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Bistable liquid crystal (LC) shutters have attracted much interest due to their low energy consumption and fast response time. In this paper, we demonstrate an electrically tunable/switchable biostable LC light shutter in biological optics through a three–step easy–assembly, inexpensive, multi–channel shutter. The liquid crystal exhibits tunable transparency (100% to 10% compared to the initial light intensity) under different voltages (0 V to 90 V), indicating its tunable potential. By using biomedical images, the response time, resolution, and light intensity changes of the LC under different voltages in three common fluorescence wavelengths are displayed intuitively. Particularly, the shutter's performance in tumor images under the near–infrared band shows its application potential in biomedical imaging fields. [ABSTRACT FROM AUTHOR]

This paper introduces a tunable metamaterial absorber (MA) based on polymer network liquid crystal (PNLC) in the terahertz (THz) frequency band. Under the optimal polymerization condition, through electrical control of the orientation of the PNLC embedded in the frequency selective surface, the resonant frequency of the absorber can be tuned from 416.5 to 405.0 GHz, corresponding to fractional frequency bandwidth of 2.8%. The experimental results show that the proposed MA based on the PNLC offers an adjustment time of 10 ms and recovery time of 85 ms, which is significantly faster than the tunable metamaterial devices based on conventional nematic liquid crystal (LC). [ABSTRACT FROM AUTHOR]

A diffraction grating of polymer-dispersed liquid crystal (PDLC) with polarization-selective characteristics is investigated. Electrically controllable gratings are produced using In-Plane Switching (IPS) electrodes. Indium tin oxide (ITO) electrodes with a stripe pattern are used to generate a horizontal electric field parallel to the substrate on a single glass substrate. It is known from the experimental results that the number of diffraction orders can be controlled by applied voltage. Except for the zeroth order, the consistently highest intensity can be obtained for every other order of diffraction, and the polarization direction of the diffraction is perpendicular to the direction of the electrode stripes. The polarization direction of the zeroth order diffraction is parallel to the direction of the electrode stripes. Therefore, it can be used as a filter for light polarization. [ABSTRACT FROM AUTHOR]

The electric-field-driven liquid crystal lens (ELC) induces the lens effect by the spatially non-uniform distribution of the refractive index. A scheme to analyse the performance of the ELC lens for the lights of various incident angles is devised by the calculation of the phase through the ELC lens and the determination of light ray directions from these phases. The calculated results show that the ELC lens changes the incident light of the plane wave into a focused wave and the focal distance becomes shorter for larger incident angles. [ABSTRACT FROM AUTHOR]

A liquid crystal (LC) lens is used to change the propagating direction of the incident light at the lens-shaped boundary of two different media and shows strong dependence on the polarisation direction of the incident light. The performance of the cylindrical LC lens is investigated in the situation that the directions of the incident linear polarisation and the axis of the cylindrical lens are not parallel. Two LC cell configurations were considered where LC directors aligned twisted or homogeneously. In case of the twisted LC configuration, the polarisation direction of the propagating light follows the twist of the LC directors if the directions of the incident linear polarisation and the LC alignment on the side of the lower substrate are parallel. For these two configurations, the LC lens can control the propagating light in the LENS-ON state. In case of homogeneously aligned LC configuration, polar angle distributions of LC directors are not uniform due to the lens-shaped boundary. So in the LENS-OFF state where LC directors rotate under the driving voltage, a LC domain boundary may occur in some cases. [ABSTRACT FROM AUTHOR]

We report optical interferometric studies of a wedge-shaped nematic liquid crystal cell. Interference fringes were observed when a nematic liquid crystal cell was placed in one of the arms of Mach–Zehnder interferometer. In the case of homogenous gap cell, the fringe contrast remained unaffected on applying voltage. However, in the case of wedge-shaped cell, the fringe contrast was found to degrade under an applied electric (DC) field and it became poorer at higher voltages. At higher voltages the fringe contrast improved where complete switching occurred. The degradation in fringe contrast due to wedge-shaped cell structure might find applications for speckle reduction in future laser-based rear projection displays. [ABSTRACT FROM AUTHOR]

A new liquid crystal lens design is proposed to improve the recovery time with a ring-and-pie electrode pattern through a suitable driving scheme and using dual-frequency liquid crystals (DFLC) MLC-2048. Compared with the conventional single hole-type liquid crystal lens, this new structure of the DFLC lens is composed of only two ITO glasses, one of which is designed with the ring-and-pie pattern. For this device, one can control the orientation of liquid crystal directors via a three-stage switching procedure on the particularly-designed ring-and-pie electrode pattern. This aims to eliminate the disclination lines, and using different drive frequencies to reduce the recovery time to be less than 5 seconds. The proposed DFLC lens is shown effective in reducing recovery time, and then serves well as a potential device in places of the conventional lenses with fixed focus lengths and the conventional LC lens with a single circular-hole electrode pattern. [ABSTRACT FROM AUTHOR]

In this study, we applied the non-contact photo-polymerisation method in fabricating pseudo-polyimide (PI) liquid crystal (LC) devices with vertical alignment copolymer films, in the hope of solving the problems associated with traditional rubbing alignment processes. We mixed different photo-polymerisation-type acrylic monomers and negative dielectric anisotropy LC at proper weight percentages, and induced phase separation behaviour between the photo-polymerisation-type acrylic monomers and the LC molecules in the LC mixture solution through ultra-violet light irradiation, thereby forming copolymer films with vertical alignment effects. We further conducted preliminary characterisation of the alignment mechanism, photo-polymerisation, etc., of the LC molecules and investigated their interactions with alkyl acrylic monomers with different carbon chain lengths (the analogue of alkyl side chains in traditionally used vertical alignment-type PI) and electro-optical properties such as the transmittance, contrast ratio, and response time. [ABSTRACT FROM AUTHOR]

Four laterally difluorinated phenyl tolane liquid crystals were synthesised and their physical properties evaluated. These compounds exhibit a fairly small heat fusion enthalpy (ΔH ∼ 12.56 kJ mol-1), which is favourable for mixture formulations. For comparison, four lateral difluoro cyclohexane tolane homologues are also studied. We doped 10 wt% of each compound into a commercial negative mixture N1 and measured the birefringence, visco-elastic coefficient and figure of merit. Birefringence varies very little between homologues, but the visco-elastic coefficient increases as alkyl chain length increases. The refractive indices of these guest-host mixtures at 1550 nm are also measured. [ABSTRACT FROM AUTHOR]

Dual frequency liquid crystals (DFLCs) provide a possibility to achieve sub-millisecond rise and decay times and are useful for various electro-optical devices. In most of DFLC mixtures, ester compounds are utilised in order to obtain a low crossover frequency. However, the ester linking group does not provide π electron conjugation so that the DFLC mixtures usually exhibit a limited birefringence (Δn < 0.3 at λ = 633 nm). For phase modulation a lower birefringence demands a thicker cell gap which, in turn, increases the response time and operating voltage. This problem becomes more severe as the wavelength increases. In this paper, we introduce a series of isothiocyanato phenyl tolane positive compounds with high birefringence and moderate dielectric relaxation frequency. The dielectric and electro-optical properties of these compounds are investigated. We also formulated a DFLC mixture using these new positive compounds which exhibits Δn = 0.39 at λ = 633 nm and 0.33 at λ = 1550 nm. [ABSTRACT FROM AUTHOR]

We review the recent progress in developing high performance dual frequency liquid crystal (DFLC) materials. A new figure of merit is derived for evaluating the DFLC performance. The dielectric relaxation and electro-optical properties of several high performance compounds, which possess a positive dielectric anisotropy at low frequencies and a relaxation frequency in the kilohertz region, are presented. Potential applications of these compounds are discussed. Compounds with a negative dielectric anisotropy were also compared and discussed. Two experimental DFLC mixtures were demonstrated with one intended for room temperature operation and another for elevated temperature operation. [ABSTRACT FROM AUTHOR]

An experimental phase diagram for two types of nematic liquid crystals and their binary mixtures was established by polarised optical microscopy and differential scanning calorimetry methods. The mixtures comprised a tolane-base liquid crystal, 4-heptyl-3-fluoro-4-isothiocyanatotolane (7TOLF), and a biphenyl-base nematogen, 4-heptyl-3-fluoro-4-isothiocyanatobiphenyl (7BF). The static Kerr effect and third-order non-linearity were investigated for 7TOLF and 7BF and their nematic mixtures within the isotropic phase. Both the compounds have a positive and large Kerr constant. The second-order phase-transition temperatures, T*, were determined for these mixtures. The linear dependence of (T - T*)- 1 on the Kerr constant is found to be in good agreement with the predications of the Landau-de Gennes model. The third-order non-linear susceptibility,χ(3), values were determined for these mixtures. [ABSTRACT FROM AUTHOR]

Normal-mode polymer stabilised cholesteric liquid crystal (Ch-LC)/aerosil particles (APs) composites are prepared by photo-polymerisation of the photo-polymerisable monomer in the electrically induced homeotropically oriented cholesteric phase of a photo-polymerisable monomer/Ch-LC/APs mixture. On the application of an electric field, the composites can change from a light scattering state to a transparent state accompanied by the texture changing from a focal conic texture to an electrically induced homeotropic texture. Moreover, the process was greatly influenced by the addition of the APs. The focus of this study is on the effects of the APs on the electro-optical performance and the morphology of polymer network of the composites. The effect of the chiral dopant concentration on the electro-optical performance is also studied. [ABSTRACT FROM AUTHOR]

We have synthesised and evaluated the physical properties of some high birefringence, lateral difluoro-terphenyl compounds and mixtures. These mixtures exhibit a high birefringence (Δn∼0.24) in the visible spectral range and a relatively low viscosity. When doped in commercial mixtures, the difluoro-terphenyl compounds enhance the mixture's birefringence while causing almost no penalty to the rotational viscosity. These mixtures are particularly attractive for achieving fast response times in thin-cell liquid crystal displays. [ABSTRACT FROM AUTHOR]

A high birefringence (Δn = 0.292 at λ = 633 nm, 25°C) and low crossover frequency (<1 kHz at T = 25°C) dual-frequency liquid crystal (DFLC) mixture was developed. The high birefringence enabled us to use a thin liquid crystal cell, which is helpful for fast response time and low operating voltage. The initially low crossover frequency allowed us to operate the DFLC device at an elevated temperature, which significantly lowers the viscosity while keeping the crossover frequency in an acceptable range (<10 kHz). We demonstrated a 2π phase shifter at λ = 633 nm using such a DFLC and obtained a sub-millisecond response time at T∼45°C. This type of DFLC mixture together with elevated temperature operation opens a new way for achieving fast response time. [ABSTRACT FROM AUTHOR]

Some high birefringence ester liquid crystal compounds with cyanate or isothiocyanate polar groups were investigated, the relaxation frequency of which is in the kilo-Hertz region. These compounds are useful for formulating dual-frequency liquid crystal (DFLC) mixtures. The dielectric relaxation and electro-optical properties of these compounds were characterised. Potential applications of these compounds for DFLC mixtures to meet different requirements are discussed. [ABSTRACT FROM AUTHOR]

We have designed, synthesised and evaluated the physical properties of high-birefringence (Δn) isothiocyanato phenyl-tolane and terphenyl liquid crystals. These compounds exhibit a Δn∼0.36-0.52 at room temperature and wavelength λ = 633 nm. A different mesomorphic sequence was observed depending on the terminal alkyl chain length. Compounds with a longer chain show smectic and nematic phases, whereas short-chain homologues only exhibit a nematic phase. In the vicinity of the nematic-to-smectic transition, the viscoelastic coefficient decreases, instead of increases, as the temperature decreases. The observed change is as large as 75% for the 4'-(3,5-difluoro-4-isothiocyanatophenylethynyl)-4-pentylbiphenyl compound studied. This behaviour is useful for improving the response time of a liquid crystal device, if the operating temperature can be controlled accurately. [ABSTRACT FROM AUTHOR]

Liquid crystals are valuable materials for applications in beam steering devices. In this paper, an overview of the use of liquid crystals in the field of adaptive optics specifically for beam steering and lensing devices is presented. The paper introduces the properties of liquid crystals that have made them useful in this field followed by a more detailed discussion of specific liquid crystal devices that act as switchable optical components of refractive and diffractive types. The relative advantages and disadvantages of the different devices and techniques are summarised. [ABSTRACT FROM AUTHOR]

We improved the three-dimensional (3D) crosstalk level of multi-view 3D displays using a lens array with small f-number, thereby facilitating a wide 3D viewing window. In particular, we designed a polarization-dependent-switching liquid crystal (LC)-based gradient refractive index (GRIN) lens array that could be switched between 2D and 3D viewing modes. For the GRIN lens with a small f-number (1.08), we studied the effect of the interfacial curvature between the plano-concave isotropic polymer layer and the plano-convex birefringent LC layer on the aberration properties. We examined the conventional spherical, quadratic polynomial aspherical, and a high-order (fourth-order) polynomial aspherical curvature. For the high-order polynomial aspherical curvature, the achievable transverse spherical aberration (TSA = 10.2 µm) was considerably lower than that with the spherical (TSA = 100.3 µm) and quadratic polynomial aspherical (TSA = 30.4 µm) curvatures. Consequently, the angular luminance distributions for each view were sharper for the high-order polynomial interfacial curvature. We designed multi-view (43-view) 3D displays using the arrays of switchable LC lenses with different curvatures, and the average adjacent crosstalk levels within the entire viewing window (50°) were 68.5%, 73.3%, and 60.0% for the spherical, quadratic polynomial aspherical, and high-order polynomial aspherical curvatures, respectively. [ABSTRACT FROM AUTHOR]

The methods to enhance contrast ratios (CRs) in scattering-type transflective liquid crystal displays (ST-TRLCDs) based on polymer-network liquid crystal (PNLC) cells are investigated. Two configurations of ST-TRLCDs are studied and are compared with the common ST-TRLCDs. According to the comparisons, CRs are effectively enhanced by assembling a linear polarizer at the suitable position to achieve better dark states in the transmissive and reflective modes of the reported ST-TRLCDs with the optimized configuration, and its main trade-off is the loss of brightness in the reflective modes. The PNLC cell, which works as an electrically switchable polarizer herein, can be a PN-90° twisted nematic LC (PN-90° TNLC) cell or a homogeneous PNLC (H-PNLC) cell. The optoelectric properties of PN-90° TNLC and those of H-PNLC cells are compared in detail, and the results determine that the ST-TRLCD with the optimized configuration using an H-PNLC cell can achieve the highest CR. Moreover, no quarter-wave plate is used in the ST-TRLCD with the optimized configuration, so a parallax problem caused by QWPs can be solved. Other methods for enhancing the CRs of the ST-TRLCDs are also discussed. [ABSTRACT FROM AUTHOR]