A practical liquid crystal mixture for IR applications

The goal of this work was to formulate an inexpensive room temperature nematic mixture having a large birefringence in the near-IR and good switching properties. It was accomplished by mixing two liquid crystalline compounds that are easy to synthesize with a commercial liquid crystal, E7. In view of the good electro-optical properties of the mixture and the simplicity of the syntheses, it is considered to be a very practical liquid crystal material for applications in the near-IR.     

High-birefringence nematic liquid crystal for broadband THz applications

Liquid crystals (LCs) have been studied extensively in the visible range for their dielectric tunability, and the characterisation in the terahertz (THz) range has gained increasing interest due to the need for active THz modulation and switching devices. In this paper, we use THz time-domain spectroscopy to measure the frequency-dependent birefringence and the absorption coefficient of a number of commercial and non-commercial nematic LCs, including E7, BL037, MDA-98-1602, LCMS-107, GT3-23001 and 1825, over a range of bias voltages at room temperature. Furthermore, several basic components of LC mixture are analysed to establish their contributions to birefringence and theoretical model is used to fit the absorption spectra. The large tunability and low loss measured for a range of samples show that the LCs are useful tunable dielectrics for compact, efficient and broadband THz devices.     

Self-constructed stable liquid crystal alignment in a monomer-liquid crystal mixture system

Here, we demonstrate excellent liquid crystal (LC) vertical alignment without using an alignment layer printing process by introducing octadecyltrichlorosilane (OTS) into the LC mixture. Further, we investigated the alignment mechanism by analysing the surfaces of the substrates. The optimum concentration of OTS was found to be about 0.03 wt%, which is 1/100 of that in the previously reported polyhedral oligomeric silsesquioxane (POSS)–LC system. Moreover, the OTS–LC system exhibited a more stable LC alignment compared with the POSS–LC system. These differences may arise from the different strengths of surface–dopant interactions; that is, the covalent bond in the OTS–LC system and the van der Waals interactions in the POSS–LC system. We also demonstrated that the method can be used in a capillary tube, which may serve as a new method facilitating the application of LCs with curved surfaces.     

New highly anisotropic liquid crystal materials for high-frequency applications

We report on the developed liquid crystal (LC) compositions with large optical anisotropy and low melting point on the basis of synthesised quaterphenyl and quinquiphenyl LC compounds containing lateral substituents. Optical and dielectric anisotropies of synthesised compounds were studied to select the most optimal compounds for high-frequency applications. As a result of the research, base matrices with a wide range of the nematic phase were developed and their mesomorphic and physicochemical properties were investigated. Based on the experimental data, the influence of various fragments of molecules on the magnitude of permittivity in the high-frequency region, as well as on the loss tangent of LCs was established.     

Study of the molecular tilt angle and the order parameter of a ferroelectric liquid crystal mixture using IR spectroscopy

We report measurements of the molecular tilt angle of a ferroelectric liquid crystal mixture (ZLI 3654), using infra-red spectroscopy, and of the optical tilt angle using polarizing microscopy. The molecular tilt angle is found to be approximately the same for different molecular vibrations. The second rank orientational order parameter for the axis of molecular vibrations of different groups of the molecule for homogeneously aligned samples have been determined as a function of temperature. The order parameter depends slightly on the sample thickness and is about 8 per cent higher for a lower thickness (∼ 7 μm) compared to a higher thickness (∼ 12 μm). The order parameter does not change significantly at the S_C*-S_A transition. The order parameter of S_C* is found to be unaffected by the application of a constant electric field. The order parameter for the alkyl chain is found to be about three times lower than for the molecular core.     

Study of the molecular tilt angle and the order parameter of a ferroelectric liquid crystal mixture using IR spectroscopy

Abstract

We report measurements of the molecular tilt angle of a ferroelectric liquid crystal mixture (ZLI 3654), using infra-red spectroscopy, and of the optical tilt angle using polarizing microscopy. The molecular tilt angle is found to be approximately the same for different molecular vibrations. The second rank orientational order parameter for the axis of molecular vibrations of different groups of the molecule for homogeneously aligned samples have been determined as a function of temperature. The order parameter depends slightly on the sample thickness and is about 8 per cent higher for a lower thickness (～ 7 μm) compared to a higher thickness (～ 12 μm). The order parameter does not change significantly at the S_C?–S_A transition. The order parameter of S_C? is found to be unaffected by the application of a constant electric field. The order parameter for the alkyl chain is found to be about three times lower than for the molecular core.     

Reentrant isotropic phase in a discotic liquid crystal mixture

We have used X-ray diffraction and optical microscopy to study binary mixtures of 2,3,7,8,12,13-hexa-n-tetradecanoyloxy- and 2,3,7,8,12,13-hexa(4-n-undecyloxybenzoyloxy) truxenes. Our measurements confirm the existence of a reentrant isotropic phase, which is however always preceded by a mixed phase region rather than by a pure columnar phase. The nematic N_D phase displays an anomalously negative thermal expansion coefficient. We speculate on mechanisms for reentrancy in discotic mesogens.     

Negative uniaxial films from lyotropic liquid crystalline material for liquid crystal display applications

For a certain class of molecules which form a lyotropic liquid crystalline phase, the oriented molecular layers possess negative birefringence. When the liquid crystal is sheared onto a substrate to produce a thin film, the optic axis orients along the shearing direction of the material. The dried lyotropic films can be applied for improvement of the viewing angle performance of liquid crystal displays.     

Negative uniaxial films from lyotropic liquid crystalline material for liquid crystal display applications

For a certain class of molecules which form a lyotropic liquid crystalline phase, the oriented molecular layers possess negative birefringence. When the liquid crystal is sheared onto a substrate to produce a thin film, the optic axis orients along the shearing direction of the material. The dried lyotropic films can be applied for improvement of the viewing angle performance of liquid crystal displays.     

Reflective mode of deformed-helix ferroelectric liquid crystal cells for sensing applications

We present a detailed theoretical and experimental study of the reflectance response of a deformed-helix ferroelectric (DHF) liquid crystal (LC) cell to an applied voltage under cross-polarisers. Using a model based on the effective dielectric tensor approximation, we derive simple analytical formulas to design a LC cell with maximum modulation depth and optimal linearity of the electro-optical response intensity versus the electric field. Our experimental results show that the cell works at frequencies up to 10 kHz and exhibits excellent linearity, with a total harmonic distortion as low as ?70 dB. These findings suggest that DHF-LCs can be exploited to develop simple and accurate optical sensors.     

Polyfluorinated cycloalkoxyphenyl ether systems as dopants for liquid crystal display applications

The effects on response times of representative commercially available dielectrically negative (−LC) and positive (+LC) type liquid crystal mixtures doped with small quantities of various tetrafluorinated cycloalkoxyphenyl ether derivatives were investigated. The synthesis of various polyfluoroaryl systems by nucleophilic aromatic substitution processes is described and electro-optical measurements concerning the rise and decay times and voltage holding ratios of each doped LC formulation are reported. The response time of the dielectrically negative LC host was improved upon doping with each of the tetrafluorinated cycloalkoxyphenyl ether derivatives, although no improvement in the performance of the dielectrically positive LC host was achieved.     

In situ self-assembled photo-switchable liquid crystal alignment layer using azosilane monomer-liquid crystal mixture system

Self-assembled monolayers (SAMs) are a unique approach for the liquid crystal (LC) alignment in electro-optical applications such as displays. Herein, a new methodology for photo-switchable LC alignment layer using an azosilane monomer and LC mixture system in the absence of any other foreign alignment layer is presented. The azosilane monomer spontaneously separated from the host LCs, and formed a stable monolayer network on the substrate surface. Data from X-ray photo-electron spectroscopy (XPS), spectroscopic elipsometry (SE), water contact angle and LC alignment studies confirmed that, in the azosilane and LC mixture system, azosilane makes an in situ SAM that is capable of photo-switchable LC alignment layer on glass and indium tin oxide (ITO) substrates. The LCs are aligned with respect to change in the photo-isomerisation of the azo molecule.     

A two dimensional liquid crystal simulation for thin film transistor liquid crystal displays

Abstract

A two-dimensional liquid crystal simulation, whose electrode configuration corresponds to that in a thin film transistor liquid crystal display (TFT-LCD), was carried out. Simulation results show that the lateral field between buslines and pixel electrode forms a reverse tilt domain. This reverse tilt domain leads to the disclination on the pixel electrode. The distance from the pixel electrode edge to this disclination location depends on the dielectric anisotropy and elastic constant for the liquid crystal. A small dielectric anisotropy or large elastic constant makes this distance small.     

Field-induced reorientation dynamics of a surface stabilized ferroelectric liquid crystal mixture

Time-resolved measurements of electro-optical response and Fourier-transform infrared absorption have been employed to study the submolecular motion of a surface stabilized ferroelectric liquid crystal (SSFLC) mixture during a field-induced reorientation process. All observed submolecular groups were found to rotate in unison about the layer normal in a steady d.c. field. In a transient situation, the FLC director can respond rapidly to a pulsed driving field. However, the orientation distribution was found to spread at first and then slowly converge to the new direction of the FLC director. When the field is switched off, all the core groups relax to a steady-state direction with varying relaxation times. The results reveal that submolecular fragments of different molecular species in the FLC mixture move correlatively in a steady d.c. field and during a field-induced reorientation process.     

High performance dual frequency liquid crystal compounds and mixture for operation at elevated temperature

Dual frequency liquid crystal (DFLC) compounds with high birefringence and ultra-low crossover frequency have been developed. The long conjugated rigid core structure gives these compounds a high birefringence (0.3–0.4) and ultra-low Debye relaxation frequency. Using these compounds we have formulated a new DFLC mixture, designated UCF–02, for operation at elevated temperatures. The birefringence of the mixture is greater than 0.3 at 25°C and λ = 633 nm. Their initial ultra-low crossover frequency allows them to be used at elevated temperatures and significantly improves their utility.     

Nanostructure and dielectric properties of a twist-bend nematic liquid crystal mixture

We present freeze-fracture transmission electron microscopy (FF-TEM), dielectric spectroscopy and electro-optic measurements on a dimeric liquid crystal mixture, which previously was proposed to form the twist-bend nematic (N_tb) phase. Our FF-TEM studies provide a direct image of a 10.5 nm periodic structure, consistent with the expected nanoscale, heliconical twist-bend modulation of the molecular orientation. Dielectric measurements in the 100 Hz to 10 MHz range reveal three nearly Debye-type dispersion processes in the nematic and the twist-bend phase. Low frequency 8 V/µm electric fields applied on planar cells cause the optical-scale stripe texture (another characteristic feature of the N_tb phase) to disappear. Higher (>16 V/µm) fields gradually realign the heliconical axis along the electric field; it relaxes back after the field removal.     

Low-birefringence liquid crystal mixtures for photonic liquid crystal fibres application

ABSTRACT

Series of low-birefringence liquid crystal mixtures composed of alkyl alkylbicyclohexyl carbonates and modified mixtures with other compounds have been formulated, their refractive indices and electric permittivity measured upon temperature. They exhibit the ordinary refraction index n_o lower than the refractive index of silica glass in a different range of temperature. This enables to observe in photonic liquid crystal fibres (PLCFs) a change in the light propagation mechanism from photonic band gap guiding to modified total internal reflection at different temperatures. Selected applications of PLCFs are also discussed.     

Towards a new class of ferroelectric liquid crystal molecules for nonlinear optical applications

The synthesis of a new molecule specifically designed to exhibit both ferroelectric and nonlinear optical properties is presented. This molecule possesses an optically active phenyl sulphinic group, which introduces a dipole moment directly linked to the chromophore group, with a nitro group in the meta -position. The rest of the molecule comprises a tolane and a benzoate group. Such a structure results in an enantiotropic smectic C*-smectic A phase sequence in its polymorphism. A first measurement of the spontaneous polarization shows a high value. The polymorphism of the racemic homologous molecule possessing the nitro group in the ortho-position is also presented, and also shows the smectic C-smectic A phase sequence.     

Ultrafast switchable wide angle negative refraction in novel dual-frequency liquid crystal mixture

Ultrafast switchable wide angle negative refraction in a novel dual-frequency liquid crystal mixture is demonstrated experimentally. Laterally fluorinated isothiocyanato phenyl-tolane liquid crystal compound is doped into Merck MLC-2048 to acquire the high birefringence dual-frequency liquid crystal. The maximum negative refraction angle achieved is about 12° and the switching time is only 25 ms for a 40-μm cell which is two orders faster than switchable negative refraction achieved before in nematic liquid crystals. Such properties provide a feasible approach to design negative refraction devices based on liquid crystals.     

Towards a new class of ferroelectric liquid crystal molecules for nonlinear optical applications

The synthesis of a new molecule specifically designed to exhibit both ferroelectric and nonlinear optical properties is presented. This molecule possesses an optically active phenyl sulphinic group, which introduces a dipole moment directly linked to the chromophore group, with a nitro group in the meta -position. The rest of the molecule comprises a tolane and a benzoate group. Such a structure results in an enantiotropic smectic C*-smectic A phase sequence in its polymorphism. A first measurement of the spontaneous polarization shows a high value. The polymorphism of the racemic homologous molecule possessing the nitro group in the ortho -position is also presented, and also shows the smectic C-smectic A phase sequence.