Mathematical model of compensation for dispersion of light polarization states in electrooptical systems based on chiral liquid crystals

Spectral dispersion of the polarization states occurs after passage of linearly polarized light through a thin layer of a chiral nematic crystal along the helical axis. It is clearly pronounced for crystals with a high optical anisotropy and a helical pitch somewhat larger than the light wavelength. The results of numerical analysis of the spectral dependence of polarization states at the output of the liquid crystal layer and the method for compensating for the dispersion are presented for the first time. It is shown that polarization dispersion can be eliminated using phase plates of two types, one of which should have a high anomalous birefringence dispersion. The possibility of developing fast light modulators operating in a wide spectral range is discussed.     

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.     

Angular dependences of transmission spectra of chiral liquid crystals

Transmission spectra of chiral nematic liquid crystals (CNLCs) and chiral smectic С* liquid crystals (CSLCs) are studied in a wide range of angles of light propagation with respect to the direction of the helical structure axis. To this end, both a planar LC cell and a cell consisting of two substrates in the form of rectangular prisms (30° × 60° × 90°) were used, which allowed measurements of the transmission spectra at the light propagation at angles up to 65° with respect to the helical axis direction. The transmission spectra are numerically simulated; the results are in good agreement with the experimental spectra. A technique for determining the tilt angle of molecules in smectic layers is proposed based on the comparative analysis of experimental and calculated transmission spectra of CSLC.     

Liquid Crystal Auto-Induction and Amplification Function for Circularly Polarized Luminescence (CPL) with High gem-Value,and Dynamically Controllable CPL Devices

Optically active phenylenevinylene derivatives with fluorescence are employed for preparation of a cholesteric liquid crystal based circularly polarized light (CPL) emission device. The device shows intense CPL with a quite large value of the g_em-factor (= 0.6) and quick CPL light switching driven by cholesteric-nematic transition with a homeotropic alignment. This research develops the auto-induction of fluorescent chiral inducers in host liquid crystal for formation of a helical structure with CPL amplification. The present research applies a classical LC light-scattering system using chiral technology as a new method for obtaining CPL dynamic control with an intense g_em-factor.     

Photo- and thermo-induced variation of photonic properties of cholesteric liquid crystal containing azobenzene-based chiral dopant

Photooptical properties of a new cholesteric mixture containing azobenzene-based chiral dopant were investigated. Presence of a photosensitive dopant in the investigated liquid crystal enabled to effectively manipulate optical characteristics of the photonic band upon temperature change and illumination. Temperature dependence of the optical anisotropy and the orientational order parameter of the liquid crystal were determined. The obtained results are compared with existing theories. Discrete multistable change of the spectral position of the photonic band on illumination is realized.     

Development of Personal UV Biodosimeter Based on Vitamin D Photosynthesis

Abstract

A new approach to monitoring of vitamin D synthetic capacity of UV solar/artificial radiation is described. Nematic liquid crystal (LC) was converted into cholesteric phase by chiral dopant of 7-dehydrocholesterol (provitamin D₃), and the effects of UV irradiation were studied using spectral and polarized observations. Significant changes in optical characteristics of the LC films depending on UV exposure were observed as a result of UV initiated photoisomerization that changed helical twisting power of dopant molecules.     

The temperature dependent collective dynamics of liquid Rubidium

Liquids alkali metals show near the melting point at small momentum transfer values an upward bending of the dispersion relation. This so-called positive dispersion can be described within generalised hydrodynamics as a visco-elastic reaction of the liquid. There is a speculation that long-living clusters could be the physical reason behind this phenomenon. To shed light on this question a coherent inelastic neutron scattering experiment on liquid Rubidium was performed at four temperatures starting at the melting point. Distinct deviations from molecular dynamics simulation results occur at the lowest accessible momentum transfer values. With rising temperature the excitation frequencies soften at a momentum transfer value which is correlated with the dimensions of the supposed clusters.     

Physical Properties of Ferroelectric Liquid Crystals Incorporating a Boron Atom

Physical properties of a new type of ferroelectric liquid crystal incorporating a boron atom have been studied. Spontaneous polarization, tilt angle and chiral pitch of the new ferroelectric liquid crystals are compared with those of other types of ferroelectric liquid crystals. The alternation of the Ps direction and the twist sense is observed as the chiral center is moved further away from the core.     

X-ray Analysis of Chiral Thermotropic Liquid Crystal Copolyesters Based on Linear and Nonlinear Aromatic Ester Triads

A series of chiral thermotropic liquid crystalline copolyesters, based on linear mesogenic triads and nonlinear nonmesogenic triads consisting of two p-oxybenzoyl units coupled by a terephthaloyl or phthaloyl residue respectively, is studied by X-ray diffraction. Complementary data of differential calorimetry and polarizing microscopy are presented. The degree of crystallinity is found to depend significantly on the relative content of the nonlinear counit and to vary with thermal history. The occurrence of cholesteric phases is established, whose stability and breadth can be modulated on the basis of appropriate chemical compositions. The structural data of the investigated copolymer samples are compared with those previously obtained for an analogous series of copolymer samples based on the same linear aromatic triad and an isomeric triad containing a central isophthaloyl residue.

Keywords: liquid crystalline polymers, chiral thermotropic polymers, cholesteric copolyesters, 4,4'-(terephthaloyldioxy)dibenzoate mesogen, X-ray diffraction     

Investigation of a Twisted Phase between the Cholesteric and the Smectic A Phase

In a ternary mixture of two liquid crystals and a non-mesomorphic chiral dopant an intermediate state with an unusual texture was found between the cholesteric and the S_A phase which could be identified as a separate twisted mesophase. The reflection wavelength of this mesophase does not change with temperature whereas the transmission of unpolarized light decreases from 50 to 20%. The experimental findings point to the existence of a twisted grain boundary phase.     

Chiral Induction in Liquid Crystals with Dopants Derived from 1-Phenyl-2-Aminoalcohols

New chiral additives derived from 1-phenyl-2-aminoalcohols have been prepared and tested. Especially ephedrine and pseudoephedrine as chiral auxiliaries with two asymmetric C atoms offer functional groups of divergent reactivity. Dopants prepared by acylation with promesogenic agents can be used in various electro-optic devices. A specific utilization concerns bistable cholesteric liquid crystal displays with stacked double layer construction in order to enhance the luminance of reflected light. Further, the solubility in nematic mixtures, the long term phase behavior, the dopant's impact on textures and phase behavior, the electro-optic response and the photostability of cholesteric mixtures have been investigated.     

New Simple Model of a Liquid Crystal Light Valve

Abstract

A liquid crystal light valve, using a heterojunction indium oxide/silicon of high resistivity (10,000 Ω.cm) and based on the cholesteric-nematic transition is described. The characteristics of this device are determined: optimum voltage to be applied to the cell in order to produce the cholestericnematic phase change, incident light power sensitivity, spectral response, rise and decay times. It has been shown that this light valve has an incident power sensitivity of less than 10 μW.cm^?2 which passes through a maximum for an incident wavelength of 0.8 μm. This valve can be used in the near infrared; rise times of a few tens of ms are obtained.     

Chiral phases in a system of biaxial molecules

Chiral phases in a system of biaxial molecules are investigated based on a theory of biaxial liquid crystals, which we have presented in past work. Four chiral phases are identified, corresponding to a uniaxial nematic phase, two biaxial nematic phases, and a discotic phase, respectively. We find that in the chiral uniaxial nematic phase, the pitch of the chiral structures does not depend on temperature. In contrast, in the two chiral biaxial nematic phases, as well as in the chiral discotic phase, the pitch is temperature-dependent.     

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.     

Multirings Aromatic Aldehyde Liquid Crystal with Azo Linkage and Their Photosensitivity in Mesophase

Ten new rod-like aromatic aldehyde liquid crystalline molecules with azo linkage were synthesized, in which bi(trans-cyclohexyl), cyclohexyl phenyl, and biphenyl carboxylic acid mesogenic cores with terminal ethyl, n-propyl, n-butyl, and n-pentyl substituents were esterified with azo benzoic aldehyde. These molecules were designed in an attempt to construct a series of new azo liquid crystalline molecules to investigate the influence of ultraviolet (UV) light on their mesophase. All compounds have been characterized on the basis of their spectral data, differential scanning calorimeter (DSC), and hot stage polarizing optical microscope (HS-POM). All these compounds exhibited liquid crystalline phase that belonged to nematic and photosensitive properties. Their temperature ranges of mesophase are from 101 °C to 150 °C. Under irradiated 365 nm UV light, they showed photosensitivity in the solvent of methanol. Observed under HS-POM, the UV light also did change the textures of these compounds. The result showed that terminal ethyl is enough for these molecules to exhibit wider temperature range of mesophase, and these new molecules have photosensitivities observed under illumination of UV light not only in solution but also in mesophase due to the change of their structures from trans isomer to cis one.     

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.     

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.     

Novel red–blue dichroic glass containing copper nanocrystals

Copper ruby glasses are commonly produced by dissolving copper in glass (also containing a reducing agent) under relatively reducing conditions to stabilize Cu⁺, followed by a heat treatment to generate the characteristic red color attributed to a colloidal dispersion of Cu⁰ particulates. This study produced copper ruby glasses in a one-step synthesis; 0.1–0.2 wt% copper was included in an alkali borosilicate glass that was synthesized at a very reducing atmosphere. The classic red coloration was directly generated by the colloidal dispersion of elemental copper nanoparticles, (Cu⁰)_n, that formed without further heat treatment. Addition of europium to the copper-containing glass melts led to dramatic spectral changes, with the europium addition not only affecting the amount of elemental copper but also the size of these copper particulates. The net result was the production of a dichroic glass that was ruby red in reflected light and blue in transmitted light. This dichroism was stable only over a certain range of copper nanoparticle sizes, unique to particular Cu/Eu ratios in the glass and optimized at 1150 °C in the alkali borosilicate composition.     

Some Cholesteric Derivatives of S-(+)-4-(2′-Methylbutyl) Phenol

A facile synthesis of S-(+)-4-(2′-methylbutyl)phenol and its interaction with a number of different acids are used to provide a wide variety of new chiral nematic liquid crystals. These new cholesterogens display a range of liquid crystal and other physical properties, and are potentially useful for application in display device and other technologies. Of particular interest in this respect are the materials containing two chiral centres and which therefore have very small helical pitch lengths-as low as 0.1 μm.     

Subject index

An ellipsoidal model of colour centers in photochromic glass was proposed to explain an additional absorption spectrum. Absorption spectra of small ($\text{R ? 100}\text{A}\text{?}$) involving oblong and oblate ellipsoids of rotation have been analysed. The splitting of the absorption spectrum of the small silver particles with non-spherical form resulted in an absorption spectrum different from that for spherical particles. Calculation of the absorption spectra of the system of oblate ellipsoids with considerable dispersion in eccentricity and of about 20 Å in size was in good agreement with experiment. Absorption spectra of the system of oblong ellipsoids differed significantly from experimental findings, indicating that oblong particles were absent. To verify the basic theory of the colour center model, advanced experiments have been carried out on the bleaching of photochromic glass by monochromatic polarized light. The so called photo-adaptation effect has been found, that is, the absorption decreased faster at the wavelength of the bleaching light. The photo-adaptation spectral width indicated that the particle size was nearly 20 Å. The ratio of changing absorption for perpendicular and parallel orientations of bleaching and reading polarization vectors was about 0.7 in the longwave visable range, indicating that the colour were substantially anisotropic.In the thermal recovery of photochromic glass the shortwave and longwave absorptions were bleached faster, indicating the lower stability of the strong oblate particles, which specifies absorption in those parts of the spectrum.