Cholesteric liquid crystals for detection of organic vapours

Studies on the use of cholesteric liquid crystals (CLCs) as sensing phase for detection of organic vapours in air are described. Stock solutions of 1.0% (w/v) cholesteryl nonanoate (CN) and cholesteryl chloride (CC) were prepared in tetrahydrofuran. Binary mixtures, with compositions ranging from 0.18 to 0.25% of CC and 0.82–0.75% of CN, respectively, were prepared by appropriate mixing of the stock solutions. Films were cast by pipetting three 10 μl aliquots of the CLC solution mixture onto a glass disk, whose reverse side was made black to absorb unscattered light. The glass disk was adapted to the common end of a bifurcated optical fibre bundle and placed in a glass vial, which provided a headspace of organic vapours. Measurements were carried out at 27±1 °C, a temperature in which the CLC mixtures maintain their liquid crystalline properties. The responses of the CLC mixtures to vapours of ethanol, acetone, benzene, pyridine and hexane were investigated. The colour of the sensing phases depended on their compositions and exposure to organic vapours gives rise to a change in the optical characteristics of liquid crystals. It was found that the CLC layers containing 0.23–0.25% of CC had no significant change in optical properties when exposed to organic vapours and that ethanol did not cause any optical changes in the liquid crystal layers. Benzene as well as hexane always turned all the coloured liquid crystalline layers to colourless. The CLC layers exhibited different behaviours to vapours of acetone and pyridine. For example, the wavelengths of maximum scattering for the 0.19% CC layer were 530 nm in air, 545 nm in pyridine and 580 nm in acetone. The CLC layers showed reversibility. The lifetimes of these layers (interval of time in which the liquid crystalline phase exists, before crystallisation) were investigated by employing acetone and n-hexane vapours. Average lifetimes of 14–15 min were found for films in contact with these vapours, while a lifetime of 205 min was possible when the CLC film was exposed to air.     

Ferroelectric liquid crystals: Excellent tool for modern displays and photonics

The ferroelectric liquid crystals, because of their fast electro‐optical response, are one of the most important classes of liquid crystals. Here, in this review, we have summarized the different electro‐optical modes for ferroelectric liquid crystals. Clark–Lagerwall effect (surface stabilized ferroelectric liquid crystal), deformed helix ferroelectric (DHF) effect, electrically suppressed helix (ESH) mode, DHF orientational Kerr effect, and ESH diffraction modes have been discussed. All of the crucial features, that is, optics, electro‐optics, dynamics, and their dependence on material parameters, operational regime, and applications, have been reviewed.     

Electrically suppressed helix ferroelectric liquid crystals for modern displays

In this article, we disclose electrically suppressed helix ferroelectric liquid crystal (ESHFLC) that is characterized by high optical quality and fast response time at the cost of extremely small driving voltage. These unique features of the ESHFLCs are highly sensitive to the anchoring energy that should be smaller and comparable to the elastic energy of the ferroelectric liquid crystal helix. The photo alignment, which offers good control on the anchoring energy by means of the irradiation energy, is critically important to lock the optimum parameters of the ESHFLC display cell. An example of field sequential color display with the frame frequency of 240 Hz at the driving voltage of 2 V has been demonstrated.     

An outstanding holographic composite employing methyl red and fullerene C60 under the same liquid crystal structure

The results of a grating diffraction experiment in which an azo dye (methyl red, MR) and fullerene (C60) are doped at the same time to nematic liquid crystal (LC) host (E7). The change of refractive index, originated by photo-induced molecular reorientation, brings about an enhanced diffraction capability with respect to the other dye-doped LC systems containing only a single agent. MR component of the proposed system satisfies the absorption-based reorientation under the illumination of He–Cd laser while C60 constitutes the charge transfer complexes. Accessible diffraction efficiency is 15% under optimum circumstances and analyzed results establishes a better reorientation lifetime by the common usage of both additives in our hybrid system in the sense that fullerene component improves efficiency.     

Refractive‐index matching between liquid crystals and photopolymers

Abstract— Refractive indices of two photocurable polymers, NOA65 and NOA81 (Norland Optical Adhesive), and two series of Merck liquid crystals, E‐series (E44, E48, and E7) and BL‐series (BL038, BL003 and BL006), and two UCF high‐birefringence liquid‐crystal mixtures were measured using an Abbe refractometer in the visible spectral region and 15–55°C temperature range. Some liquid crystals have excellent index matching with NOA65 in the red, while some fit better in the blue spectral region. To validate this index‐matching property found in the material level, we prepared some polymer‐dispersed liquid‐crystal devices. Good correlations between material and device performances are obtained.     

Dielectric properties of liquid crystals for display and sensor applications

Temperature dependence of dielectric properties of nematic liquid crystals are investigated for two types of devices: wide temperature range liquid crystal (LC) displays and capacitor temperature sensor. Both real and imaginary components of the dielectric constant have been measured in wide frequency and temperature range including t < 0 °C as well as versus the angle θ between the directions of both magnetic and electric field in the measuring scheme. Some physical parameters of the nematic LC (NLC) dielectric relaxation have been determined. Effective values of the LC dielectric permittivity with different values of the LC pretilt angles in different parts of a complex LC cell have been simulated.     

Multifunctional glassy liquid crystals for photonics

Abstract— As an emerging class of photonic materials, morphologically stable glassy liquid crystals, were developed following a versatile molecular design approach. Glassy cholesteric liquid crystals with elevated phase‐transition temperatures and capability for selective‐wavelength reflection and circular polarization were synthesized via deterministic synthesis strategies. Potential applications of glassy cholesteric liquid crystals include high‐performance polarizers, optical notch filters and reflectors, and circularly polarized photoluminescence. A glassy nematic liquid crystal comprising a dithienylethene core was also synthesized for the demonstration of nondestructive rewritable optical memory and photonic switching in the solid state.     

Towards faster liquid crystals at lower driving voltages

Abstract— Response times and switching voltages of positive‐ and negative‐type nematic liquid‐crystal mixtures are improved by the addition of small amounts of diaryl‐ether derivatives. The effect of such doping is directly seen as a reduction in the rotational viscosities of the mixtures. Despite their non‐planar nature, the dopant molecules do not cause any detrimental changes to the black levels and the contrast ratios of the displays filled with the doped LC mixtures.     

Studying nematic liquid crystals by spectroscopic ellipsometry

Abstract— The optical characterization of liquid crystals, in a wide spectral range, is becoming a very important technical task because of their expanding applications in displays, optical telecommunications and other advanced areas of science and engineering. One of the most versatile, sensitive, and well‐established technique for the optical characterization of solid and liquid materials is spectroscopic ellipsometry. In this paper, an outline is presented on the use of ellipsometry for nematic liquid‐crystal characterization: anisotropic refractive‐indices measurements and their temperature dependence, anchoring energy, and tilt distribution inside cells will be discussed. The paper is an extended version of a previously published paper.¹     

Photoinduced ferroelectric parameter changes in liquid‐crystal systems containing chiral α,β‐unsaturated ketones

Abstract— Photoinduced changes that were revealed for ferroelectric liquid‐crystalline systems with helical supra‐molecular structures induced with chiral α,β‐unsaturated ketones have been summarized. Photoinduced inversion of the spontaneous polarization sign and a concomitant increase in its absolute value and other ferroelectric parameters were observed. The E‐Z photoisomerization of the chiral compounds was determined to be responsible for the discovered effects.     

Brightness and contrast of nematic liquid crystal bistable configuration displays

A photometer was used to measure the angular dependence of the brightness and contrast ratio of bistable configuration displays in both reflection and transmission modes. The essential result is that the field of view is not limited by any cut-off in the angular properties of the bistable configurations themselves, but is determined primarily by the conditions of illumination and properties of the reflector. The contrast ratio drops off only slightly for large viewing angles, and hence the acceptable field of view is determined primarily by the angular dependence of the brightness. The latter depends strongly on the nature of the illumination. For normally incident light, the full width at half maximum of the reflective display was 25° with a contrast ratio about 2.4. The transmission display was illuminated through a ground glass scattered by light normally incident on the ground glass. Under these conditions, the full width at half maximum was essentially the same as that of the ground glass alone, about 15dg and a contrast ratio of about 2. The photometer measurements of contrast ratio are in rough agreement with densitometer measurements of the absorption characteristics of the dichroic dye guest in the liquid crystal host in its parallel-aligned state.     

Anisotropic fluorophors for liquid crystal displays

Liquid crystal displays (LCDs) are commonly found in portable equipment. The main criticisms aimed at these displays are their asymmetric angle-of-view and dull grey appearance, which is particularly acute in low ambient lighting. However, by the inclusion of a fluorescent dye, it is possible to fabricate partially-emissive LCDs with a virtually hemispherical angle-of-view. This effect has been demonstrated succesfully for a range of dyes based on the perylene diester chromophor. An energy transfer mechanism between the liquid crystal host and dye molecule has been identified. This effect was used to enhance the fluorescence intensity.     

Multiplane displays based on liquid crystals for AR applications

Multiplane displays are capable of displaying 3D scenes with correct focus cues by creating multilayer 2D images in the display volume. Hence, such a 3D display technique could effectively address the accommodation‐vergence conflict (AVC) problem, which is prevalent in augmented reality (AR) displays. In this paper, we review the recent progress on multiplane AR displays based on liquid crystals (LCs) for AR applications. The working principle of multiplane AR displays is illustrated, the electro‐optical properties of the tunable LC devices are investigated and display prototypes are demonstrated. Finally, we discuss the prospects and challenges of multiplane AR displays based on LCs.     

Photostability of liquid crystals and alignment layers

Abstract— Photostability of liquid‐crystal (LC) materials and surface alignment layers was evaluated using a UV lamp and a blue laser beam. Both organic polyimide (PI) and inorganic silicon‐dioxide (SiO₂) alignment layers were studied under nitrogen environment. Two commercial TFT‐grade LC mixtures (low‐birefringence MLC‐9200‐000 and high‐birefringence TL‐216) were used for comparisons. Results indicate that SiO₂ alignment layers are much more robust than PI layers, and low birefringence LCs are more stable than the high‐birefringence ones. At the He‐Cd laser wavelength (λ = 442 nm), both LC mixtures and SiO₂ alignment layers are hardly damaged. To lengthen the lifetime of an LCD projector, inorganic SiO₂ alignment layers, high‐optical‐density UV filter, long cutoff‐wavelength blue filter, and short‐conjugation (low birefringence) LC materials should be considered.     

Floating drive schemes for dual-frequency addressing of complex liquid crystal displays

A circuit configuration for the dual-frequency addressing of complex liquid crystal matrix displays is described. After division of the high and multilevel drive voltages into several digital signal components, they are superposed in digital 15 V CMOS ICs. The signals so produced have an amplitude which exceeds the value of the supply voltage and which involves three or four levels. The technique is suitable for modification of existing display drive systems to reach a higher number of multiplexed lines or to allow better selection.     

Novel photoinduced alignment phenomenon of liquid crystals on aromatic polyamide films

The liquid‐crystal (LC) alignment properties of polyamide films exposed to ultraviolet (UV) light were investigated. It was found that the uniform and stable alignment of LC molecules was achieved on films of aromatic polyamides exposed to linearly polarized UV light, even though these polymers contained no common photoreactive group such as cinnamoyl, coumarin, or azo chromophore. The alignment was induced in the same direction, which was perpendicular to the electric‐field vector of the linearly polarized light. The change in the UV‐visible absorption spectra before and after UV exposure suggested that the photoreaction of aromatic polyamide occurred only on the film surface, and that even such a small change in the film was enough to induce uniform alignment of the LC molecules. Furthermore, it was suggested that the photoreaction of this system was accelerated in the presence of oxygen. This paper also deals with the effect of the chemical structure of polymers on their LC photoalignment characteristics, i.e., the sensitivity of the photoinduced LC alignment. As a result, polymer materials with excellent LC photoalignment sensitivity have been determined, which could induce the uniform and unidirectional LC alignment by irradiation of 0.2–0.5 J‐cm^?2 of linearly polarized 313‐nm light. In addition, the alignment of the LC cell was found to be thermally and optically stable.     

Optimization of the inhibition of atmospheric window emission using photonic crystals

Transfer matrix method is used to discuss the effect inhibition of photonic crystals on atmospheric window emission(ε-14 μm).According to the optical characteristics of the materials,germanium and zinc sulfide are used as the composing materials.The structure of the photonic crystals is optimized,and the optimal thickness values of the germanium and zinc sulfide are 0.63 and 1.11μm respectively while the ratio of optical thickness is 1:1 and the period is 8.The photonic crystals are prepared by evaporation ...