Twisted Nematic Research Articles

Motion of Nonsingular Walls in Plane Layer of Twisted Nematics

Motion of a nonsingular flat wall in twisted planar nematic layers is investigated for a finite strength of the surface anchoring in the cell. The measurements of the wall velocity as a function of the rotation angle of the cell plate are performed. The theoretical calculations of the corresponding dependences performed for two different model anchoring potentials reveal that for thick samples the both potentials give almost the same results. For thin samples the calculation results for these potentials differ significantly. It is shown that the measurements of the wall velocity allow the determination of the anchoring strength and may be used for the reconstruction of the shape of the actual surface anchoring potential.

Electro-optical Response of Liquid Crystal Displays with Alignment Layers of a Cellulose Derivative

Low voltage operation and fast response of liquid crystal displays (LCDs) can be achieved by using a hybrid twisted nematic (HTN) cell, which is fabricated by applying the conventional twisted nematic (TN) cell. Unfortunately, the devices exhibit a narrow viewing-angle in the same order as that of TN. It has been reported that an alignment layer made of hydroxyl propyl cellulose (HPC) results in specific orientation; the director of the LC can be parallel to the substrate surface and random in the plane without any defects. The HTN cells with HPC as an alignment layer afford wide-viewing-angle as well as low voltage operation and fast electro-optical response.

Mode switching of liquid crystals improves display performance

Mobile display devices such as portable media players, satellite navigators, and laptop computers demand good readability in both indoor and outdoor settings. These stringent constraints make transflective LCDs (displays that are readable under both bright sunlight and low-light situations) a promising technology. In transflective LCDs, each pixel is divided into two parts, one transmissive and the other reflective. The limited suitability of LCDs due to their narrow viewing angles can be overcome by introducing in-plane (IPS) and fringe-field horizontal switching (FFS) modes. However, manufacturing horizontal-modeswitching LCDs requires complicated and expensive fabrication processes.1, 2 Transflective LCDs operating in a single mode and manufacturedwith a so-called single-cell-gap structure are very desirable to reduce production costs. But they require in-cell retardation layers to prevent image inversion between the reflective and transmissive modes.1, 2 Moreover, these retarders, too, have a number of significant problems, including increased operating voltage and unreliability of the materials used for their fabrication. A new optical configuration for a transflective IPS LCDwas recently reported,3 which employs a twisted-nematic liquidcrystal (TNLC) cell with a single-cell-gap structure and without in-cell retarders. Although the transmissive-mode viewing angle performance is significantly improved, the resulting reflectivemode display capability is poor, and both modes exhibit different electro-optic characteristics. Here, we propose an optical TNLC cell design for a single-cellgap transflective display in FFS mode (see Figure 1). A systematic manufacturing procedure simultaneously achieves optimal display performance of both reflection and transmission. The wavelength dispersion of the TNLC cell is suppressed effectively by introducing a half-wave plate (HWP), the best conditions Figure 1. Optical configuration of the proposed transflective LCD. HWP: Half-wave plate. QWP: Quarter-wave plate. LC: Liquid Crystal. TN: Twisted nematic.

Micro-optical polarizer with high efficiency

Based on liquid crystal polymers (LCP) a compact micro-optical polarizer with high efficiency has been designed and realized. By using cylindrical microlens arrays made of LCP and index matching layer, a complete separation of both polarization components of the unpolarized input light is achieved. Combined with a patterned twisted nematic (TN) cell, allowing a rotation of only one polarization component, a polarizer with high efficiency (72,5% (measured) is realized. Simulations and measurements are presented.

Numerical Simulation of a Twisted Nematic Cell Constructed From Micropatterned Substrates

A numerical model of the director field of a nematic in contact with a micropatterned surface has been developed. For a surface that consists of alternate homeotropic and planar stripes, it is found that differences between the elastic constants (of similar magnitude to those measured in commonly used materials) lead to considerable variation in the surface pretilt promoted by a micropatterned surface. It is furthermore shown that the director configuration adopted by a nematic in a Twisted Nematic (TN) cell constructed from orthogonally micropatterned substrates is sensitive to small changes in the elastic constants and the response to an applied field is calculated.

Single Cell Gap Transflective Liquid Crystal Display without Sub Pixels Separation

Twisted nematic (TN) liquid crystal display (LCD) is optimized for transflective liquid crystal displays. This new configuration does not separate the pixel into two sub-pixels of transmissive and reflective mode. Each pixel is partially transmissive and reflective with the help of a semi transparent mirror. The electro-optical characteristics in the transmissive mode and reflective mode match each other, which indicate that the transmissive and reflective functions can be controlled using the same voltage in a transflective LCD. TN LCD configuration exhibits a high contrast ratio, wide viewing angles, fast response time, and suitable for high quality transflective thin film transistor (TFT) LCDs.

Rewritable Liquid Crystal Gratings Fabricated Using Photoalignment Effect in Dye-Doped Poly(vinyl alcohol) Film

In this paper, we present rewritable liquid crystal (LC) gratings based on azo dye-doped polyvinyl alcohol films. The mechanism of such rewritable characteristics is attributable to the irradiation of these films with a suitable linearly polarized light beam which reorients dyes and then aligns LCs. Two gratings, one with a 90° twist nematic (TN)/homogeneous periodical alignment and the other with a 45° TN/-45° TN reversely twisted periodical alignment, are fabricated. The polarizations of the beams diffracted from these two gratings are studied. A single model using the Jones matrix representation is formulated and used to analyze these two gratings. Experimental results are consistent with theoretical results and show that the gratings can be used as polarization beam splitters and are electrically switchable.

Fast Switching Liquid Crystals for Color-Sequential LCDs

High birefringence (0.3 < Deltan < 0.4) and relatively low-viscosity liquid crystal mixtures containing isothiocyanato tolane and isothiocyanato terphenyl liquid crystals are developed. A twisted-nematic (TN) 1.6-mum thin cell for color-sequential liquid crystal display with ~1-ms response time is demonstrated.

Molecular dynamics simulation of a nanoscopic nematic twist cell

We present molecular dynamics simulations of a nanoscopic nematic twist cell confined within two bounding substrates with conflicting anchoring conditions. The results of our simulations show that the torque transmitted through the cell drops significantly below a certain critical cell's thickness, thus confirming the predictions of the continuum Landau theory extrapolated down to the nanoscopic scale [F. Bisi, E. G. Virga, and G. E. Durand, Phys. Rev. E 70, 042701 (2004)].

Glassy photomechanical liquid-crystal network actuators for microscale devices

In light-driven liquid-crystal network (LCN) actuators, large performance improvements are obtained by varying the orientation of the molecular director through the thickness of the film actuator. Experiments show that sub-millimeter bending radii are achieved using a splayed molecular orientation. Systems with a splayed or twisted nematic (TN) director profile drive greater amplitude and faster bending than uniaxial planar systems with the same chemical composition. The bending radii of these systems are predicted using a simple model including effects of light intensity, material composition and actuator thickness.

Transflective twisted‐nematic liquid‐crystal display with double twisted‐nematic cell and twisted liquid‐crystal retarder

Abstract— Two configurations, (i) a double‐cell‐gap twisted nematic (DTN) liquid‐crystal display (LCD) and (ii) a single‐cell‐gap twisted‐nematic (TN) liquid‐crystal display (LCD) using a twisted LC retarder, were optimized for transflective liquid‐crystal displays. For the DTN configuration, both the single‐cell‐gap approach and the double‐cell‐gap approach were considered. The optimized configurations exhibit a high contrast ratio, wide viewing angles, and achromatic (black/white) switching in both the transmissive and reflective modes. They are easy to fabricate and also possess a perfect dark state. Both are suitable for high‐quality transflective TFT‐LCDs.

51.1: Invited Paper: Viewing‐Angle Compensation of TN‐ and ECB‐LCD Modes by Using a Rod‐Like Liquid Crystalline Polymer Film

AbstractWe have developed a liquid crystalline retardation film technology by using a rod‐like liquid crystalline polymer (LCP) for various LCD modes. In particular, considerable improvements in viewing angle performance have been achieved for the Twisted Nematic (TN) and the transmissive/transflective Electrically Controlled Birefringence (ECB) modes by using hybrid aligned nematic film ‘NH Film’.

Comparison of optical compensations based on discotic liquid crystals with linear and non-linear orientation for wide viewing angle twisted nematic liquid crystal displays

Optical compensation film using discotic liquid crystals has been commercialized to suppress light leakage of twisted nematic (TN) liquid crystal (LC) cells in the dark state when the viewing directions are off-axes. In the conventional film, the tilt angle variation of the discotic LC layers is linear, whereas that of the rod-like LC layers in the TN cell is non-linear with twist alignment. We investigated the optical compensation effects in terms of iso-contrast ratio, gray scale inversion and color shift with linear and non-linear orientation in a tilt angle or twist and non-twist orientation on TN-LC cells. The results showed that the optical compensation with non-linear orientation exhibited an improved performance in suppressing light leakage in the dark state.

Biphotonic recording effect of polarization gratings based on dye‐doped liquid crystal films

This study investigates a holographic recording effect of biphotonic polarization gratings (BPGs) written on dye‐doped liquid crystal (DDLC) films. One linearly polarized green light (λG = 514.5 nm) and a polarization‐modulated interference pattern formed by two mutually coherent orthogonal (±45° with respect to the polarization of the green light) polarized red lights simultaneously excite the DDLC films to generate a BPG. The formation of BPGs depends primarily on the dichroism of the dye molecules and a sequence of mechanisms: photoisomerization, anisotropic adsorption and inhibition of dye adsorption. The cis‐isomer absorbance‐modulated distribution associated with the red polarization‐modulation pattern induces the dye adsorption‐modulated pattern, in turn, yielding the permanent BPG which generates a modulated twisted nematic (TN) structure pattern in the sample. It is found that each BPG is verified to be electrically switchable and thermally erasable. The switching time is in the order of milliseconds. Additionally, the recording time to form a BPG decreases with increasing intensity of the green pump beam.

Diffraction Properties of Nematic Phase Gratings with Photoregulated Liquid Crystal Cells

A detailed analysis of the recently proposed [Appl. Phys. Lett. 87 (2005) 161112] nematic phase gratings with photoregulated liquid crystal cells is presented. Nematic phase gratings with photoregulated photo-cross-linkable polymer liquid crystals were prepared and characterized. The phase grating consists of homogeneous and 90°-twisted nematic (TN) areas and shows unique polarization conversion in the diffraction process. The diffraction properties, including higher-ordered diffractions, were well explained by means of the elastic continuum theory, Jones calculus, and diffraction theory.

Liquid-crystalline physical gels

Liquid-crystalline (LC) physical gels are a new class of dynamically functional materials consisting of liquid crystals and fibrous aggregates of molecules that are called "gelators". Liquid-crystalline physical gels, which are macroscopically soft solids, exhibit induced or enhanced electro-optical, photochemical, electronic properties due to the combination of two components that form phase-separated structures. In this tutorial review, we describe the materials design and structure-property relationships of the LC physical gels. The introduction of self-assembled fibers into nematic liquid crystals leads to faster responses in twisted nematic (TN) mode and high contrast switching in light scattering mode. Furthermore, the LC physical gels can be exploited as a new type of materials for electro-optical memory. This function is achieved by the control of reversible aggregation processes of gelators under electric fields in nematic liquid crystals. Electronic properties such as hole mobilities are improved by the introduction of fibrous aggregates into triphenylene-based columnar liquid crystals. The incorporation of photochromic azobenzenes or electroactive tetrathiafulvalenes into the chemical structures of gelators leads to the preparation of ordered functional materials.

Liquid Crystal Alignment on Rubbed Polyvinyl-Cinnamate with Overdose of UV Light

Liquid crystal (LC) alignment properties and the optical writing and erasing have been reported in the LC cell with rubbed Polyvinyl-Cinnamate (PVCi) alignment film. The azimuthal anchoring energy of the PVCi surface increased by the UV light exposure. The optical writing of twisted nematic (TN) orientation domains was demonstrated on the homogeneous orientation background in the LC cell with the UV dosage less than 2 J/cm2 from a super high pressure Hg lamp. The TN orientation changed to the homogeneous orientation again with the overdose of the UV light and the written image erased, if the LC material with the UV absorption around 313 nm of the line spectrum of the Hg lamp was used. On the other hand, when the LC without the UV light absorption was used, the LC alignment did not change and the written image was kept with the UV dosage more than 30 J/cm2.

Photoalignment Material of High Thermal Resistance with Hydroxyl Aromatic Polyimide for Liquid Crystal Display

We synthesized photoalignment material of high thermal resistance with hydroxyl aromatic polyimide, and studied the liquid crystal (LC) aligning capabilities on the photopolymer layers. Also, electro-optical (EO) performances for the twisted-nematic (TN)-liquid crystal display (LCD) photoaligned with linearly polarized UV exposure were investigated. A good LC alignment with UV exposure on the photopolymer surface can be obtained. However, the low pretilt angles were obtained below 1°. The voltage-transmittance curve without backflow bounce in the photoaligned TN cell with UV exposure was observed. The response time of photoaligned TN cell was measured about 24 ms. The residual DC voltage of photoaligned TN cell will be discussed.

Electro-optical Characteristics of Twisted Nematic (TN)-Liquid Crystal Display (LCD) on a Polyimide Surface Exposed to an Ion Beam (IB) with New Type Equipment

We studied liquid crystal (LC) alignment with ion beam (IB) on polyimide and electro-optical characteristics of twisted nematic (TN)-liquid crystal display (LCD) on the polyimide surface using obliquely ion beam (IB) exposure with new IB type equipment (DuoPIGatron type ion gun). A good uniform alignment of the nematic liquid crystal (NLC) alignment with the ion beam exposure on the polyimide surface was observed. In addition, it can be achieved the good EO properties of the ion-beam-aligned TN-LCD on polyimide surface. Also the EO characteristics of the ion-beam-aligned TN-LCD on a polyimide (PI) surface with ion beam exposure using new type IB equipment is almost the same as that of ion-beam-aligned TN-LCD on a polyimide (PI) surface with ion beam exposure using Kaufman-type Ar ion gun.

프로젝션 TV 적용을 위한 액정 디스플레이의 열적 안정성에 관한 연구

We have investigated electro-optical characteristics in three kinds of twisted nematic (TN) cells on the polyimide surface. The threshold voltage and the response time of thermal stressed TN cells were same that of no thermal stressed TN cells. There were little change of value in these TN cells. On the other hand, transmittance of no thermal stressed TN cells were better than that of thermal stressed TN cells. Transmittances of TN cells on the polyimide surface decreased by increasing thermal stress time. Moreover, the residual DC of the thermal stressed TN cells increased as increasing thermal stress temperature and time. Therefore, thermal stability of TN cells were decreased gradually by giving high thermal stress for a long time.