Self-Regulation of Infrared Using a Liquid Crystal Mixture Doped with Push-Pull Azobenzene for Energy-Saving Smart Windows.

A self-regulating liquid crystal (LC) smart window whose reflectance changes with ambient conditions is demonstrated. Thermally or optically induced switching between the transparent state and a near-infrared (NIR) reflective state can be used for energy-saving windows. Reflection of NIR can reduce the energy used for cooling, while remaining transparent to visible light. By changing the initial alignment of LCs, the window can be switched between hazy-opaque and IR-reflective states to be used for privacy windows.

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.

Formation of Polymer Walls through the Phase Separation of a Liquid Crystal Mixture Induced by a Spatial Elastic Energy Difference.

We propose a method to form polymer walls without the use of a photomask in a liquid crystal (LC) cell by phase separation of an LC mixture induced by a spatial elastic energy difference. When an in-plane electric field is applied to a vertically aligned cell filled with a mixture of LC and a reactive monomer (RM), a high spatial elastic energy is induced along the direction perpendicular to the interdigitated electrodes. RMs move to the boundaries where the elastic energy is very high and an in-plane component of the applied electric field exists, which results in the phase separation of the LC/RM mixture. We have shown that we can form polymer walls by applying ultraviolet light irradiation to the LC cell. These polymer walls can function as alignment layers. We observed morphological patterns of the polymer structure through polarized optical microscopy, scanning electron microscopy, and atomic force microscopy. The polymer walls formed in an LC cell can affect the orientation of LCs in the lateral direction. Bistable switching of a polymer-walled cell could be achieved by using three-terminal electrodes where both vertical and in-plane electric fields can be applied. Vertical anchoring with the alignment layer on each substrate allows LC molecules to remain vertically aligned after removal of the applied vertical electric field. Furthermore, in-plane anchoring with the formed polymer walls allows the LC molecules to remain homogeneously aligned after removal of the applied in-plane electric field. The proposed method for the formation of polymer structures could be a useful tool to fabricate LC cells for various applications. As a bistable phase-grating device, the diffraction efficiency of a polymer-walled cell was comparable to that of a pure-LC cell. Its operating voltage was 44% lower than that of a pure-LC cell owing to in-plane anchoring provided by the polymer walls. Moreover, it can be operated with very low power because it does not require power to maintain the state. In addition, the total response time of a polymer-walled cell was approximately 68% shorter than that of a pure-LC cell because all switching was forcibly controlled by applying an electric field.

Fabrication of a Single-Substrate Flexible Thermoresponsive Cholesteric Liquid-Crystal Film with Wavelength Tunability.

Recently, single-substrate flexible liquid crystal (LC) devices have attracted considerable attention because they can provide desirable shapes, small weight, flexibility, and rollability. In this work, we fabricate a flexible single-substrate thermoresponsive cholesteric LC (CLC) film by a facile method called photoenforced stratification method. Our fabricated single-substrate CLC film consists of microscale polymer containers filled with a CLC solution. Our results showed that the temperature response of the fabricated single-substrate CLC film depends on the chiral material doped into the CLC solution. The single-substrate ultrathin CLC film exhibits very high flexibility and robustness without performance reduction. The fabricated flexible single-substrate CLC film may pave the way for the development of novel technologies for thermoresponsive devices with changeable shapes and designs.

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).

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.

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.

New linear solvation energy relationships for empirical solvent scales using the Kamlet-Abboud-Taft parameter sets in nematic liquid crystals.

The practical application of liquid crystals (LCs) as anisotropic and ubiquitous solvents is undoubtedly lucrative. Therefore, defining solvent polarity parameters as demonstrating the effects of anisotropic LC media on the photo-physical behavior of solute molecules is increasingly sought to determine their suitability for specific areas. For this fundamental reason, a spectroscopic method was used via Kamlet-Abboud-Taft (KAT) polarity functions to determine the solvatochromic polarity (SP) parameters for different LCs regarding high and low dielectric anisotropy (Δε) at different temperatures and LC phases, both isotropic and anisotropic. According to empirical solvent polarity parameters, our LCs were categorized as a dipolar hydrogen bonding donor solvent. Moreover, typical and overall matrix anisotropy polarity parameters as variations of the SP parameter values between the isotropic and anisotropic phases were sorted according to Δε magnitude. Finally, we introduced the linear solvation energy relationships of empirical solvent scales with the KAT parameters sets for the first time in nematic LCs with the well-established correlations.

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.

A Low Voltage Liquid Crystal Phase Grating with Switchable Diffraction Angles.

We demonstrate a simple yet high performance phase grating with switchable diffraction angles using a fringe field switching (FFS) liquid crystal (LC) cell. The LC rubbing angle is parallel to the FFS electrodes (i.e. α = 0°), leading to symmetric LC director distribution in a voltage-on state. Such a grating exhibits three unique features: 1) Two grating periods can be formed by controlling the applied voltage, resulting in switchable diffraction angles. In our design, the 1st diffraction order occurs at 4.3°, while the 2nd order appears at 8.6°. 2) The required voltage to achieve peak diffraction efficiency (η~32%) for the 1st order is only 4.4 V at λ = 633 nm as compared to 70 V for a conventional FFS-based phase grating in which α ≈ 7°, while the 2nd order (η~27%) is 15 V. 3). The measured rise and decay time for the 1st order is 7.62 ms and 6.75 ms, and for the 2nd order is 0.75 ms and 3.87 ms, respectively. To understand the physical mechanisms, we also perform device simulations. Good agreement between experiment and simulation is obtained.

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.

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.

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.

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.

Fast fringe-field switching of a liquid crystal cell by two-dimensional confinement with virtual walls.

We report a simple method for reducing the response time of a fringe-field switching liquid crystal cell by using two-dimensional confinement of the liquid crystals. Through both numerical calculations and experiments, we show that the switching speed can be increased by several fold in a fringe-field switching cell by simply using a rubbing angle of zero, which causes virtual walls to be built when an electric field is applied between the interdigitated electrodes and the common electrode, without requiring additional fabrication steps or complicated drive schemes. Furthermore, the devices fabricated with this method exhibit a reduced color shift and excellent dynamic stability, even with a high applied voltage and under external pressure.

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.

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.

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.