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.     

Magnetic Field Induced Deformations in Nematic Liquid Crystals

Abstract

Unlike most organic materials, liquid crystals respond readily to magnetic fields. This response originates in the anisotropic magnetic susceptibility of nematics, and the relative ease with which they undergo orientational deformations. Because of the large optical birefringence of liquid crystals, these orientational deformations are easy to observe. An interesting demonstration is to place a small rare earth magnet on a nematic cell between crossed polarizers which is illuminated from below. As the magnetic field reorients the liquid crystal, beautiful interference colours appear, indicating director deformations.     

Effects of electrohydrodynamic instability in smectic C liquid crystals

The effects of electrohydrodynamic instability (EHDI) in smectic C liquid crystals in an electric field are studied. The objects of study are oriented layers of para-hydroxybenzoic acid possessing a smectic phase. The observed effects are divided into three groups according to the character of the motion of molecular centers of mass and the orientation of the director and the smectic C liquid crystal layers. The instabilities of the azimuthal and Kapustin-Williams domains are experimentally found. The experimental cell consists of two glass plates with tin dioxide electrodes separated by Mylar spacers of a specified thickness. Various schemes of domain rotation are considered. The EHDI is found to depend on the layer geometry.     

Birefringence of binary liquid crystalline mixtures of MBBA and PPMAECOBA in TCM,interferometric assessment

Abstract

The mutual influence of two mesogenic components, namely a nematic liquid crystal N-(p-methoxy-bezylidene)-butylaniline (MBBA) and a lyotropic liquid crystal poly-(phenyl-methacrylic) ester of cetyloxybenzoic acid (PPMAECOBA) in tetra-chloromethane (TCM) is studied in this paper. The ordinary and extraordinary refractive indices of the mixtures, nematic liquid crystal and lyotropic liquid crystal were measured at 589.3?nm with a Rayleigh interferometer and some electro-optical parameters were computed. The main refractive indices, the birefringence, the main normalized polarizabilities and their difference are dependent on the mixture volumetric concentrations, proving the existence of the collective orientational interactions between the two types of liquid crystals.     

Theoretical and Practical Studies on Effects of External Electrostatic Electric Field on Nucleation and Growth Kinetics of Protein Crystals

The crystallization technique where an electric field is applied is an extremely powerful tool to control the crystallization processes of various materials. In particular, the method with application of an external electrostatic electric field can have a significant effect on the phase equilibrium of the liquid and solid phases. This review demonstrates that the crystallization processes of proteins are significantly impacted by the application of an external electrostatic electric field: (1) Control of both the increase and decrease in the nucleation rate can be achieved by changing the applied frequency of the external electrostatic electric field. (2) The effect of the external electrostatic electric field on the nucleation rate can be controlled by regulating the thickness of the electric double layer (EDL) formed at the interface. (3) The quality of the grown crystals can be improved by an increase in the step free energy under application of an external electrostatic electric field at 1 MHz. The effect of the external electrostatic electric field on nucleation and growth kinetics during crystal growth of proteins is also discussed based on a thermodynamic perspective.     

Fredericks Transition in Pure and Nanodoped Surface Stabilized Ferroelectric Liquid Crystals

Recently, ferroelectric liquid crystals (FLCs) doped with nanoparticles have attracted a wide interest not only from a scientific but also from a practical point of view and there is continuously growing interest in the effects caused by doping the ferroelectric nanoparticles. Furthermore, the presence of external fields exhibits a very interesting transition, reported by recent experiments. In this work, we investigate the response of a surface stabilized ferroelectric liquid crystal (SSFLC) doped with ferroelectric nanoparticles to an applied electric field. We assume that the smectic layers consist of uniform planes with a fixed orientation and the system is free from dislocation of constant layer thickness due to nanoparticles. We obtain the threshold field of orientational transition and the maximum deviation of the polarization vectors for a pure and a doped SSFLC medium. Then, we discuss the Fredericks transtion of the system and formation of inhomogeneous texture. It is found that the ferroelectric nanoparticles have significant influences in ordering behavior of a SSFLC medium and the threshold fields are critically changed by doping nanoparticles in the SSFLC, which is fundamental to operation of many liquid crystal devices.     

PVT法生长4H-SiC晶体及多型夹杂缺陷研究进展

作为第三代半导体材料的典型代表,碳化硅因具备宽的带隙、高的热导率、高的击穿电场以及大的电子迁移速率等性能优势,被认为是制作高温、高频、高功率以及高压器件的理想材料之一,可有效突破传统硅基功率半导体器件的物理极限,并被誉为带动“新能源革命”的绿色能源器件。作为制造功率器件的核心材料,碳化硅单晶衬底的生长是关键,尤其是单一4H-SiC晶型制备。各晶型体结构之间有着良好的结晶学相容性和接近的形成自由能,导致所生长的碳化硅晶体容易形成多型夹杂缺陷并严重影响器件性能。为此,本文首先概述了物理气相传输(PVT)法制备碳化硅晶体的基本原理、生长过程以及存在的问题,然后针对多型夹杂缺陷的产生给出了可能的诱导因素并对相关机理进行解释,进一步介绍了常见的碳化硅晶型结构鉴别方式,最后对碳化硅晶体研究作出展望。     

FLEXOELECTRIC INSTABILITY IN NEMATIC LIQUID CRYSTAL BETWEEN COAXIAL CYLINDERS

Abstract

The stability of the equilibrium configurations of a nematic liquid crystal confined between two coaxial cylinders is analysed when a radial electric field is applied and the flexoelectric effect is taken into account. The threshold for perturbations depending only on the radius r in the cylindrical coordinate system and strong boundary conditions is studied. A new type of orientational transition caused by pure flexoelectric effect is found.     

Local Field Anisotropy in Nematic Liquid Crystals

The anisotropy of the local electric field in the interior of ordered fluids provides a measure of the anisotropy of the molecular distribution. In nematics, both the orientational order parameter and the anisotropy of the local field can be calculated from refractive index and density measurements; results are presented for the liquid crystal butyl p-(p-ethoxyphenoxycarbonyl) phenyl carbonate. It is found that the anisotropy of the local field is very nearly proportional to the orientational order parameter. A simple model incorporating steric intermolecular repulsion predicts this behaviour. The ratio of the local field anisotropy to the uniaxial and biaxial order parameters is simply related to the length-to-breadth ratio of the effective molecular hard core.     

Molecular Dynamics Study of the Nematic Liquid Crystals

Since nematic liquid crystals have found a wide use for solving a number of practical problems, the theoretical prediction of thermodynamic and transport properties of liquid crystal systems, as well as an explanation of the structure and dynamic characteristics on the basis of reasonable molecular models have become a point of great importance. The modern molecular- statistical theories, e.g. Maier-Saupe theory and its modifications, use rough approximations (mean field approximations for example) and therefore may lead to wrong conclusions.     

名义纯及掺杂铌酸锂晶体内偏置场的实验研究

铌酸锂晶体的内偏置场对铁电应用、电光应用和非线性光学应用等均有直接影响。本工作建立了铌酸锂(LN)晶体内偏置场测试方法,对同成分铌酸锂(CLN)晶体、近化学计量比铌酸锂(nSLN)晶体、掺杂铌酸锂(doped LN)晶体的内偏置场和矫顽场进行测量。结果表明,CLN晶体内偏置场最高(E_int=2.53 kV/mm),nSLN晶体的内偏置场大幅降低,其中富锂熔体法生长和气相输运平衡(vapor transport equilibration, VTE)法结合得到的nSLN晶体的内偏置场最小,与CLN晶体相比降低了约两个数量级;掺杂铌酸锂晶体的内偏置场与CLN晶体相比也普遍降低,其中掺6.5%(摩尔分数)Mg的CLN晶体的内偏置场约为CLN晶体的四分之一,掺7%(摩尔分数)Zn的CLN晶体的内偏置场约为CLN晶体的六分之一。最后对组分和掺杂影响内偏置场的因素进行了简要分析。     

Thermal relaxation calorimetry on ethanol at low temperatures

Ethanol exhibits a very interesting polymorphism presenting different solid phases: a fully-ordered (monoclinic) crystal, a (bcc) plastic crystal which by quenching becomes an orientationally-disordered crystal with glassy properties, and the ordinary amorphous glass. Therefore, it appeared as a good model system to investigate low-temperature properties of glasses, including the role played by orientational vs translational disorder. In the present work, we extend and improve previous measurements by implementing higher-accuracy calorimetric methods at low temperatures (especially, two complementary versions of the thermal relaxation method). We have employed these new measuring methods to study the possible effect of water impurities on the calorimetric and thermodynamic behavior of the different solid phases of ethanol.     

Simulation of electrooptical effects and dynamics of ferroelectric liquid crystals

The behavior of ferroelectric liquid crystals in an external electric field is simulated numerically. The equations that describe the dynamics of the director of a liquid crystal are derived within the continuum theory of elasticity with due regard for compressibility of smectic layers, finite anchoring energy, and dielectric properties of orienting coatings and external elements of a real electric circuit. These equations make the basis for simulation of the electrooptics of ferroelectric liquid crystals. The specific features and mechanisms of the surface-stabilized bistability and hysteresis-free electrooptical switching (the V-shape effect) are discussed.     

Flux process for growth of large crystals with application to beryllia

A procedure is described for flux-growth of beryllia crystals, applicable to other crystals. Growth is promoted in a solvent flux by continuous transport of BeO solute in a steady-state thermal gradient. Prepared seeds are mounted on a support structure, submerged in the flux at thermal equilibrium, and rotated during the growth period. The character of growth depends on multi-parameter growth conditions including proper seed crystal structure and preparation, flux composition and impurities, temperature, and operational procedures.     

Bowlic and Polar Liquid Crystal Polymers

Bowlic liquid crystals are made up of polar molecules. Both main-chain and side-chain bowlic polymers are possible. Exactly solvable discrete models describing the phases and conformations of these bowlic and other polar liquid crystal polymers are presented. For the ideal one-dimensional case the model is equivalent to the 1D Ising model. Susceptibility and other properties are calculated. Wave propagation and solitons in these polar polymers are discussed. Possible highly conducting and high T_c superconducting liquid crystal polymers are proposed.     

Dual nature of the orientational effect of ultrasound on liquid crystals

The new model of thresholdless distortion of the orientational structure in a homeotropic layer of nematic liquid crystal with free ends in ultrasonic field has been experimentally substantiated for the first time. The model is constructed within the concepts of nonequilibrium thermodynamics and statistical hydrodynamics of liquid crystals for the frequency range in which the elastic and viscous wavelengths are, respectively, longer and shorter than the layer thickness. The main regularities of the phenomenon, which relate the conditional effect threshold to the ultrasonic frequency and layer thickness, have been established based on the experimental data for (20–150)-μm-thick layers in the frequency range of 0.1–9 MHz. These data are compared with the results of numerical calculations, performed taking into account two mechanisms of liquid crystal structure distortion (convective and nonlinear relaxation ones).     

Study of the surface heterogeneity of topographic and electric nature by atomic force microscopy by the example of triglycine sulfate

Triglycine sulfate crystals with an ideal (010) cleavage plane are used as model objects to reveal problems in interpreting atomic force microscopy (AFM) images of surfaces with nonuniform charge distribution. Specific microrelief features of two types are found: lenslike formations with different contrast and rounded protrusions/valleys of different size but fixed height. An analysis of their evolution with a change in temperature and under an electric field and mechanical impacts has made it possible to separate relief elements from the crystal domain structure. The interpretation proposed is confirmed by the multimode AFM data. The specific features of the images of dynamic domains and aged domains (which cannot undergo polarization reversal) are studied. The domain-wall width found in the AFM measurements depends on the technique used and the specificity of probe-surface interaction; it varies from 9 to 2000 nm. The most reliable data on the domain-wall width in triglycine sulfate crystals are provided by piezoelectric force microscopy, according to which the wall width does not exceed 30 nm.     

Captured Orientational Order in Polymer Network Assemblies

Abstract

The “marriage” between low molecular weight liquid crystals and polymers burgeoned in the 1980s with the idea of dispersing liquid crystal droplets in a rigid polymer matrix to create an electrically controllable light scattering medium. The orientation of the liquid crystal droplets, and hence the refractive index match and scattering, can be systematically controlled with an electric field. Today, dispersions of liquid crystals and polymers are found in many forms depending on the concentration of polymer, which can be as large as 70% or as small as 1%. The systems most understood are those of larger concentration where the liquid crystal is segregated out in the form of droplets randomly distributed throughout the surrounding polymer (see page 2). Dispersions of liquid crystals and polymers differ from macroscopic bulk liquid crystals because of the large surface-to-volume ratio and symmetry breaking non-planar geometry imposed by the polymer. Their composite nature profoundly affects the ordering of the liquid crystal, and their susceptibility to external fields makes them suitable for many new electro-optic applications, as well as intellectually challenging to study from the basic science perspective.     

A contribution to the study of the influence of impurities on the growth and some physical properties of TGS single crystals

Single crystals of TGS doped with Ni²⁺, Cu²⁺ and Fe³⁺ have been prepared under identical conditions by the method of temperature decrease (below the phase transition point). Presentation of the influence of impurities on the development of individual crystal faces is based on goniometric measurements. The influence of impurities on the domain structure of TGS is documented by microphotographs of domain structures and by investigation of the variation with time of the average domain width. It is shown that the distribution of impurities (between the liquid and solid phases) is approximately proportional to the concentrations of metal glycino chelates in the solution. From the results concerning the influence of impurities on various physical properties follows that the degree to which the properties are affected depends not only on the impurity concentration but also on the specific intensity of their action which is due to variations in the binding forces in the structure of TGS.