Liquid crystal cell asymmetrically anchored for high transmittance and triggered with a vertical field for fast switching.

The optical performance of an asymmetrically surface-anchored liquid crystal (LC) cell driven with three-terminal electrodes is demonstrated. The transmittance of an asymmetrically anchored cell is considerably higher than that of a symmetrically anchored cell. However, the slow response of an asymmetrically anchored cell makes its practical application difficult. In this work, we demonstrate that the slowest GTG response time from a high to low grey level in an asymmetrically anchored cell can be reduced to less than 0.7 ms by applying a vertical trigger pulse with three-terminal electrodes while maintaining the high transmittance of an asymmetrically anchored cell.

Low-power control of haze using a liquid-crystal phase-grating device with two-dimensional polymer walls.

We propose a two-dimensional (2D) polymer-walled liquid-crystal (LC) phase-grating device, which can be used to control the haze with a very low power. 2D polymer walls can be formed in an LC cell through ultraviolet light irradiation while applying an in-plane electric field through phase separation induced by the spatial elastic energy difference. The transparent and translucent states can be realized by applying vertical and in-plane electric fields to the 2D polymer-walled LC cell, respectively. The cell can be operated with a very low power as the transparent [translucent] state is maintained even after the applied vertical [in-plane] electric field is removed. It consumes power only during state switching. The fabricated device exhibits outstanding performances, such as a very low operating voltage (< 10 V), low haze (< 2%) in the transparent state, high haze (> 90%) in the translucent state, and short switching time (< 2 ms), compared to those of other bistable LC devices, which can be used to control the haze.

Independent control of haze and total transmittance with a dye-doped liquid crystal phase-grating device.

This paper presents a dye-doped liquid crystal (LC) phase-grating cell that is switchable between transparent, dark, and opaque states. The device can control haze and transmittance independently. Initially, LC and dye molecules are twist-aligned to make the cell opaque but haze-free due to the absorption of incident light without scattering. Switching to the transparent state could be achieved by applying a vertical electric field, whereas switching to the opaque state could be achieved by applying an in-plane electric field. It exhibited several advantages, such as a low switching voltage (<18 V) and fast response time (<30 ms).

Enhancement of absorption and haze with hybrid anchoring of dye-doped cholesteric liquid crystals.

We demonstrated that hybrid anchoring of dye-doped cholesteric liquid crystals (ChLCs) could be used for the simultaneous control of haze and transmittance. Hybrid anchoring of ChLCs can be obtained by the vertical anchoring at one substrate surface and planar anchoring at the other substrate surface. In a ChLC cell with hybrid anchoring, the LCs near the planar alignment layer are in the planar state, while those near the vertical alignment layer are in the focal-conic state. In the initial opaque state, the incident light can be absorbed by the LC mixture in the planar state and scattered by the LCs in the focal-conic state. The ChLC cell with hybrid anchoring exhibited lower transmittance and haze value than those of a focal-conic state in a ChLC cell with vertical anchoring. The cell can be switched to the transparent state without use of a complicated drive scheme at a driving voltage significantly lower than that for a double-layered cell.

Switching between transparent and translucent states of a two-dimensional liquid crystal phase grating device with crossed interdigitated electrodes.

We report an electrically-switchable two-dimensional liquid crystal (LC) phase grating device for window display applications. The device consists of the top and bottom substrates with crossed interdigitated electrodes and vertically-aligned LCs sandwiched between the two substrates. The device, switchable between the transparent and translucent states by applying an electric field, can provide high haze by the strong diffraction effect thanks to a large spatial phase difference with little dependence on the azimuth angle. We found that the device has outstanding features, such as a low operating voltage, high transparency, and wide viewing angle characteristics in the transparent state and high haze in the translucent state. Moreover, we achieved submillisecond switching between transparent and translucent states by employing the overdrive scheme and a vertical trigger pulse.

Sunlight-switchable light shutter fabricated using liquid crystals doped with push-pull azobenzene.

We propose a sunlight-switchable light shutter using liquid crystal/polymer composite doped with push-pull azobenzene. The proposed light shutter is switchable between the translucent and transparent states by application of an electric field or by UV irradiation. Switching by UV irradiation is based on the change of the liquid crystal (LC) clearing point by the photo-isomerization effect of push-pull azobenzene. Under sunlight, the light shutter can be switched from the translucent to the transparent state by the nematic-isotropic phase transition of the LC domains triggered by trans-cis photo-isomerization of the push-pull azobenzene molecules. When the amount of sunlight is low because of cloud cover or when there is no sunlight at sunset, the light shutter rapidly relaxes from its transparent state back to its initial translucent state by the isotropic-nematic phase transition induced by cis-trans back-isomerization of the push-pull azobenzene molecules.

Electro-optical characteristics of an in-plane-switching liquid crystal cell with zero rubbing angle: dependence on the electrode structure.

When an electric field is applied to in-plane switching (IPS) and fringe-field switching (FFS) cells with zero rubbing angle, virtual walls are built such that the switching speed can be increased several-fold. In this study, we investigate the dependence on the interdigitated electrode structure of the electro-optical characteristics of IPS and FFS cells with zero rubbing angle. We found that when the rubbing angle is zero, the single-layered IPS electrode structure provides a higher transmittance than the double-layered FFS electrode structure because of the reduced width of dead zones at domain boundaries between interdigitated electrodes. Single-layered IPS electrodes not only minimize the transmittance decrease but also provide a shorter response time than double-layered FFS electrodes, although the operating voltage is higher and fabrication requires a more precise rubbing process. The transmittance decrease due to the zero rubbing angle in an IPS cell can be minimized using optimization of the electrode structure while retaining a short response time.

Interdigitated pixel electrodes with alternating tilts for fast fringe-field switching of liquid crystals.

We propose an interdigitated pixel electrode structure with alternating tilts for fast fringe-field switching of liquid crystals (LCs). In contrast to an LC cell, where the pixel electrodes are parallel to the LC alignment direction, this device does not require a non-zero pretilt angle, owing to an obliquely applied electric field; thus, it can retain a much wider viewing angle by aligning the LCs without a pretilt. In addition to a short response time and wide viewing angle, the proposed device allows a much larger deviation of the LC alignment direction, which is essential for mass production. Moreover, LCs with negative dielectric anisotropy can be used to minimize the transmittance decrease.

Effect of two-dimensional confinement on switching of vertically aligned liquid crystals by an in-plane electric field.

We investigated the two-dimensional (2-D) confinement effect of liquid crystals (LCs) on the switching of vertically aligned LCs by an in-plane electric field. When an in-plane field is applied to a vertical alignment (VA) cell, virtual walls are built at the center of the interdigitated electrodes and at the middle of the gaps between them. The LC molecules are confined not only by the two substrates but also by the virtual walls so that the turn-off time of a VA cell driven by an in-plane field is dependent on the pitch of the interdigitated electrodes as well as the cell gap. Therefore, the turn-off time of a VA cell driven by an in-plane field can be reduced simply by decreasing the pitch of the interdigitated electrodes as a result of the enhanced anchoring provided by the virtual walls. The experimental results showed good agreement with a simple model based on the 2-D confinement effect of LCs.

Near-zero pretilt alignment of liquid crystals using polyimide films doped with UV-curable polymer.

We propose an alignment method for the near-zero pretilt angle of liquid crystals (LCs) using polyimide films doped with a UV-curable polymer. The near-zero pretilt angle can be obtained by UV curing of reactive mesogen monomers mixed with planar alignment material while a vertical electric field is applied to an LC cell assembled after the rubbing process. We demonstrated that the pretilt angle can be decreased from 2.390° to 0.082° by employing the proposed method.

Elimination of image flicker in a fringe-field switching liquid crystal display by applying a bipolar voltage wave.

Recently, low-frequency driving of liquid crystal display (LCD) panels to minimize power consumption has drawn much attention. In the case in which an LCD panel is driven by a fringe-field at a low frequency, the image flickering phenomenon occurs when the sign of the applied electric field is reversed. We investigated image flickering induced by the flexoelectric effect in a fringe-field switching (FFS) liquid crystal cell in terms of the transmittance difference between frames and the ripple phenomenon. Experimental results show that image flicker due to transmittance difference can be eliminated completely and that the ripple phenomena can be reduced significantly by applying a bipolar voltage wave to the FFS cell.

Elimination of light leakage over the entire viewing cone in a homogeneously-aligned liquid crystal cell.

We propose an optical compensation scheme that uses uniaxial films for perfect elimination of light leakage over the entire viewing cone in a homogeneously-aligned liquid crystal cell. Uniaxial films with different dispersion characteristics are used so that they can compensate one another to achieve achromatic optical compensation. Owing to the rotational symmetry of the polarization change on the S(2)-S(3) plane of the Poincaré sphere, we can eliminate the light leakage at all azimuth angles. The contrast ratio of the proposed configuration for white light is higher than 3000:1 at a polar angle of ± 85°, irrespective of the azimuth angle.

Elimination of image flicker in fringe-field switching liquid crystal display driven with low frequency electric field.

Recently, a low frequency driving of a fringe-field switching-liquid crystal display (FFS-LCD) draws much attention to minimize the power consumption. In the low frequency driving of FFS-LCD, an image flickering effect occurs when the sign of the electric field is reversed. We suggested a method to eliminate the image flickering effect by doping small amount of bent-core liquid crystal (BLC) molecules. The BLC molecules have an opposite sign of flexoelectric polarization and reduce the flexoelectric polarization of the host liquid crystal. By adding 2.0 wt% of BLC, the total transmittance during a positive and a negative electric field could be balanced and the image flickering effect was not observed by eyes.

Achromatic wide-view circular polarizers for a high-transmittance vertically-aligned liquid crystal cell.

We propose an optical compensation scheme through which we can eliminate the off-axis light leakage in a vertically-aligned liquid crystal cell with circular polarizers. In this scheme, four uniaxial films with complementary dispersion characteristics are used to compensate one another, resulting in achromatic effective phase retardation for off-axis angles. By using the proposed optical compensation, a contrast ratio higher than 2000:1 can be realized over the entire 55° viewing cone in a multi-domain vertical-alignment liquid crystal cell with circular polarizers.

Fast bistable switching of a cholesteric liquid crystal device induced by application of an in-plane electric field.

We propose a method for fast bistable switching of cholesteric liquid crystals. Fast switching from the focal conic to the planar state can be achieved by applying an in-plane electric field to the device for a short time. The in-plane field induces a transient state, which relaxes rapidly to the initial planar state. We demonstrated that the switching time from the focal conic to the planar state could be reduced from 150 to 5 ms by applying an in-plane field instead of a vertical field. We achieved a total response time of less than 10 ms. The proposed device is applicable to a reflective display and to other optical switching devices requiring both fast response time and low power consumption.

Fast switching of nematic liquid crystals over a wide temperature range using a vertical bias electric field.

We propose a drive scheme using a three-terminal electrode structure for submillisecond switching of nematic liquid crystals (LCs). A vertical bias electric field is continuously applied to the LCs, whereas an in-plane electric field controls the switching to the bright state. Applying the proposed scheme to a homogeneously aligned nematic LC cell yields a submillisecond response time of 0.7 ms at room temperature and 4.9 ms at -20°C.

Light shutter using dichroic-dye-doped long-pitch cholesteric liquid crystals.

We propose a light shutter device using dichroic-dye-doped liquid crystals (LCs) whose Bragg reflection wavelength is set to be infrared by controlling the pitch of cholesteric liquid crystals (ChLCs). A dye-doped long-pitch ChLC cell is switchable between the dark planar state and the transparent homeotropic state. It has the advantages of high transmittance, low operation voltage, and an easy fabrication process relative to previous LC light shutter devices. The proposed light shutter device is expected to achieve high visibility for transparent organic light-emitting diode displays and emerging smart windows, which can be used in airplanes, cars, and other similar applications.

Achromatic optical compensation using dispersion of uniaxial films for elimination of off-axis light leakage in a liquid crystal cell.

We propose an achromatic optical-compensation method using uniaxial films to eliminate the off-axis light leakage at the dark state in a homogeneously aligned liquid crystal cell. Three uniaxial films with different dispersion characteristics are used so that they can compensate each other to achieve achromatic effective phase retardation at off-axis. The retardation values are optimized with the aid of the Poincaré sphere and through numerical research. A contrast ratio of higher than 2000∶1 is predicted over the entire ±60° viewing cone for a homogeneously aligned LC cell with zero pretilt angle.

In-plane switching of vertically aligned negative liquid crystals for high transmittance and wide viewing angle.

We propose a method for in-plane switching of vertically aligned negative liquid crystals (LCs) for high transmittance and wide viewing angle. By applying an in-plane electric field using a double-layered electrode structure, LC molecules can be rotated by the vertical as well as the in-plane components of the applied field over the entire region so that high transmittance can be achieved. The threshold voltage difference can be obtained simply by varying the electrode structure, which can reduce the off-axis gamma shift in a multidomain vertical alignment LC cell.

Reflective dual-mode liquid crystal display possessing low power consumption and high contrast ratio under ambient light.

We propose a reflective dual-mode liquid crystal display (RD-LCD) which has advantages of long memory retention time and high contrast ratio. The proposed device adopts ideal bistable characteristics, a cell thickness over pitch (d/p) of 0.25. It can realize long memory retention time, thereby reducing power consumption. In addition, an optical configuration for the RD-LCD makes low light leakages at dark state and shows good dispersion characteristics in both dark and bright states over the entire visible ranges. We experimentally confirmed retention time over 6 months in memory mode and memory and dynamic contrast ratios of 47:1 and 43:1 under ambient light, respectively. As a result, the proposed RD-LCD demonstrates convincingly that it is a candidate for green display.