Twisted Nematic Liquid Crystal Cell Research Articles

Electrooptical Properties of Carbon-Nanotube-Doped Twisted Nematic Liquid Crystal Cell

We investigated the electrooptical properties of a carbon nanotube (CNT)-doped twisted nematic (TN) liquid crystal (LC) cell. Experimental results reveal that the doped CNTs influence the elastic constant of LC–CNT dispersion. Using a small amount of CNT dopant, the field-on response time of the LC cell is nearly invariant; the threshold voltage of the cell increases due to the increase in the elastic constant of LC–CNT dispersion. At a higher CNT concentration, the marked increase in the dielectric anisotropy of LC–CNT dispersion markedly decreases the field-on response time and threshold voltage of the LC cell. The field-off response time of this cell decreases with increasing CNT concentration due to the increase in elastic constant and the slight increase in viscosity of LC–CNT dispersion. The field-on and field-off response times of the LC cell are reduced simultaneously when the LC host is doped with a moderate amount of CNT dopant.

Pixel-isolation walls of liquid crystal display formed by fluorinated UV-curable polymers

A pixel isolation wall of liquid crystal display is fabricated by utilizing the excellent phase separation between liquid crystal molecules and fluorinated UV-curable polymers. Due to the strong immiscibility of the fluorinated monomers, the segregation of the material occurs effectively to form a polymer wall during the UV exposure. Twisted nematic liquid crystal cells with fluorinated polymer wall are fabricated for the comparison with other devices made of ordinary polymer wall. The fluorinated polymer wall device exhibits enhancement of contrast due to the suppression of the light leakage through the polymer wall as well as reduced saturation voltage by anchoring enhancement.

Fast determination method of cell thickness and time-dependent twist angle in twisted-nematic liquid-crystal cells

We propose a determination method of a cell thickness d and a time-dependent twist angle Φt in a twisted-nematic liquid-crystal (TN-LC) cell using circularly and homogeneously aligned LC cells and multiple wavelengths of incident light. Influences of a spectrum distribution on determining Φt and d are theoretically discussed, compared with a monochromatic laser light source. It is found that the influence of the spectrum distribution becomes stronger with an increase in the thickness of TN-LC cells. We propose a compensation method of the influence of the spectrum width in order to accurately determine Φt and d of TN-LC cells. Results of the determination of Φt and d of two TN-LC cells are presented, and the availability of the proposed determination method using three interference filters is confirmed experimentally.

Optimal design of achromatic quarter-wave plate using twisted nematic liquid crystal cells

A novel configuration of achromatic quarter-wave plate using two twisted nematic liquid crystal (TNLC) cells is presented. The 4 × 4 Mueller matrix is used to describe the optical properties of the liquid crystal cells. The universal tabu (Utabu) search method is used for optimizing the structural parameters. Simulation results indicate that the designed structure is capable of turning a linearly polarized light into perfectly circularly polarized light and vice versa in wavelength range 1200–1650 nm. The manufacturing tolerances of cell gap, twisted angle, etc. are good.

Electro-Optical Characteristics of a Twisted Nematic Liquid-Crystal Cell Doped with Carbon Nanotubes in a DC Electric Field

Electro-optical effects of twisted nematic (TN) cells of pristine and carbon-nanotube-doped liquid crystal were investigated. In comparison with the characteristics of an undoped TN cell, both the threshold de voltage and the ioncharge effect are reduced by the addition of either single-walled (SW-) or multi-walled (MW-) carbon nanotubes (CNTs) as a dopant in the TN cell. Experimental evidence indicates that MWCNTs are a superior additive to SWCNTs.

Cell parameter determination of a twisted-nematic liquid crystal display by single-wavelength polarimetry

Optical transmittance of a 90-twisted nematic liquid crystal cell inserted between crossed or parallel polarizers is neither sensitive to the molecular twist sense nor to the exchange between the extraordinary and the ordinary birefringence axis at the input face of the cell. In this article, the equivalence between a twisted-nematic liquid crystal cell and the combination of a retardation wave plate and a polarization rotator is fully exploited to understand the physical origin of these ambiguities. We determine, in a simple way, cell parameters by means of standard cell transmittance measurements when both crossed and parallel polarizers are rotated simultaneously. We show that this procedure, which is subjected to the aforementioned ambiguities, can be completed by additional polarimetric measurements. In particular, we demonstrate that the twist angle, twist sense, retardation, and the rubbing direction can be univocally determined by experimental measurement of the Stokes parameters of the light transmitted through the cell at one wavelength. Laboratory results corresponding to a commercial liquid crystal display panel are carried out.

Determination of Director Profile of Twisted Nematic Liquid Crystal Cell with Tilted Surface Alignment by Renormalized Transmission Ellipsometry

Applying renormalized transmission ellipsometry (RTE), the evaluation of a director profile in the case of a twisted nematic cell with a relatively high pretilt angle was performed. To determine the director profile, we proposed a novel technique for determining four cell parameters, namely, the cell gap d, the pretilt angle θp, the twist angle of the director alignment φt, and the angle between the axis of the polarizer and the entrance liquid crystal director φi. To date, several methods of measuring the average tilt angle of a twisted nematic (TN) cell have been proposed, but our method of simultaneously determining the four cell parameters is considered to be quite effective in the evaluation of liquid crystal displays.

The formula of anchoring energy for a nematic liquid crystal

The interface energy for a nematic liquid crystal (NLC) is considered as the sum of potential energy between LC molecules and molecules of the substrate surface, and a formula for anchoring energy is derived by elementary principles. The anchoring energy for a NLC should have two terms, the first term is the same as the Rapini–Papoular expression, the second is related to the normal of interface and resultes from the biaxial property of a NLC induced by interface. Hence there are two anchoring coefficients, W 1 and W 2. We demonstrate that W 1 is equal to the tilt angle strength Aθ , and W 2 corresponds to the difference between Aθ and the azimuthal strength Aϕ . Thus Aθ –Aϕ is due to the biaxial property of the NLC near the interface. Applying this formula to the twisted NLC cell, we discuss the threshold and saturation field, as well as the maximal tilt angel θ m with respect to Aθ /Aϕ . Previously proposed formulae are discussed from our point view.

On the determination of order parameters for homogeneous and twisted nematic liquid crystals from Raman spectroscopy

Traditional approaches to the use of Raman spectroscopy as an aid to the determination of local order parameters in liquid crystalline materials have employed polarizations of the excitation source and/or the analyser which are orthogonal to the liquid crystalline director. The present paper describes a Raman study, which seeks to take advantage of the additional information available from examining the complete range of orientations of the director in relation to these polarization directions. A theory is developed which shows how it is possible to use this additional information to derive more reliable values of the P 2 and P 4 local order parameters in homogeneous and twisted nematic liquid crystal cells.

Achromatic polarization switch using a film-compensated twisted nematic liquid crystal cell

A broadband polarization switch consisting of an active twisted nematic liquid crystal cell and two passive uniaxial compensation films is analysed. The conjugate gradient method was used for optimizing the device parameters. Simulation results indicate that the polarization switch exhibits a broad bandwidth and high contrast ratio. The manufacturing tolerances on cell gap, twist angle and compensation films thickness are reasonably large.

Cell parameter measurement of a twisted-nematic liquid crystal cell by the spectroscopic method

We propose a measurement method to determine the cell parameters which are a cell gap thickness, a twist angle and dispersion for a twisted-nematic liquid crystal cell by the spectral method. This method finds the variation point of transmission with respect to the wavelength λ. The proposed method can be applied to the determination of cell thickness and twist angle in the low cell gap as well as to the high cell gap.

Liquid crystal alignment by Holographic surface relief grating inscribed on azo-polymer film

We found that the surface relief grating(SRG)made the nematic liquid crystal(LC)align along the grating direction. The SRGs were photo-fabricated on a liquid crystalline azobenzene side-chain polymer film by the conventional holographic method. The twisted nematic liquid crystal(TNLC)cell was prepared using two SRG substrates or two rubbed polyimide substrates as alignment layers, and the LC alignment was investigated by measuring the transmittance of a 90°-TNLC cell located between crossed polarizers. The LC aligning process was studied in-situ using the hybrid LC cell with the rubbed polyimide substrate and the SRG substrate as the reference and the test alignment layers, respectively. The in-situ transmission data show that the topological effect by the SRG formation and the intermolecular interaction effect by the azo chromophore alignment compete with each other.

A New Polyimide with Multifunction of Alignment and Planarization

We developed new material, modified polyimide (MPI) that functions as both an alignment layer for the liquid crystal (LC) and a planarization layer (called overcoat (OC) layer) covering color filter layer. This functioning organic layer is very important to reduce process step and a cost when manufacturing in-plane switching (IPS) and fringe-field switching (FFS) devices since on top substrate of both devices, the alignment layer is coated over the OC layer. The MPI showed excellent aligning capability and high transparency with moderate degree of planarization. In addition, the fabricated twisted nematic LC cell showed similar electro-optic characteristics to those with conventional polyimide.

A simple method for measuring the cell gap of a reflective twisted nematic LCD

A simple method is demonstrated for measuring the cell gap of reflective twisted nematic (RTN) liquid crystal (LC) cells. This method utilizes a single laser beam and crossed polarizer configuration such that Fabry-Perot effect and surface reflection are eliminated. Experimental results agree well with theory.

Raman scattering studies of homogeneous and twisted-nematic liquid crystal cells and the determination of 〈 P2〉 and 〈 P4〉 order parameters

Measurements of the intensities of scattered Raman radiations in different orthogonal linear polarization conditions is a well-established method for the elucidation of molecular ordering in condensed phase materials. Application of this method to liquid crystal systems, however, has been shown to lead to discrepancies from other experimental methods employed for the determination of molecular distribution functions, in particular for the 〈 P 4〉 order parameter which, with the larger term 〈 P 2〉, serves to characterise such systems. The present paper describes initial studies of spontaneous Raman scattering in homogeneous and twisted nematic liquid crystal cells which seek to overcome shortcomings in this approach by measuring the Raman intensities of these ordered systems as a function of the angle of rotation around the scattering direction. The two- and fourfold symmetry patterns generated for different polarizations of incident and scattered radiation as the sample is rotated from 0 to 360° are shown to be sensitive functions of both the 〈 P 2〉 and 〈 P 4〉 order parameters. Theoretical expressions derived on the basis of a stick model (one relevant component of the molecular polarizability only, α zz ) are shown to account for the observed patterns satisfactorily, though a fuller treatment is necessary to account for the finer details of the observed distribution of intensities.

Liquid-crystal polarization controller arrays on planar waveguide circuits

We have developed thin liquid crystal (LC) arrays for use as simple polarization-controller arrays on planar lightwave circuits (PLCs). The 30-μm thick LC arrays are vertically inserted into trenches cut across PLC substrates. Two such LC cells that are in line and parallel to each other, one of which has a direction of alignment that is rotated by 45/spl deg/ relative to that of the other, are able to convert an arbitrary input polarization to a desired one. Twisted-nematic LC cells can switch the polarization of input light between the TE and TM modes with an extinction ratio of 15 dB.

Low-Cell-Gap Measurement by Rotation of a Wave Retarder

A simple method of measuring the cell gap of a low-cell-gap twisted nematic liquid crystal (TNLC) cell is proposed. Unlike the conventional methods which make use of a rotatable polarizer and analyzer or a compensator set between a pair of crossed polarizers, the proposed method uses a wave plate set on top of a motor by which we rotate the plate by 360°. With the proposed method, we can measure the cell gap of TNLC cells with a low cell gap below 4 µm. The proposed method can also be applied to single-polarizer reflective LC cells.

Measurements of Cell Parameters in Reflective Liquid Crystal Cells by using a Stokes Parameter Method

We propose an application of Stokes Parameter Method (SPM) for determining cell parameters of reflective twisted nematic (TN) liquid crystal (LC) cells by measuring Stokes parameters of the LC cells. The relationship between the Stokes parameters of the reflection light of the reflective TN LC cells and cell properties such as the thickness and twist angle are given on Jones matrix representation. The cell properties of the reflective LC cell are discussed and compared with those of the transmissive LC cell. The cell thickness of the reflective TN LC cell can be measured, but the twist angle cannot be determined by this method.

Cell Gap Measurement Method for Single-Polarizer Reflective Liquid Crystal Cells

A novel method to measure the cell gap of a single-polarizer reflective twisted nematic liquid crystal (TN LC) cell by using a spectrometer is proposed. By rotating a LC cell, we can measure the cell gap of a LC cell as well as the wavelength dependence of retardation Δnd.

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°.