Aligned Liquid Crystal Cell Research Articles

Analytic twist angle measurement in liquid crystal cells

Previously we have described a method to measure residual twist angle in homogeneously aligned liquid crystal cells by comparing experimental results with simulated data in an electric field. This allows determination of a small twist angle to an accuracy of 0.02°. In this paper, an analytic method is described whereby it is possible to determine the small twist angle directly from the measured transmission without simulation. A simple equation for the twist angle independent of any material parameters is derived and its reliability and accuracy confirmed by comparison with the results of the previous method.

P‐122: Enhanced Dynamic Response Time of Alq3‐Doped Vertical Alignment Liquid Crystal Cell

AbstractWe mixed Alq3 in negative dielectric anisotropy liquid crystal to improve the electro‐optical characteristics of vertical alignment cell. The fabricated vertical alignment cell shows lower operating voltage and faster dynamic response time than pure vertical alignment cell without decrease of the transmittance. We analyzed the physical characteristics of Alq3‐doped liquid crystal in terms of elastic constant and rotational viscosity. Consequently, the fast dynamic response time is caused due to the increase of the elastic constant in the Alq3‐doped liquid crystal cell.

Small twist angle measurement in homogeneously aligned liquid crystal cells

During the liquid crystal display manufacture a small twist angle is usually generated. This causes light leakage in the black state and degrades the electro-optical properties of the cells. Therefore, an accurate twist angle measurement is necessary to improve the manufacturing process and the cell properties. In this paper, a simple and accurate method for the homogeneously aligned cells is described. Interestingly, peak-shaped unique voltage-transmission curves are obtained for twisted cells in both simulated and experimental studies. Comparison of the curves shows that small twist angles of less than 1° can be measured with an accuracy of about ±0.02°.

Thermally induced light leakage in in-plane-switching liquid crystal displays

In this paper, we report the thermally induced light leakage in in-plane-switching liquid crystal displays (IPS LCDs) that have a built-in small-angle deviation (<4 deg) between the polarization axis of a polarizer and the rubbing directions for liquid crystal (LC) alignment. We have found that the thermally induced light leakage depends strongly on the phase retardation which is a function of cell gap, wavelength, and birefringence of LCs used in the IPS LC cells. In addition, we have measured the light leakage of vertically aligned LC cells, which is independent of temperature and the deviation angle. A temperature-scanning method is proposed to reduce the light leakage of the IPS LCDs.

Response time characteristics of optical shutter of vertical alignment liquid crystal cell for obliquely incident light

The temporal transmittance of a liquid crystal (LC) cell shutter was investigated for various incident angles to verify the possibility of a fast switching time. For a mono-domain vertical alignment (VA) cell, the switch OFF time in which the LC restores to its initial alignment state under no driving voltage is found to relate to the angle of the incident light. A decrease of switch OFF time is observed when the azimuth angle of the incident light approaches the azimuth angle of the LC optical axis. For a 4-domain VA cell, switch OFF time is observed not to decrease for any obliquely incident angle.

Optimization of an optical configuration in a vertical alignment liquid crystal cell for wide viewing angle

We propose an optical structure for a vertical alignment (VA) liquid crystal (LC) cell with a wide viewing angle. The proposed LC cell consists of an A-plate and two C-plates for optical compensation. Optical compensation and optimization to eliminate off-axis light leakage in the entire visible wavelength range are performed on a Poincaré sphere using the Stokes vector and the Muller matrix method. After optimizing the wavelength dispersion of the retardation films that are used, we prove that the proposed VA LC cell can improve the viewing angle and contrast ratio by calculating optical characteristics, particularly in diagonal directions.

Nanosecond Laser Pulse-Induced Refractive Index Changes in Anthraquinone-Doped Liquid Crystal

The liquid crystal response to instantaneous strong laser pulses is related to its electronic, orientational, as well as thermal nonlinear optical properties. In this work, the transients of the refractive index variation in a pentyl-cyanobiphenyl (5CB) liquid crystal doped with an anthraquinone dye are studied with nanosecond laser pulse excitation at 532 nm. The magnitude of the response and its sign are studied as functions of the dye concentration and the ac electric field applied to a planarly or homeotropically aligned liquid crystal cell.

Effect of the electrode edge on the viewing angle property of a patterned vertical alignment liquid crystal cell

This paper investigates the effect of the electrode edge of a patterned vertical alignment (PVA) liquid crystal (LC) device on the viewing angle characteristics. In general, a transmissive LCD applies an LC layer with half‐wave retardation for a bright state and with zero retardation for a dark state. The retardation of the LC layer would be distorted in each point, however, when a voltage is applied because of the non‐uniform voltage distribution in the electrode edge effect. In this paper, the feasibility of the full effect of the electrode edge on the viewing angle property is considered, and the optical viewing angles of the VA LCD with a uniform half‐wave LC layer and the PVA LCD with a practical non‐uniform LC layer are compared.

Vertical alignment nematic liquid crystal cell controlled by double-side in-plane switching with positive dielectric anisotropy liquid crystal

We propose a vertical alignment liquid crystal cell controlled by double-side in-plane switching with a positive dielectric anisotropy liquid crystal material. In this structure, the pixel electrodes are interdigital electrodes, which are deposited on the bottom and top substrates parallel to each other. In the voltage-off state, the liquid crystal molecules are vertically aligned under the crossed polarizers; hence the cell appears to be dark. When an in-plane field is applied, the liquid crystal molecules tilt down to the left or right along the field direction, which is at 45° with respect to the input polarizer, giving rise to maximum transmission of the incident light. We fabricated the proposed cells, which show higher transmittance and lower driving voltage compared with the conventional vertical alignment configuration with a positive dielectric anisotropy liquid crystal. The improved electro-optical properties are due to the stronger in-plane field compared with the configuration having one-side electrode. In this device, the response time and viewing angle characteristics are almost the same as the conventional structure.

Controllable-Viewing-Angle Display Using a Hybrid Aligned Nematic Liquid-Crystal Cell

We have developed a “controllable-viewing-angle (CVA)” device consisting of a hybrid aligned nematic (HAN) liquid-crystal (LC) cell and an negative C-plate. The CVA device can control the transmission of light at predetermined azimuth angles and at the polar angles without changing the transmission of light in the normal direction. By arranging the CVA device on top of a LC panel, we have realized a simple controllable-viewing-angle display. In particular, the CVA device using a HAN LC cell realizes a stronger light-shielding effect and better control over a wider range of viewing angles than possible with a homogeneous aligned LC cell.

Surface Polymer-Stabilized Vertically Aligned Liquid Crystal Cells with Various Polymer Wall Structures

Vertically aligned liquid crystal (LC) surrounded by polymer wall, which is also known as locked-super homeotropic (LSH) mode has several advantages such as rubbing- and spacer-free, wide-viewing angle and stable LC dynamics against external pressure. However, the LSH mode shows slow response time due to the instantaneous collision between the LC molecules when a vertical electric field is applied, because the reorientation of chiral doped vertically aligned LC is only determined by the polymer wall. We found that the slow response time can be improved by surface polymer-stabilized technique where surface pretilt angle is defined on alignment layers. This technology can be used in any shape of polymer wall. (PACS 42.79.Kr, 85.60.-q).

Switching of Vertical Alignment Liquid Crystal Cell Doped with Carbon Nanotubes

We investigate switching characteristics of a vertical alignment (VA) liquid crystal (LC) cell doped with minute amounts of carbon nanotubes (CNTs). The doped CNTs increase the effective elastic constant of the LC–CNT mixture, decreasing the fall time of the cell. However, the doped CNTs disturb the LC alignment, generating complex defects in the cell under voltage supply. The flow motion of the defects generates an optical bounce on the rising edge of the optical response curve of the cell, increasing the rise time of the cell. A step-voltage driving scheme is demonstrated to eliminate the optical bounce and effectively improve the rise time of the cell. Under the CNT doping condition and the step-voltage driving scheme, the response time of the VA LC cell can be reduced to 50% of that of the pristine cell under the conventional driving scheme.

Tunable Frequency Selective Surfaces and Negative-Zero-Positive Index Metamaterials Based on Liquid Crystals

We utilize the properties of aligned nematic liquid crystal (LC) cells in the design of: (i) a new type of metamaterial whose index of refraction is tunable from negative, through zero, to positive values and (ii) micron-scale metallodielectric and all-dielectric tunable frequency selective surfaces (FSSs). The metamaterial is constructed by randomly doping a liquid crystal substrate with coated dielectric (non-magnetic) spheres and can be utilized over a large spectral range. FSSs with a liquid crystal superstrate are synthesized using conventional and genetic algorithm methods to exhibit broadband tunable filter characteristics at mid-infrared (mid-IR) wavelengths. These LC tunable FSS structures can be used to develop a new class of infrared/optical switches for terahertz applications.

P‐196: Vertically Aligned Nematic Liquid‐Crystal Display with Double‐Sided In‐Plane Switching Using Positive Liquid‐Crystal Materials

AbstractWe propose a vertical alignment liquid crystal cell controlled by double‐Side in‐plane switching with a positive dielectric anisotropy liquid crystal material. In this structure, the pixel electrodes are interdigital electrodes which are deposited on the bottom and top substrates parallel to each other. In the voltage‐off state, the liquid crystal molecules are vertically aligned under the crossed polarizers, hence the cell appears to be dark. When the in‐plane field is applied, the liquid crystal molecules tilt down to the left and right along the field direction which is at 45° with respect to the input polarizer, giving rise to transmission of the incident light. Proposed structure shows higher transmittance and lower on‐voltage compared to the conventional vertical alignment configuration with a positive dielectric anisotropy liquid crystal. The improved electro‐optical properties are due to the stronger in‐plane field compared with the configuration having one‐Side electrode. In this device, the response time and viewing angle characteristics almost the same as the conventional structure.

Dynamics of molecular exchange between aligning adsorbed film of liquid crystal and the bulk

The liquid crystal (LC) originally filled in thin cells is replaced by a different LC having a drastically different pretilt angle. The mechanism of desorption/adsorption of LC molecules from/on the adsorbed LC film at the substrate changes the composition of the adsorbed layer and thus the pretilt angle. The time dependence of molecular pretilt angle at solid surfaces of homogeneously aligned LC cells is measured. The characteristic molecular exchange time depends on the coefficients of desorption of the first LC. The results presented here provide an important insight into the molecular exchange between solid surface and bulk fluid.

Switching of polymer-stabilized vertical alignment liquid crystal cell

This work investigates the switching characteristics of the polymer-stabilized vertical alignment (VA) liquid crystal (LC) cell. The experimental results reveal that the fall time of the cell declines as the monomer concentration increases because the vertically-aligned polymer networks accelerate the relaxation of the LC molecules. Furthermore, the formed polymer networks impede the growth and annihilation of LC defects, suppressing the optical bounce in the time dependent transmittance curve of the cell when the voltage is applied to the cell, substantially reducing the rise time of the cell. A step-voltage driving scheme is demonstrated to eliminate completely the optical bounce and hence improve further the rise time of the VA LC cell. The rise times of the pristine and the polymer-stabilized VA LC cells under the step-voltage driving scheme are less than 50% of those under the conventional driving scheme.

Trapping of defect point to improve response time via controlled azimuthal anchoring in a vertically aligned liquid crystal cell with polymer wall

Conventional multi-domain vertically aligned liquid crystal (LC) cells have defect points due to the collision of LC directors during the formation of multiple domains. In addition, the location of defects changes with time resulting in a slow response time. This paper proposes a robust vertically aligned LC cell, where the LCs are locked by polymer walls, and the azimuthal anchoring on the surface of the alignment layer is controlled by the polymerization of a UV curable reactive mesogen monomer. As a result, the defect points are trapped at a single position, resulting in a greatly improved response time.

High-Precision Measurement of the Polar Anchoring Strength of Several Vertical Alignment Layer Materials by Psi Delta Hybrid Aligned Liquid Crystal Cell Method

With the increasing popularity of vertically aligned mode liquid crystal displays (VA-mode LCDs), the evaluation of the surface polar anchoring strength of the vertical alignment layer has become increasingly important. The surface polar anchoring strength is believed to be one of the most crucial elements that influence the response property of VA-mode LCDs. Therefore, a quantitative evaluation of the relationship between the anchoring strength and the response property of a VA-mode LC cell is a key issue. In this paper, we discuss the relationship between the polar anchoring strength and the response speed of a VA-mode LC cell referring to the results of an evaluation by the psi delta hybrid aligned liquid crystal cell method.

An optical configuration for vertical alignment liquid crystal cell with wide viewing angle

We propose an optical configuration of a vertical alignment (VA) liquid crystal (LC) cell to eliminate the light leakage in the diagonal direction. VA LC cell has an excellent contrast ratio in the normal direction due to the no phase‐retardation. However, change of the phase‐retardation occurs in all directions, which causes the light leakage and deteriorates the characteristics of the dark state. We designed the LC cell structure composed of multiple combinations with two A‐plates and two C‐plates in order to achieve wide viewing property on the Poincaré sphere. From calculations, we show that the proposed structure can improve the viewing angle characteristics by compensating for the light leakage in all directions.

Patterned Vertical Alignment Liquid Crystal Cell with Crossed Stripe-Electrode Patterns

We propose crossed stripe-electrode patterns for a vertically-aligned liquid crystal (LC) cell. The proposed electrode structure forms the radial LC distribution with the point singularity S = + 1 in the presence of the applied electric field, resulting in the wide viewing angle characteristics as well as the reduced color-shift compared to the conventional patterned vertical alignment (PVA) cell. Moreover, the radial LC distribution with the point singularity results in an increased transmittance compared to the conventional PVA cell, because the region with zero transmittance only occurs at the singular point.