Transparent windows that insulate infrared (IR) light entering indoor spaces promise to reduce energy consumption. However, a long-standing challenge for an energy-efficient window is achieving compactness and flexibility simultaneously. In this paper, we report the fabrication of a transparent, flexible, ultrathin, and thermally-insulated IR reflector using a coatable chiral-nematic liquid crystal polymer and polyvinyl alcohol (PVA) as an alignment layer. PVA is transparent, biocompatible, thermostable, noncorrosive, and cost-effective. The fabricated IR reflector shows high optical transparency (similar to 91%) and low haze (approximate to 9.8%) values in the visible spectrum of light. Moreover, it significantly reduces greenhouse gas emissions by preventing energy consumption for cooling systems and lighting in indoor spaces; specifically, it reduces electricity consumption in indoor spaces for lighting and cooling systems under hot weather conditions. To demonstrate the mechanical stability of the fabricated reflector, many mechanical, flexibility, and bending stability tests were conducted. The results show that our proposed IR reflector is a potential candidate for designing desirable shapes for many applications, such as anti-IR devices and energy-efficient windows, for achieving significant environmental and economic benefits. (C) 2021 Elsevier B.V. All rights reserved.

A self‐regulating liquid crystal (LC) smart window whose reflectance can be controlled by 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.

A dye-doped cholesteric liquid crystal smart window with a haze-free opaque state is demonstrated. We designed a random stripe pattern in the cholesteric phase, known as the fingerprint (FP) texture, as an initial state (opaque state). In the theoretical calculation of the LC alignment states, although the transmittance of the FP state is slightly higher than that of the twisted state (planar state), it is much lower than homogeneously aligned states. Moreover, we confirmed the proposed LC cell shows the lowest operating voltage (4 V) among guest-host liquid crystal cells because of vertical anchoring at the surfaces.

A liquid crystal (LC) phase grating devices have been studied actively because of their outstanding features, such as the high diffraction efficiency, large diffraction angle, no diffraction at the initial state, and the simple fabrication process. It can be used to control the haze value owing to its high diffraction efficiency. Although it can be operated with low power, power consumption needs to be further reduced because it requires power to maintain the diffraction. To reduce the power consumption in a phase grating device, bistable operation, which consumes power only while it is being switched between the states, is necessary. In this paper, we will introduce bistable LC phase grating devices which can provide a translucent state with a high haze value thanks to its strong diffraction. Moreover, it can be operated with very low power as the transparent [translucent] state is maintained even after the applied vertical [in-plane] electric field is removed. We believe that these devices can be one of the new candidates for power-saving smart window or window display applications.

We proposed a new method for designing graded index lens using liquid crystal infiltration into annular photonic crystals. Applying an external nonuniform voltage in the transverse direction perpendicular to the direction of light propagation yields different orientation of liquid crystal molecules inside the photonic crystal unit cells. As a result, a gradient refractive index was modulated. We numerically investigate focusing properties of the designed graded index structure using plane-wave expansion and finite-difference time-domain methods. The gradient refractive index profile was adjusted by varying the nonuniform voltage excitations, which consequently altered the focal distance of the graded index structure. A wide tuning range of 1856 nm was achieved for focal distance by the proposed graded index structure. This feature can be implemented for planning a flat lens with tunable focal distance based on electro-optic effect. These achievements may have future applications in some optical devices such as near-field imaging and scanning.

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.

Disc–shaped liquid crystals with interesting structural features are an important research field in the science and technology. Function of these strongly depends on their molecular structure and the media that surrounds them. This paper highlights spectral features of three symmetrical disc–shaped liquid crystals with similar structures and dissimilar substituent groups in various surrounding media to recognize their interactional and structural similarities and differences. Determining the type of molecular interactions in these liquid crystals and their effect is of great importance in accurate determination of host–guest interactions for diverse applications in polymer–dispersed liquid crystal displays, elastomers and optical industries, and so on. For this purpose, spectral variations were analyzed through KAT (Kamlet–Abboud–Taft) model and the dipole moments were measured through UV–visible spectroscopic technique, too. The dipole moment results reveal that the electron cloud resides in the central part of these liquid crystals. The experimental data were validated via DFT (Density Functional Theory) calculations. Both experimental and theoretical data assist to better identify the spectroscopic features of these liquid crystals and give useful information about their performance in various media that can be valuable for designing applicable guest–host or mixture liquid crystal systems.

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).

It is well-known that orientational order of nematic liquid crystals (NLCs) is converted during phase transition from the long-range order (LRO) in the nematic phase to the short-range order (SRO) in the isotropic state. Finding the initial temperature range of SRO in the isotropic state can be a challenging issue because most of the physical constants disappear after this phase transition. For the first time, to resolve this issue, we introduced a new method based on the electro-optic Kerr (EOK) effect in NLC mixtures. In this method, the coherence length (ξ), which was qualitatively estimated by employing the pre-transitional temperature (T⁎) according to the de Gennes phenomenological theory, and the refractive index (n) were correlated with the Kerr constant (B) at the same temperature in the isotropic phase. Because molecular interaction in the NLCs is one of the essential factors in the investigation of the orientational ordering, we used n and ξ as described intra- and intermolecular interactions, respectively. In other words, n attributes to the polarization and dipole moment as an important parameter of the intramolecular interaction and ξ demonstrates the intermolecular interaction and domains of SRO interaction in the isotropic phase. Finally, the initial temperature range of SRO in the isotropic phase was found from the temperature dependence with B of the normalized values of influential coefficients n and ξ at the similar temperatures. Our proposed method appropriately can provide a suitable approach to distinguish the preservation of the anisotropy effect in the isotropic phase of NLCs.

Tristate switching of a liquid-crystal (LC) device among transparent, haze-free dark, and high-haze dark states is presented. In the haze-free dark state, LCs and dye molecules are twisted so that the incident light is absorbed without light scattering. In the high-haze dark state, LCs and dye molecules are randomly oriented by electrohydrodynamic instability so that the incident light is not only absorbed but also simultaneously scattered. Depending on the application, the optical characteristics of the proposed LC device can be tuned by employing an LC material with the appropriate birefringence value. Since the polymer structure is not used, it shows a clear transparent state with little haze. Owing to its simple switching process and excellent optical performance, the proposed LC device is very promising for the development of a multipurpose switchable window, which can be used for energy saving and privacy protection.

It is well known that a relatively large Kerr effect in the isotropic phase of nematic liquid crystals (NLCs) is related to the existence of short-range ordering (SRO). Thus, the conversion of the long-range ordering (LRO) behavior in the nematic phase to the SRO in the isotropic phase of NLCs can characterize the nonlinear electro-optic Kerr effect. To show conversion from LRO to SRO, we employ an empirical solvent polarity scale (ETN). Interestingly, ETN originates from the solvatochromic polarity (SP) parameters, which demonstrates the quantitative value for describing the molecular interaction in each temperature point, even in the isotropic phase of NLCs. Our results show that the higher ETN value leads to a larger Kerr constant (B) owing to the larger short-range molecular interaction between rigid rod-like NLCs. To demonstrate a linear relationship of the B value with the ETN and SP parameters, a new correlation is introduced for the first time. Our investigations and newly introduced parameters strongly indicate that short-range molecular interactions still exist in the isotropic phase of NLCs, even when the macroscopically observable LRO behaviors disappear.

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.

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.

Light shutter technologies that can control optical transparency have been studied extensively for developing curtain-free smart windows. We introduce thermally and optically switchable light shutters using LCs doped with push-pull azobenzene, which is known to speed up thermal relaxation. The liquid crystal light shutter can be switched between translucent and transparent states or transparent and opaque states by phase transition through changing temperature or photo-isomerization of doped azobenzene. The liquid crystal light shutter can be used for privacy windows with an initial translucent state or energy-saving windows with an initial transparent state.

We introduce 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 with little dependence on the azimuth angle owing to a large spatial phase difference. This device exhibits outstanding features, such as a low operating voltage, high transmittance, and wide viewing angle in the transparent state and a high haze in the translucent state. In addition, the LC device can provide sub-millisecond switching between the transparent and translucent states with the use of an overdrive scheme and a vertical trigger pulse.

In this paper, we present the effects of curing temperature on switching between the transparent and translucent states in a polymer-stabilized liquid-crystal (PSLC) cell. When cured at a low temperature, polymer structures are formed without disturbing the initial alignment of LCs so that haze in the transparent state is dramatically reduced. Moreover, we can achieve very fast switching between the haze-free transparent and high-haze translucent states because there is a little degradation in the performance of the transparent and translucent states when the polymer concentration is increased. We demonstrated an ON-OFF response time of less than 3 ms and a gray-to-gray response time of less than 6 ms in a PSLC cell.

We demonstrate an ion-doped liquid-crystal (LC) cell that can provide a very low specular transmittance in the opaque state using the electro-hydrodynamic effect. In the opaque state, the LC and dye molecules are oriented randomly in planes parallel to the substrates because of the electrohydrodynamic effect. We found that the total transmittance of an ion-doped LC cell is almost the same as that of a light shutter based on light absorption and its haze value is the same as that of a light shutter based on light scattering. Moreover, an ion-doped LC cell can be fabricated without an ultraviolet curable process. Owing to its excellent opaque state properties, an ion-doped LC cell can be used in see-through display and smart-window applications.

The temperature dependence of optical birefringence, order parameter, effective geometry parameter, and normalized polarizability was determined for nematic liquid crystal mixtures by measuring the refractive indices. The effects of the adjustable parameters obtained from temperature-dependent refractive indices were analyzed appropriately for the crossover temperature and order parameter values. Besides, a four-parameter power law expression method was used for the optical birefringence measurement to address inappropriate fits in the weak first-order phase transition region. From this method, the effective second order phase transition temperature T** was determined by showing the slightly higher values than phase transition temperature. The value of the order parameter critical exponent A obtained from this method showed a good agreement with the mean field theory by demonstrating closeness to the tricritical hypothesis (beta = 025). As a consequence, the measurement of the temperature-dependent optical birefringence was successfully verified the pre-transitional phenomena accurately, and it can facilitate an insightful description of that region. (C) 2018 Elsevier B.V. All rights reserved.

Self-switching smart windows, which can be switched between the transparent and opaque states by ambient conditions, have applications in buildings, automobiles, and switchable sunglasses. In particular, technologies for transmittance control without light scattering are highly desirable owing to their ability to control the throughput of sunlight and solar heat through a window without blocking the view. In this study, a self-shading window based on liquid crystals doped with push-pull azobenzene with a haze-free opaque state for transmittance control is demonstrated. For the realization of the haze-free opaque state (or a low-transmittance state), the isotropic phase was used, which can be induced either thermally or optically.