crystal director field on the slit of the patterned vertical alignment cell

In this letter the authors studied the method to control the position of the generated defect on the slit of the patterned vertical alignment liquid crystal (LC) cell with dynamic stability. The authors modeled the LC director field with a defect on the slit and proposed an advanced wing pattern with a defect trap to stick the defect outside the active area. This shape can prevent the generation of the defect on the slit in the active area even if the authors increase the operation voltage, so that the LC dynamics on the slit is very stable and optical transmittance becomes higher.

Reverse tilt domains in liquid crystal cells with a splayed director configuration

When an electric field is applied to a nematic liquid crystal cell with splay alignment layers, which is realized by mechanical parallel rubbing, the system exhibits abnormal reverse tilt domains. Even though the system has pretilt angle due to symmetry breaking by rubbing, existence of singular point in a cell by splay geometry and shift of that by locally asymmetric pretilt angle configurations result in the abnormal reverse tilt domains. On the basis of Oseen-Frank theory, dependence of singular point upon the abnormal reverse tilt domains is theoretically investigated for a splay alignment liquid crystal cell. By using the Dickman’s Q-tensor method [SID Int. Symp. Digest Tech. Papers 24, 638 (1993)], liquid crystal director field is calculated for the splay-aligned liquid crystal cell that an electric field is applied to. We presume a model to explain the abnormal reverse tilt domains, and derive the conditions required for the generation and elimination of these domains.

Nanosphere dispersed liquid crystals for tunable negative-zero-positive index of refraction in the optical and terahertz regimes

An analysis of aligned nematic liquid crystal cells containing core-shell nanospheres shows that it is possible to devise a new type of metamaterial whose index of refraction is tunable from negative, through zero, to positive values. The design parameters for the constituents can be scaled for application in the optical as well as very long wavelength (e.g., terahertz and microwave) regions.

Dopant-Induced Contrast Enhancement of Vertically Aligned Liquid Crystal Cells

A simple method is developed for aligning the laterally difluorinated high birefringence liquid crystal (LC) compounds and mixtures in a homeotropic cell. Adding a few percent positive dielectric anisotropic or nonpolar LC material not only enhances the contrast ratio but also improves the overall figure-of-merit. Several negative LC compounds with a wide variation of birefringence are used to investigate this phenomenon. Molecular modeling and experimentation are demonstrated to support this concept.

Vertically Aligned Liquid Crystal Cell with Image-Enhanced Reflector for High-Contrast Transflective Displays

We propose vertically aligned liquid crystal cell structures with image-enhanced reflectors for transflective displays. Liquid crystal (LC) molecules are aligned vertically so that an excellent dark state can be achieved. Wide-band quarter-wave design is employed to realize broad spectral characteristics over the entire range of visible wavelengths. The display performance in the transmissive mode can be markedly improved compared with that of homogeneous cells.

Cell Gap-Dependent Transmittance Characteristic in a Fringe Field-Driven Homogeneously Aligned Liquid Crystal Cell with Positive Dielectric Anisotropy

Transmittance characteristic in a homogeneously aligned liquid crystal (LC) cell driven by a fringe-electric field is investigated as a function of cell gap using the LC with positive dielectric anisotropy. In this device, the fringe-electric field drives the LCs to rotate so that the dielectric torque is electrode-positional dependent, which results in electrode-position dependency in the LC's rotating angle. As the cell gap decreases to 2 µm, more LCs are affected by surface anchoring, and the LCs above the center of electrodes, in which the LCs are twisted by elastic force between neighboring molecules, are less twisted compared to the 4 µm cell. Consequently, when the cell gap decreases from 4 to 2 µm, the transmittance also decreases even though the cell retardation value remains the same.

Modeling of Defects Nucleation in the Inhomogeneous Liquid Crystal Director Field

With the fast Q-tensor method, which can model the defect dynamics in a liquid crystal director field, the nucleation and dynamical behavior of defects is modeled. In order to model the defect, hormeotropic aligned liquid crystal cell with step inhomogeneous electrode which has a height of $1\;{\mu}m$ is used. From the simulation, we can observe the nucleation and line of the defect from surface inhomogeneity and the experiment is performed for confirmation. The experimental result is compared with numerical modeling in order to verify the simulation of the defect nucleation.

P-181L: Late-News Poster: High-Precision Measurement of Polar Anchoring Strength and Elastic Constant using Hybrid Alignment Nematic Cell for Improvement of Response Property of VA-mode LCD

The surface polar anchoring strength is the most crucial element which governs the response property of VA-mode LCD. We devised a new measurement method for the surface polar anchoring strength and the elastic constant using a hybrid alignment liquid crystal cell. We measured the angle dependency of the phase difference in the direction perpendicular to the rubbing direction of HAN cells and demonstrated that these parameters can be measured with a high degree of accuracy.

P-126: Polar Anchoring Energy Measurement of Vertically Aligned Liquid Crystal Cells

A simple high electric field method is developed for measuring the polar anchoring strength of a vertically aligned nematic liquid crystal cell. Both theoretical analyses and confirming experimental results are presented. Buffed polyimide vertically aligned liquid crystal cells are used as examples.

Direction-tunable cholesteric phase gratings

In suitable conditions, the homeotropic aligned cholesteric liquid crystal (ChLC) cell can be in fingerprint texture when abruptly switching off the applied electric field. The stripe orientation depends not only on the thickness-to-pitch (d∕P0) ratio, but also on the applied driving voltage. In this paper, the ChLC phase grating with the field-controllable grating orientation is realized and the operational mechanism of this device is presented.

Vertical alignment liquid crystal cell with optically compensated splay configuration of the liquid crystal

We have observed a phenomenon associated with a transition from vertical alignment to an optically compensated splay structure. With rubbed homeotropic alignment in parallel directions, the device shows vertical alignment but the liquid crystals (LCs) are twisted 180° in the absence of an electric field. Depending on the voltage applied, two different configurations of LCs are possible. After applying a critical voltage, the LC configuration becomes splayed such that the middirector lies parallel to the substrate and around it, and a hybrid structure forms symmetrically. A method for obtaining the transition and the electro-optic characteristics of the device is discussed.

PHASE MODULATION TIME DEPENDENCE MEASUREMENT

Phase modulation using liquid crystals has wide-ranging applications including correlation [1] and telecommunications [2,3]. At present, relatively slow (order 5ms) nematic cells demonstrate the advantage of continuous phase modulation but are sensitive to polarisation of the input light [4,5]. The alternative is to use faster ferroelectric cells which exhibit polarisation insensitivity but show only binary phase modulation [2]. This work demonstrates a novel technique for measuring the phase response time, which allows evaluation of different cell topologies. Rise and fall times are calculated in the Fourier plane from a wave-front splitting interferometer. Phase modulation measurements of in-house cells have been made for planar and hybrid aligned liquid crystal cells using nematic liquid crystals. The results demonstrate the capabilities of in-house cells, and also show agreement with theoretical predictions of cell behaviour.

On the non-symmetric planar aligned NLC cell

The planar aligned nematic liquid crystal cell with different anchoring for the two substrates (i.e. a non-symmetric NLC cell) is investigated by an analytical method. We deduce the basic equations and the boundary conditions of the tilt angle θ of the LC director. Expressions for threshold and saturation magnetic field are obtained, and numerical results of these two quantities with variation in anchoring parameters of the two substrates are given. A symmetry breaking parameter Δ is introduced and the relations between Δ and applied field, as well as the two sets of anchoring parameters are discussed in detail. A feasible experimental plan for measurement of anchoring strengths of a series of different substrates is proposed.

P‐121: Optical Anisotropic Compensator by Polymer Stabilization Technique

AbstractPolymer stabilization technique was applied for making optical anisotropic compensators for the purpose of controlling the wave length dispersion characteristic. Quarter wave compensators with hybrid aligned structure were prepared by this technique using liquid crystals (LCs) with different optical birefringence (Δn). The wavelength dispersion of the resulting compensators depended on the Δn of the LC used. The compensator using LC with the smallest Δn was found to give almost same retardation value over the wide range of visible light. Hybrid compensators were combined with hybrid aligned liquid crystal cells for reflective‐ LCD application. The hybrid compensator with very low wavelength dispersion gave the reversed dispersion characteristics very close to the ideal one at black state.

High contrast ratio of a vertically aligned liquid crystal cell using photocrosslinking alignment

A photo-induced alignment layer for LCDs has been successfully fabricated using the polarized UV photoreaction of a photo-crosslinkable polymer with the incident UV light at some angle with respect to the cell normal. The surface alignment and electro-optic properties were investigated for various UV exposure times. The homeotropic alignment layer showed a discrete anisotropic dichroic ratio, its surface morphology became smoother as the UV exposure time increased, and the LCs were arranged in a perpendicular direction to the PUVL direction with a proper pretilt angle. The cell showed no defects under cross-polarized microscopy and the contrast ratio was as high as 550:1 in transmittance. The contrast ratio pattern was found to be very similar to that of dichroic ratio as a function of UV exposure time and depended upon the frequency change to some degree. The response time was also investigated.

Application of a Circularly Homogeneously Aligned Liquid-Crystal Cell to Real-Time Measurements of Twist Angles in Twisted-Nematic Liquid-Crystal Cells

A novel system which employs a pair of circularly homogeneously aligned liquid crystal cells (CH-LC cells) for measuring the time-dependent twist angles of twisted- nematic liquid crystal (TN-LC) cells is proposed. A unique polarization-converting property of a CH-LC cell is discussed. A scheme of calculating the twist angle is derived by the Jones matrix. The applicability of the proposed system is confirmed by measuring the time-dependent twist angle of a TN-LC cell with partially saponified polyvinyl alcohol (PVA) films in real time. Two factors, on which the measurement accuracy depends, are discussed: deviations of the polarizer and the analyzer from both x and y-axes and the resolution of the measured transmission image. The measurement accuracy in this paper is estimated to be within about 1°.

Liquid Crystal Alignment on Polymer Films

The rubbed polyimide (PI) and PI LB films were fabricated and treated to align the nematic liquid crystal (LC) 5CB. The rubbed polyimide (PI) films aligned LC cell shows a higher LC pretilt angle than the cell aligned by PI LB films. Atomic force microscope (AFM) observations indicate that the rubbed PI films have a needle-like structure, while PI LB films have a groovy structure. We think that the needle-like structure plays a major role in producing the higher LC pretilt angle.