Advanced electro-optic properties of nematic liquid crystal doped with high birefringence liquid crystal molecules

The study of fast response nematic liquid crystal display material mixture and synthesis about compounds has always been an important subject in recent years. In this article, one type of high birefringence liquid crystal based on phenyl-diacetylenes was synthesized by sonogashira coupling reaction. A research is conducted on the effect of electro-optic properties of a widely used nematic liquid crystal mixture doped with the prepared high birefringence liquid crystal molecule which concentrations ranging from 1% to 5 wt%. While the results show that the steepness of electro-optic curves first increase and then decrease with the prepared high birefringence liquid crystal molecule content increasing, when the content of high birefringence liquid crystal molecule reaches 3%, electro-optic properties of the mixture liquid crystals are excellent.     

Acousto-optic effect in a nematic liquid-crystal layer under the binary effect of sound and viscous waves

The optical effect in a liquid crystal cell containing a homeotropic layer of nematic liquid crystal (NLC) is analyzed. An NLC layer, located between crossed polaroids and opaque in the absence of external effect, is cleared after irradiation by an ultrasonic beam with a sharp spatial boundary. This enlightenment is suggested to be caused by the reorientation of crystal molecules in the acoustic flows that arise under the binary effect of the layer compression in the irradiated region and the viscous waves propagating from the layer boundaries. The flows were calculated taking into account the stress caused by the velocity convection and crystal structure relaxation. An expression is derived for the cell transparency, and the relative role of the convection and relaxation processes in the effect is determined.     

On the nature of the orientational effect of ultrasound on nematic liquid crystal

Experimental substantiation of the validity of the model of orientational distortion in a homeotropic layer of nematic liquid crystal under an ultrasonic beam with a sharp boundary is presented for the first time. The model is constructed within the concepts of nonequilibrium thermodynamics and statistical hydrodynamics, taking into account the processes of structural relaxation of the mesophase. It establishes the relationship between the characteristics specifying the homeotropic structure deformation (layer thickness, ultrasound frequency, parameters of the molecular micromodel of liquid crystal, and its material constants) and the layer transparency for a linearly polarized light beam. The calculation results are compared with the experimental data in the frequency range of 0.1–3 MHz.     

Helical disclinations in a cylindrical capillary with nematic liquid crystal

A new type of linear disclinations in a cylindrical capillary with a nematic liquid crystal (NLC) has been studied. It can be considered as a generalized linear disclination of the unit strength. The exact analytical expressions for the elastic field components, lines of force, and the free energy of deformation have been derived.     

The effect of concentration and molecular weight of polycarbonate on the elastic constants of a nematic liquid crystal solvent

A polarized upconverting luminescence in a novel upconverison nanorods (UCNRs)/liquid crystal network polymer (LCN) composite film is proposed. The UCNRs were fabricated by hydrothermal method, which were further modified to be introduced into LCN precursors with good dispersion. The orientation of UCNRs in LCN precursors was found to be strongly influenced by the configuration of LC molecules which could be modulated by external electric field. The UCNRs/LCN composite films in planar and homeotropic configuration were fabricated by UV photopolymerization. It is demonstrated that upconverting fluorescence emission from the UCNRs/LCN film with planar orientation is polarization-dependent, while the emission from the UCNRs/LCN film with homeotropic alignment is not polarized. These observations are ascribed to the directional arrangement of the UCNRs enabled by the LC molecules alignment and the crystal symmetry of the UCNR matrix. This work paves a different avenue to the construction of the upconversion nanoparticles/polymer composite system with prospective application value in anti-counterfeiting optical films.     

Analysis of optical and thermal properties of double hydrogen bonded liquid crystal binary mixtures

Eighteen binary mixtures are prepared from three thermotropic double hydrogen bonded liquid crystal complexes formed by Mesaconic Acid (ME) and various alkyloxy benzoic acids, namely ME + 5BAO, ME + 11BAO and ME + 12BAO where 5BAO, 11BAO and 12BAO represents pentyloxy benzoic acid, undecyloxy benzoic acid and dodecyloxy benzoic acid respectively. Two sets of binary mixtures (ME + 12BAO with ME + 5BAO and ME + 11BAO with ME + 5BAO) are formed by varying their molar ratio in steps of 0.1 to 0.9 which leads to the formation of eighteen binary mixture complexes. Existence of different energy bands in the binary mixtures is identified from the analysis of Fourier Transform Infrared Spectroscopy (FTIR). Phases such as nematic and smectic C are observed in all the binary mixtures which are confirmed through Polarizing Optical Microscopy (POM). Transition temperatures corresponding to the individual mesophases are further verified using Differential Scanning Calorimetry (DSC). Phase diagram of two sets of binary mixtures are constructed with the aid of POM and DSC data. Optical tilt angle measurement; a primary order parameter; is calculated in smectic C phase of the binary mixtures and the resultant data obtained is fitted to critical exponent value is in concurrence with the Mean Field theory predicted value. Thermal studies such as stability factor, order of phase transitions and specific heat capacity value of the mesophases are extracted from the DSC thermograms.     

Behavior of a nematic liquid crystal in oscillatory flow at weak surface anchoring

The effect of the surface viscosity on the orientational dynamics of a nematic liquid crystal in an oscillatory flow has been studied. For the shear and the Poiseuille flows, the approximate analytical solutions of the nematodynamic equations are obtained at small flow amplitudes. The frequency range of the oscillatory flow is determined, in which the surface viscosity strongly affects the optical response of a nematic layer. The results obtained are compared with the data of direct numerical simulation of the nematodynamic equations and the method for the experimental determination of surface viscosity is proposed.     

Domain structure of a planar layer of a nematic liquid crystal in oscillating Couette flow

The spatially periodic structure arising in a planar layer of a nematic liquid crystal in oscillating Couette flow is described theoretically. The analysis of this effect is based on the hydrodynamic equations for nematic liquid crystals, from which a self-consistent system of equations is singled out for perturbations of hydrodynamic variables. It is shown that the structure and threshold parameters of the effect depend on the oscillation frequency and the layer thickness through the scaling combination ωh ². The dependence of the configuration of distortions on the viscosity α₃ is analyzed.     

Wavelength-tuneable laser emission from a dye-doped achiral nematic liquid crystal dispersed into a chiral polymer scaffold

ABSTRACT

We compare the emission characteristics of a thin-film liquid crystal (LC) laser created using a polymer-stabilized, dye-doped chiral nematic LC to that of an LC laser that was fabricated using an achiral, dye-doped nematic refilled into a chiral polymer scaffold that was templated from the same chiral nematic host. Both lasers exhibit wavelength tuning upon the application of an external electric field. However, for the templated sample, tuning is found to occur across a broader wavelength-range for the same electric field amplitude. We discuss the benefits of the templated approach and how it can be used to circumvent dye bleaching that may occur during photo-polymerisation.     

Investigation of thermo-electro-optic properties of some mixed nematic liquid crystals

The 6CB/8CB/8OCB liquid crystal mixture has been studied by Differential Scanning Calorimetry (DSC), polarised optic microscopy (POM), Semiconductor Characterization System, and Ultra-violet spectrophotometry (UV). DSC and POM results indicate that the 6CB/8CB/8OCB mixture exhibits liquid crystalline properties. The capacitance-voltage and conductance-voltage measurements were performed in the frequency range of 200-500 kHz and in the temperature range of 30oC–50oC. The 6CB/8CB/8OCB mixture showed an extremely large positive dielectric anisotropy. The molar absorptivity ? for the 6CB/8CB/8OCB mixture was calculated and found to be higher than the absorptivity values of the binary mixtures due to the alkyl chain length with H-aggregation.     

Optical and X-ray studies of a four component cyano liquid crystal mixture: Comparison of properties with that of a similar mixture

Optical and X-ray studies have been conducted on a four component liquid crystal mixture (code name 2013) with cyano–end groups in all the four components and a lateral substitution of fluorine atom in one of them. The variations in the optical birefringence with temperature, have been investigated and the thermal dependence of polarisability anisotropy and orientational order parameter have been evaluated. X-ray studies have been conducted to determine the effective molecular characteristics such as apparent molecular length and inter molecular distance and their variations with temperature. From nature of molecular alignment as envisaged from X-ray photographs the order parameter variation with temperature has been determined and compared with macroscopic order parameter variation as determined from birefringence studies. The results have been compared with these obtained for another structurally similar four-component liquid crystal mixture (code name 2014) with isothiocyanato end group in place of cyano.     

X-ray studies of binary mixtures of smectics A1 with a nematic gap and reentrant nematic phase

Binary mixture of 80BCAB—10TPCHB system are investigated. It is shown that the dimerization of 80BCAB compound in the matrix of 10TPCHB takes place as it was in 7CBB—10XPCHB systems. The generalized conclusion is drawn that such a behaviour is common for homologous series the members of which with shorter aliphatic chains yield the smectic A₁ phase and those with longer chains the A_d one.     

Spatially periodic reorientation of liquid crystal director at planar Freedericksz transition

Conditions for the formation of a spatially periodic structure in a liquid crystal cell at the threshold planar-planar director reorientation in a dc electric field are considered. The dependences of the threshold and the wave number of the arising structure on (i) the polar and azimuthal anchoring energy at the surface of the cell substrates, (ii) the ratio r of the elastic constants of the liquid crystal, and (iii) the value of the flexoelectric parameter v are calculated. It is shown that the range of the parameters r and v corresponding to the spatially periodic director reorientation depends strongly on the anchoring energy. The range of allowable values of the parameter r narrows with a decrease in the azimuthal anchoring energy and expands with a decrease in the polar anchoring energy, while the range of allowable values of the parameter v widens with decreasing azimuthal anchoring energy and depends nonmonotonically on the polar anchoring energy.     

Optical and laser properties of Nd:LuVO4 crystal

Polarized absorption spectra of Nd:LuVO₄ crystal were measured at room temperature. The optical parameters of π polarization and σ polarization were calculated by Judd‐Ofelt theory. Meanwhile, the phenomenological intensity parameters: Ω₂, Ω₄, and Ω₆ were obtained, then the parameters were used to calculate the luminescence parameters of Nd:LuVO₄ crystal, including oscillator strengths, fluorescence branching ratio, radiative lifetime and integrated emission cross‐sections. Experiments of a‐cut and c‐cut Nd:LuVO₄ lasers were also performed, and opt‐opt conversion efficiency and slope efficiency were 40.3%, 50.5% for a‐cut and 23.6%, 30.9% for c‐cut, respectively. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Refracting properties of lyotropic nematic calamitic mesophase under magnetic field

The present study found the time-dependent extraordinary refractive index and birefringence of lyotropic nematic calamitic mesophase for different temperatures during its orientation under magnetic field. Based on the experimental results, the average angle between the mesophase directors and magnetic field was calculated to estimate the orientation dynamics of the mesophase during the director rotation from planar to homeotropic orientations. On the other hand, the textural properties were investigated during the orientation. The ordinary and extraordinary refractive indices were also measured depending on temperature to obtain the orientational order parameter of the mesophase. Using the macroscopic refractive index results, the microscopic polarizability was studied as well.     

Optical probing of low-pressure solution grown GaN crystal properties

The structural and optical properties of self-nucleated crystals grown by a near atmospheric pressure solution growth method are presented. High-resolution room temperature Raman scattering studies demonstrate that stress-free crystals with low free-electron background have been produced. Low and room temperature photoluminescence experiments confirm the presence of shallow donors and an unknown shallow acceptor. Large relative intensity variations of the emission bands assigned to recombination process involving donors and acceptor, resulting from significant changes in the incorporation and/or activation of defect associated with each recombination channel, reflect major changes in the intrinsic material properties.     

Dielectric relaxation and electro-optical switching behavior of nematic liquid crystal dispersed in poly(methyl methacrylate)

Polymer dispersed liquid crystal composite films were prepared from poly(methyl methacrylate) and nematic liquid crystal E44 by solvent induced phase separation method. In the present investigation we report effect of liquid crystal concentration on the electro-optical and dielectric properties of the composite films. The results were interpreted in terms of phase separation of liquid crystal and polymer, shape and size of liquid crystal droplet, interfacial charge layer effect, liquid crystal loading and miscibility of liquid crystal in the polymer matrix. The miscibility between two phases at interface was investigated by employing Fourier‐Transform Infrared Spectroscopy and differential scanning calorimetry. Morphological study showed that liquid crystal phase is embedded in a spongy poly(methyl methacrylate) matrix and homogeneous distribution increased with increasing E44 content. Electro optical behavior of these composite films under the condition of an externally applied AC electric field (0–200 Vp-p, 50–1000 Hz) and wide range of temperature was determined using He–Ne laser (wave length 632.8 nm) as a light source. It was found that Poly(methyl methacrylate)/E44 (30/70) wt.% composite has more significant properties than the other concentrations. The performance of all composites showed variations with respect to applied voltage as well as temperatures. Dielectric measurement of polymer dispersed liquid crystals has been carried out in the frequency range from 20 Hz to 20 MHz and over the temperature range from 24 °C to 100 °C. The Maxwell–Wagner effect due to interfacial charge accumulation between boundaries of liquid crystal droplets and surrounding of polymer matrix has been observed.