Physical properties of chlorinated liquid crystals

Dielectric, optical and electro-optical properties of four chlorinated nematic liquid crystal compounds and a eutectic mixture were characterized. Some chlorinated liquid crystals are found to exhibit a wide nematic range, modest dielectric and optical anisotropies, low viscosity and small UV absorption. Potential application of mixtures containing chlorinated liquid crystals for information displays in the visible spectral region is foreseeable.     

Nematic Liquid Crystals as Media for Real-Time Holography

We report on a novel type of orientational photorefractive devices consisting of dye-doped or pure nematic liquid crystals. These systems can be employed as the recording media for real-time (dynamic) holography. Optical addressing of liquid crystal molecules is facilitated due to the natural tendency of molecular self-assembling in nematics and their easy reorientation upon action of an electric field. This field can be produced by incoming light via photoconductivity either in the bulk or at the surface of the nematic liquid crystal layer. We present an experimental study of dynamic self-diffraction of light on thin phase holograms formed in a nematic liquid crystal panel. Optically addressed spatial light modulators designed by us can be the active elements of image processing systems and in this communication we demonstrate the optical correlator performing image recognition.     

Liquid crystal devices for optical communication and information processing systems

This paper reviews liquid crystal optical devices including tunable filters, photonic switches and spatial light modulators, and examines their application to optical communication and information processing systems. Optical processing has three major advantages: massive parallelism, high speed and broad wavebands. Compared with typical optical materials including silica, semiconductors and inorganic electrooptic crystals used in communication systems, a liquid crystal has a number of outstanding features including its large refractive index change and large polarization rotation power at a low voltage. These features make it possible to develop new optical components.     

Phase modulation using different orientations of a chiral nematic in liquid crystal over silicon devices

Modulation of the intensity of light by high quality reflective micro-displays is predominantly carried out by liquid crystal over silicon (LCoS) spatial light modulator (SLM) technology for applications such as pico-projectors. Wider use of these devices, in applications such as computer-generated holography and optical correlation, is limited by their phase modulation ability and illumination polarisation state dependence. These devices rely on planar or twisted nematic liquid crystals to modulate the light, but due to their viscoelastic properties they are inherently slow. Research into the use of the polymer stabilised blue phase has already shown that it can offer high speed phase modulation. However, other chiral nematic orientations are yet to be compared in LCoS devices. In this article, we demonstrate that polymer-stabilised chiral nematic liquid crystal electro-optical effects can offer phase modulation in silicon backplane devices. The uniform standing helix and focal conic textures are shown to be independent of the input light polarisation state and the uniform lying helix is shown to be polarisation dependent. These optical responses are then compared with that of the blue phase to identify a suitable orientation for further development in LCoS technology in order to find a high-speed, full phase modulating material.     

Enhanced dielectric and electro-optical properties of a newly synthesised ferroelectric liquid crystal material by doping gold nanoparticle-decorated multiwalled carbon nanotubes

In this article, a newly synthesised ferroelectric liquid crystal (FLC) material, namely LAHS 22, has been characterised. The characterisation of the FLC material has been performed using dielectric relaxation spectroscopy, differential scanning calorimetry and polarisation optical microscopy. We observed an enhancement in the dielectric and electro-optical properties of the FLC material by incorporating gold nanoparticles (GNPs)-decorated multiwalled carbon nanotubes (MWCNTs). The GNPs-decorated MWCNTs cause an increment in dielectric dispersion (up to kHz), absorption, spontaneous polarisation and rotational viscosity of the FLC material. The pure and GNPs-decorated MWCNTs doped FLC cells were analysed by means of various dielectric spectroscopic and optical measurements. The observed enhancement in the dielectric and electro-optical properties of the FLC material has also been studied with concentration of GNPs-decorated MWCNTs in FLC material. The GNPs-decorated MWCNTs/FLC composites are not only of fundamental importance, but also useful materials for device applications such as liquid crystal displays and memory devices.     

Unidirectional Alignment of Surface-Grafted ZnO Nanorods in Micrometer-Thick Cells Using Low-Molecular-Weight Liquid Crystals

Inorganic nanomaterials such as nanotubes and nanorods have attracted great attention due to their anisotropic properties. Although the alignment control of inorganic nanomaterials is key to the development of functional devices utilizing their fascinating properties, there is still difficulty in achieving uniform alignment over a large area with a micrometer thickness. To overcome this problem, we focused on liquid crystals (LCs) to promote the alignment of anisotropic nanomaterials, taking advantage of the cooperative motion of LCs. We present the uniform, one-dimensional alignment of ZnO nanorods along the direction of LCs in micrometer-thick cells by grafting nematic LC polymers from the nanorod surfaces to provide miscibility with the host LCs. Polarized optical microscopy and polarized UV–visible absorption spectroscopy revealed the unidirectional alignment of nematic LC polymer-grafted ZnO nanorods parallel to the nematic host LCs.     

基于定向电纺纤维膜的可调制偏振片的制备

基于定向静电纺丝技术制备了高取向性有序纤维薄膜, 利用有序纤维对液晶分子取向的诱导, 构建了可调制散射型偏振片. 填充混合液晶的有序聚甲基丙烯酸甲酯纤维薄膜在可见光范围内, 表现出明显的偏光特性. 混合液晶中光敏性偶氮液晶4-正丁基-4'-甲氧基偶氮苯在360 nm紫外光照射下进行顺反异构转变, 诱导混合液晶发生从各向异性到各向同性的相变. 利用混合液晶光致相变与有序纤维的耦合, 实现了薄膜偏光特性的光控切换.     

New functional polymers for liquid crystal displays review of some recent developments

Polymeric materials play an important role in the construction and performance of liquid crystal displays. New functional polymers are developed to improve the displays on brightness, power efficiency and viewing angle performance. Polymer films with a controlled molecular structure and architecture provide new polarization optics to convert non‐polarized light into polarized light with higher efficiency than the traditional polarizers based on stretched iodine doped poly(vinyl alcohol). Other films provide a polarization compensation function to maintain an angular invariant net optical retardation of the display device and therewith a better contrast to wider viewing angles. Color generation by non‐absorbing methods is believed to improve on the display brightness. Special control of the liquid crystal alignment by photo‐sensitive orientation layers, polymer protrusions or photo‐formed polymer walls provide multiple director patterns within a single pixel to average out angular LC effects.     

Polarised UV-induced homogeneous alignment for fringe-field switching liquid crystal display with a new azobenzene monomer

A novel preparation method of homogeneous alignment polymer film (HAPF) was proposed by polymerisation of the monomer, 4,4?-di-methacryloyl-oxy azobenzene (4,4?-DMOAz), being dissolved in the liquid crystal (LC) material of positive dielectric anisotropy. For obtaining the homogeneous alignment, exposure of the polarised UV light was carried out to the LC cell above the nematic to isotropic transition temperature of the LC material. The fringe-field switching (FFS) mode LC cell with the HAPF formed from the monomer 4,4?-DMOAz (FFS-HAPF-LC cell) exhibited enough level of alignment state, electro-optical and response properties compared with the FFS-LC cell carrying the conventional polyimide-type alignment layer. FFS-HAPF-LC cell can be expected to be useful for next-generation displays such as flexible LC displays.     

Reliability of liquid crystal cell and immiscibility between dual-curable adhesives and liquid crystal

The dual-curable adhesive used to attach thin film transistors (TFTs) to color filters in the construction process of liquid crystal display (LCD) panels requires fast curing by UV irradiation and strong bond strength after thermal-curing. In addition, it is necessary to consider the immiscibility of the dual-curable adhesives with the liquid crystal, because they come directly into contact with the liquid crystal without curing process in the large LCD panel production. In this study, dual-curable adhesives based on partially acrylated epoxy acrylate oligomers were prepared and investigated with nematic liquid crystals using gas chromatography (GC), polarized optical microscopy and the measurement of the transmittance of the liquid crystal.As the concentration of CC bonds was increased, the immiscibility was enhanced due to the fast curing rate of the partially acrylated epoxy acrylate oligomers and reduced visual contamination was observed in the polarized optical microscope images. Moreover, the transmittance of the liquid crystal cells was not changed before and after the dual-curing of the adhesives and was maintained for 100 h.     

The application of the phase transition in nematic liquid crystals for the optical detection of volatile organic compounds

It is demonstrated that optical transducers can take advantage from the outstanding optical properties of nematic liquid crystals. The measurements point out that the birefringence and the phase transition of a liquid crystal can be exploited for the threshold sensing. For that purpose a device consisting of an orientated liquid crystalline film between two crossed polarizers was used. The nematic liquid crystal was also used as a sensitive coating material on an integrated optical Mach-Zehnder interferometer. The calibration experiments were carried out with volatile organic solvents and different calibration curves were measured for meta-xylene and para-xylene.     

The determination of molecular parameters from dielectric measurements on nematic liquid crystals

The physical properties of liquid crystal phases (mesophases) depend in a very sensitive manner on the structure of the constituent liquid crystal-forming molecules (mesogens). Thus a small change in molecular structure can result in a dramatic change in the corresponding liquid crystal properties. The dielectric responses of materials reflect the electric properties of molecules, and so analysis of dielectric measurements on liquid crystals in terms of molecular properties should lead to the development of structure/property relationships for such mesophases. This paper explores the problems of determining molecular parameters from dielectric measurements on nematic liquid crystals. Specifically, the difficulties in defining the internal electric field and knowing the value of the order parameter are addressed. Experimental results on a range of liquid crystals show that short range correlations are important in determining both the dielectric and optical response of nematic liquid crystals. It is concluded therefore that more sophisticated molecular theories, which take account of molecular shape and volume, need to be developed before reliable structure/property relationships can be established for liquid crystals.     

Optical activity of chiral thin film and liquid crystal hybrids

Using the glancing angle deposition (GLAD) technique, we have fabricated porous, chiral thin films with optically anisotropic helical microstructures that exhibit optical phenomena such as wavelength specific rotation of linearly polarized light. Initial research has shown that the porosity of the films allows for the addition of nematic liquid crystals (NLCs) to the films for promising applications in dynamically switchable devices, while simultaneously enhancing the optical properties of the film. This study describes the fundamental optical behaviour of LC-filled chiral thin films in relation to material, porosity, structure and thickness. It was found that for SiO₂ films, the addition of NLCs increased the effective rotatory power by two-fold when compared with results from the film without added LCs. The rotatory power of Al₂O₃ and MgF₂ films, while being similarly increased by the addition of LCs, exhibited a reversal in sign, or direction of rotation, for the visible wavelength spectrum investigated. The effects of film porosity and structure were studied by varying the angle of incidence from 83° to 86°; it was found that the greater porosity of the films deposited at larger angles allowed for more filling by the LCs and thus a larger increase in rotatory power. Finally, the addition of LCs was observed to shift the wavelength of peak rotation towards smaller values.     

High-Reflective Templated Cholesteric Liquid Crystal Filters

Cholesteric liquid crystals (CLCs) have been widely applied in optical filters due to Bragg reflection caused by their helical structure. However, the reflectivity of CLC filters is relatively low, commonly less than 50%, as the filters can only reflect light polarized circularly either left- or right-handedly. Therefore, a high-reflective CLC filter with a single-layer template was proposed which may reflect both right- and left-handed polarized light. The CLC filters of the red, green, blue color were fabricated by the templating technology, which show good wavelength consistency. Additionally, a multi-phase liquid crystal filter with high reflectance was demonstrated by the single-layer templating technology. The templated CLC or multi-phase liquid crystal filters show great potential applications in the optical community, reflective display, and lasing.     

Alignment modulation of azobenzene-containing liquid crystal systems by photochemical reactions

Recent progress in alignment modulation of azobenzene-containing liquid crystal systems by photochemical reactions has been reviewed by dividing the modulation methods into two types: phase transitions (order–disorder change) and change of liquid crystal directors (order–order change). First, photochemical phase transitions and alignment changes of liquid crystals in guest/host mixtures and polymers are summarized. Then, alignment control of liquid crystals by linearly polarized light and photoactive surface layers is discussed. Finally, recent applications of alignment change and photochemical phase transitions of liquid crystals in holographic technology and photomechanical effects are introduced. In addition, future possible applications for a variety of practical devices, such as display devices, optical switching and reversible optical image storage, are mentioned.     

Star-shaped supramolecular ionic liquid crystals based on pyridinium salts

Series of novel star-shaped liquid crystals containing [1,1?-biphenyl]-4,4?-diyl diisonicotinate mesogens with various counterions (Br^?, B-SO₃^?,C-SO₃^?,H₂PO₄^?, BF₄^?) have been synthesized and characterized, which display a nematic phase. The molecular structures of the liquid crystals, thereof consisting of rod-like mesogens linked together by different long alkyl spacers to a small disc-like core of central benzene-1,3,5-triyl triisonicotinate, have been fully characterized by ¹H NMR. Their thermal and supramolecular organization behaviours have been studied by combining polarized optical microscopy, differential scanning calorimetry and small-angle X-ray scattering. These investigations showed that the mesophase temperature range increased with increasing alkyl chain length and the clearing point decreased with increase of anion size. These star-shaped liquid crystals have a long and ordered molecular structure, the electron delocalization of the π-π stacking conjugation effect and the electrostatic attraction of ionic make them have good ferroelectric properties and ionic conductivity properties. The interest in preparing ionic liquid crystal with a nematic phase lies in the technological applications as it is well known that the nematic phase has the highest fluidity of all liquid crystalline phases and hence the possibility to align it by applying an external electric/magnetic field, commonly used in electro-optical devices.     

Investigation of electro-optical characteristics of photo-aligned TN-LCDs on PCEMA surfaces

In this work, a novel photo-alignment material, poly(cinnamolyethyl methacrylate) (PCEMA), was synthesized by photo-dimerization. We investigated the electro-optical characteristics of twisted nematic (TN)-liquid crystal displays (LCDs) photo-aligned with linearly polarized UV light irradiation at normal direction on the PCEMA surfaces. Excellent voltagetransmittance characteristics were observed. The threshold voltage of the photo-aligned TN-LCD decreases with increasing UV irradiation time. Additionally, response times are almost the same as for a rubbing-aligned TN-LCD.     

Investigation of electro-optical characteristics of photo-aligned TN-LCDs on PCEMA surfaces

In this work, a novel photo-alignment material, poly(cinnamolyethyl methacrylate) (PCEMA), was synthesized by photo-dimerization. We investigated the electro-optical characteristics of twisted nematic (TN)-liquid crystal displays (LCDs) photo-aligned with linearly polarized UV light irradiation at normal direction on the PCEMA surfaces. Excellent voltagetransmittance characteristics were observed. The threshold voltage of the photo-aligned TN-LCD decreases with increasing UV irradiation time. Additionally, response times are almost the same as for a rubbing-aligned TN-LCD.     

Liquid-crystalline physical gels

Liquid-crystalline (LC) physical gels are a new class of dynamically functional materials consisting of liquid crystals and fibrous aggregates of molecules that are called "gelators". Liquid-crystalline physical gels, which are macroscopically soft solids, exhibit induced or enhanced electro-optical, photochemical, electronic properties due to the combination of two components that form phase-separated structures. In this tutorial review, we describe the materials design and structure-property relationships of the LC physical gels. The introduction of self-assembled fibers into nematic liquid crystals leads to faster responses in twisted nematic (TN) mode and high contrast switching in light scattering mode. Furthermore, the LC physical gels can be exploited as a new type of materials for electro-optical memory. This function is achieved by the control of reversible aggregation processes of gelators under electric fields in nematic liquid crystals. Electronic properties such as hole mobilities are improved by the introduction of fibrous aggregates into triphenylene-based columnar liquid crystals. The incorporation of photochromic azobenzenes or electroactive tetrathiafulvalenes into the chemical structures of gelators leads to the preparation of ordered functional materials.     

Stability of liquid crystal micro-droplets based optical microresonators

Liquid crystal (LC) based tuneable optical microresonators are potential for being used as crucial components in photonic devices. In this article, we report experimental studies on LC micro-droplets dispersed in several dispersing media. We find that the size of the micro-droplets formed in a low refractive index and optically transparent perfluoropolymer are most stable with time than commonly used dispersing media. Using a negative dielectric anisotropy nematic liquid crystal, we show that the whispering gallery mode optical resonance properties such as the quality factor and the free spectral range of stable micro-droplets are independent of the strength of the applied electric field. The optical resonance properties under applied field are significantly different than that of the liquid crystals with positive dielectric anisotropy and are explained based on the elastic deformation of the micro-droplets.