Electric Field-Induced Phase Transitions in Electro-Optic Cell with Dielectrically Negative Chiral Nematic Liquid Crystal

Behaviour of a chiral nematic mixture with a negative dielectric anisotropy was studied between two electrodes with homeotropic anchoring in the presence of an a.c. electric field. By changing the sample thickness d and field strength E at constant cholesteric pitch p, the phase diagram in the parameter plane (d/p, E) was determined. Usefullness of this diagram for realization of positive contrast dichroic LC displays with nematic-cholesteric phase change is discussed.     

Modulated Microdomain Switching of Nematic Liquid Crystals

Abstract

Modulated microdomain switching of nematic liquid crystals have been investigated. This display mode used for light switching of micron-sized domains regions using a complicated electric field with special mesh electrode structures. Under He-Ne laser irradiation, contrast ratio was 22:1 for homeotropic alignment cell and transmittance change was no dependent of incident polarization condition. Under white light illumination, the contrast ratio was 4.3:1.     

Electro-Optical Properties in Polymer-Stabilized Blue Phase of n-OCB Homologue Chiral Nematic Liquid Crystal Mixtures

In this study, the chemical structures and physical property dependencies of the electro-optical properties were investigated in polymer-stabilized blue phase (BP) using the four kinds of 4-n-alkyloxy-4′-cyanobiphenyl (n-OCB) homologue chiral nematic LCs. It was confirmed that the optical response time and Kerr coefficient in polymer-stabilized BP was varied with the molecular parity as determined by the alkyl chain length of the n-OCB homologue chiral nematic LC mixtures.     

ENHANCED FLEXOELECTRIC SWITCHING BEHAVIOUR IN THE CHIRAL NEMATIC PHASE

Abstract

In order to develop materials that exhibit enhanced flexoelectric switching in the chiral nematic phase we have identified mesogenic units that display inherently strong flexoelectric coupling capabilities. Here we examine the oxycyanobiphenyl (OCB) moiety: homologues from the nOCB series exhibit significant electro-optic switching effects when doped with a highly chiral additive. Here we have examined lower dielectric anisotropy materials, since they allow the flexoelectric response to be extended to high field amplitudes. We show that dielectric coupling strength can be low in symmetric bimesogenic molecules. The flexoelectric response of such a molecular structure is tested by doping a homologue from the series CBOnOCB with a chiral additive: very significantly we find that the optic axis is rotated through 2φ = 45° in < 50 μs on reversing the polarity of the field (amplitude E = ±6 V μm^?1). Subsequently we have synthesized room temperature chiral nematic materials that exhibit 2φ > 90° at E = 10 V μ^?1.     

PHYSICAL PROPERTIES OF SOME NOVEL NEMATIC BIMESOGENS FOR THE FLEXOELECTRIC EFFECT

Abstract

In a chiral nematic liquid crystal, the flexoelectric effect consists of a fast and linear coupling with an applied electric field. One difficulty to overcome is the unwinding of the helix that occurs at higher fields due to dielectric coupling. The use of bimesogens, which possess very low molecular dielectric anisotropy can improve flexoelectric characteristics. New bimesogen compounds have recently been synthesised that exhibit switching angles of 45° for applied fields of about 9 V.μm^?1. In this paper, results from dielectric, electro-optic and dynamic light scattering measurements are reported for the new bimesogenic mixture. The dielectric anisotropy Δε changes sign with temperature and its values range between -0.2 and 0.3 for the temperature range studied. For Δε weakly positive, no electric field Freedericksz transition could be induced but Williams domains are observed instead. The large decrease in the bend elastic constant to viscosity coefficient ratio is attributed to a large increase in the bend viscosity coefficient.     

New fabrication approach to develop a high birefringence photo-crosslinked film based on a sulfur-containing liquid crystalline molecule with large temperature dependence of birefringence

ABSTRACT

We present a new fabrication approach to achieve a high birefringence film by means of photopolymerization based on an alkylthio-containing rod-like liquid crystalline molecule exhibiting large temperature dependence of birefringence. We designed a new reactive mesogen having alkylthio linkages (BPM–S). It was found that BPM–S had a larger increment of birefringence with decreasing temperature, relative to commercially available alkoxy analog LC242. This result could be thought to be due to enhanced intermolecular attractive interaction for an alkylthio mesogen implied by the proximity of laterally neighboring molecules and cybotactic nematic tendency based on wide-angle X-ray measurement. The uniaxially-aligned photo-polymerized film for BPM–S showed higher birefringence than that for LC 242.     

Electro-Optical Properties of Electric Field-Induced-Polymer Stabilization at Blue Phase of Chiral Nematic Liquid Crystal

Polymer stabilization of the blue phase induced a wide temperature range of more than 60 K, but the high driving voltage and the large switching hysteresis compared to the low molecular weight blue phase of the chiral nematic liquid crystal. In this study, we newly proposed electric field-induced-polymer stabilization of blue phase (EFIPSBP) which the electric field was applied to the blue phase during polymer stabilization and investigated the driving voltage and the switching hysteresis using various voltage-applied EFIPSBP cells. We confirmed that the driving voltage and the switching hysteresis of the EFIPSBP cell were decreased compared to those of conventional polymer stabilized BP at 8OCB chiral nematic liquid crystal mixture. This could be suggested as a potential method for improving the driving voltage and the switching hysteresis of conventional polymer stabilized BP.     

Monte Carlo Study of Slot-waveguide Liquid Crystal Phase Shifters

Abstract

We present detailed Monte Carlo simulations of a simple model of a nematic liquid crystal slot waveguide shifter, investigating the effect of an applied electric external field. The simulations are based on the Lebwohl-Lasher lattice spin model with boundary conditions chosen to mimic the planar alignment as in Silicon Organic Hybrid waveguides and the homeotropic anchoring appropriate for Polydimethylsiloxane polymer walls. The external field is modeled by adding a term to the Hamiltonian which describes its coupling to the mesogenic molecules. We have investigated the effect of the external field on the optical transmission and the ordering across the cell.     

On the Stability of Travelling Waves Arising in Nematic Liquid Crystals

Abstract

The stability of travelling waves which occur when a nematic liquid crystal is subjected to crossed electric and magnetic fields has been studied previously where conditions on a control parameter q for stability to occur have been given. This article is concerned with the behaviour of the stable perturbations as time increases. For each of the three travelling wave solutions we can determine the long-term monotonic or oscillatory behaviour of the perturbations using knowledge of the spectrum of the operator governing the perturbation equation.     

Photoswitching of Helical Twisting Power by Chiral Diarylethene Dopants

Abstract

Photochromic chiral compounds having two diarylethene units were synthesized in an attempt to use them as dopants for photoresponsive liquid crystals. Stable photoswitching of the photochromic dopants induced large pitch changes of chiral nematic liquid crystals composed of K-15 and a small amount of the chiral dopants.     

A fast-switching vertically-aligned liquid crystal device based on a multi-functional mesogenic photocrosslinker

Abstract

A vertically-aligned liquid crystal (LC) device with fast-switching LC molecules and excellent electro-optical properties is proposed. The proposed method employs a mixture of a polyimide to achieve the desired vertical alignment, and a mesogenic photocrosslinker to obtain the fast LC response. More specifically, the photocrosslinkers were photopolymerized by UV irradiation under an electric field to pre-tilt the LC molecules. In addition, upon increasing the number of functional groups in the photocrosslinker, faster switching properties were exhibited due to enhanced pre-tilting. Following UV irradiation, the proposed system also exhibited stable homeotropic alignment properties similar to those of conventional polyimides.     

Design of a Multi Mode Interferometer using LC:PDMS technology

Abstract

Multimode interference (MMI) couplers have received more attention in recent years due to their significant role played in telecommunications. Their unique features are compactness, relaxed design constrains, large optical bandwidth. In this paper a theoretical analysis and design of a MMI acting as multi/demultiplexer based on liquid crystals (LC) embedded in polydimethylsiloxane (PDMS) channels named LC:PDMS waveguides is presented. Such a device is capable of operating at two different wavelengths, typically used in erbium-doped fiber optical amplifiers, 980 and 1550?nm with an extinction ratio higher than 11?dB. The LC:PDMS MMI operation is tuned by applying an electric field by using coplanar electrodes whose fabrication process is described.     

The Effect of Rhodamine 6G on Phase Transitions in Cholesteryl Ester Mixtures

Compensated cholesteric mixtures were the subject of a whole range of studies.^1-6 Due to their characteristic properties (the existence of a temperature at which compensation occurs and the dependence of the critical field on temperature), the orientation of guest molecules by compensated cholesteric hosts⁷ shows some particular features in comparison to the orienting mechanisms acting within nematic liquid crystals^8-15 or within chiral nematic mixture.^16,17 When the colour of sample varies due to the influence of the electric field and temperature the guest dye contributes to it as well the compensated cholesteric host.

In this paper we report the modification of compensated cholesteric mixture properties due to the introduction of Rhodamine 6G as guest.     

Highly Birefringent Nematic Liquid Crystals for Reflective STN Applications

Abstract

An important parameter of any LCD containing portable electronic device is a low power requirement. The reflective STN display mode offers a moderate to high information content display with low power consumption. One method to realise colour in this mode is to use a large retardation (dΔn) value in the cell. For a cell gap of 6 μm, and a retardation of ?1.5, a birefringence (Δn) of greater than +0.2 is necessary. To achieve such a value in an STN LC mixture, materials with broad nematic range, good solubility and a Δn value of <+0.25 are required.

A study of the trans-cyclohexyldifluorophenyltolane core structure was made and a large number of dialkyl, alkenyl-alkyl and alkoxy materials were synthesised and characterised. These materials possess low to moderate melting points, broad purely nematic mesophases, a high Δn and very good solubility in nematic LC hosts. They are suitable materials for inclusion in mixtures for colour reflective STN displays.     

Liquid crystal–gold nanoparticle composites

Nanoparticles (NPs) have emerged as extremely promising materials to alter and improve the properties of liquid crystals (LCs) used, for example, in device applications. In this paper, we summarise recent work from our lab that aims to provide a fundamental understanding of structure–property and composition–property relationships governing LC–NP interactions, which may point to new directions for major improvements in the efficiency of LCs used in display applications. A variety of LC hosts (phases) doped with surface-functionalised gold NPs have been systematically studied ranging from one-dimensionally ordered nematic over two-dimensionally ordered smectic to three-dimensionally ordered columnar phases. Significant progress with respect to LC–NP interactions was made for NP-doped nematic phases. Here, the observation of an unusual texture for Au NP-doped nematic LCs, that is, the formation of birefringent stripe textures and the induction of homeotropic alignment of the nematic LC similar to chiral finger (or fingerprint) textures, provided the basis for numerous experimental studies using Au NPs with different core sizes and surface functionalities.     

Photonics of liquid-crystal structures: A review

The original results of studies of the electro-optical and laser effects which have been performed at the Laboratory of Liquid Crystals of the Institute of Crystallography, Russian Academy of Sciences, over the last few years are reviewed. Cholesteric liquid crystals as vivid representatives of photonic structures and their behavior in an electric field are considered in detail. The formation of higher harmonics in the periodic distribution of the director field in a helical liquid crystal structure and, correspondingly, the new (anharmonic) mode of electro-optical effects are discussed. Another group of studies is devoted to bistable light switching by an electric field in chiral nematics. Polarization diffraction gratings controlled by an electric field are also considered. The results of studies devoted to microlasers on various photonic structures with cholesteric and nematic liquid crystals are considered in detail. Particular attention is given to the new regime: leaky-mode lasing. Designs of liquid crystal light amplifiers and their polarization, field, and spectral characteristics are considered in the last section.     

Dynamic photorefractivity guided by single-pulse voltage

The dynamic photorefractivity in a cell with photoconducting orienting layers, filled with a nematic liquid crystal (LC) 6CHBT and a mixture of anthraquinone dyes AD1 and AD2, has been investigated. The single-pulse mode, in which the polarity and amplitude of a dc electric field applied to an LC cell are switched for a fixed time interval, has been used. The scheme of dynamic self-diffraction of low-power laser beams was used in the experiment. The dependences of the width and intensity of diffraction pulses on the bias and switching voltages have been investigated. It is established that the width and intensity of the diffraction pulse arising after initial voltage recovery depends also on the switching pulse width. At the optimal width of the control pulse, the diffraction efficiency increases by two orders of magnitude.     

Specific features of electromechanical conversion in various liquid crystalline phases

Electromechanical effect in various liquid crystalline phases—nematic, smectic A, and smectic C—is studied. It is shown that a liquid crystal (LC) sample in the nematic phase does not differ from an electromechanical converter containing an isotropic dielectric and exhibits no features characteristic of LCs at electromechanical conversion. A phenomenon similar to the piezoelectric effect may occur in ferroelectric LCs due to their biaxiality.     

Application of Infrared ATR Spectroscopy to Liquid Crystals. III

Infrared absorption measurements of nematic N-(p-ethoxybenzylidene)-p'-cyanoaniline (EBCA) under a DC electric field were performed by means of our previously reported ATR method. DC voltages up to 15V were applied to the nematic liquid crystal 15 μm thick between a silicon ATR prism (probing electrode) and an In₂O₃-coated glass electrode (opposite electrode). By using this method the orientational behavior of molecules in boundary layers about 1 μm thick at the Si electrode was investigated.

The threshold voltage for the dielectric orientation varied from 2 to 6 V depending upon the boundary conditions at the opposite electrode, and the maximum degree of order of homeotropic orientation was found to be 0.6–0.7. A storage effect, in which a homeotropic texture induced by the electric field remains unaltered even if the field is removed or reversed, was observed.     

Dielectric Behavior of Nematic Liquid Crystals under DC Electric Field

Dielectric behavior of nematic liquid crystals of p-methoxybenzylidene-p-n-butylaniline and p-ethoxybenzylidene-p-n-butylaniline have been investigated under DC electric field in relation to their optical behavior. Both dielectric permittivity and transmitted light intensity decrease at a threshold voltage of external DC electric field, and the latter reaches a constant low value at higher field, while dielectric permittivity increases via minimum to approach a constant value at higher field strength of about 2 × 10⁴ V/cm. Such a change of dielectric permittivity under DC electric field is characteristic of nematic and supercooled states, and exhibits large dependence on both temperature and sample thickness. This dielectrical observation is discussed in terms of dielectric anisotropy of the nematic liquids oriented to the field, and the disturbance of molecular orientation due to the dynamic scattering at higher DC electric field.