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.     

Helical Sense and Pitch of Cholesteric Liquid Crystals

The rotatory sense of a circularly polarized light transmitted through a cholesteric liquid crystal cell was studied by measuring the retardation of a birefringent plate placed upon the cell. Furthermore, the dependence of helical pitch on composition is described for various binary cholesteric and nematic-cholesteric mixtures. The helical sense of eleven cholesterics was determined by the experimental results. As a result, it is shown, for example, that cholesteryl chloride has a right-handed helical structure and cholesteryl propionate has a left-handed helical structure.     

Functionalisation of cholesteric liquid crystals by direct laser writing

Cholesteric liquid crystals selectively reflect circularly polarised light with the same handedness as the helix. Because of their sensitivity to external stimuli, such as heat and electrical fields, various applications utilising their tunability have been proposed. Tuning is usually performed in the bulk, meaning that cholesteric liquid crystals usually possess a single pitch throughout the medium. However, when the helical structure is locally modulated, different optical properties arise, such as tunable photonic defect modes and multiple reflection bands. Here we show a technique to locally modulate the helical structure of cholesteric liquid crystals on a submicron scale, based on two-photon excitation direct laser lithography. Two examples of cholesteric liquid crystal structures with modulated helical structures will be presented.     

Flexible Structures Based on a Short Pitch Cholesteric Liquid Crystals

We report on the realization and characterization of electro-responsive and pressure sensitive polydimethylsiloxane (PDMS) conductive photonic structures combined with the reconfigurable properties of short pitch cholesteric liquid crystals (aligned in Grandjean configuration). By combining ion-implantation process and surface chemistry functionalization, we have overcome the insulating properties of PDMS and induced long range organization of cholesteric liquid crystals, thus controlling both diffraction and selective Bragg reflection of light by means of external perturbations (electric field, pressure). We have characterized our devices in terms of morphological, optical and electro-optical properties.     

Influence of Curing Frequency on the Morphology and the Electro-Optical Property of Polymer-Stabilized Cholesteric Textures

The effect of curing frequency on the morphology of polymer networks and the electro-optical property of normal-mode polymer-stabilized cholesteric textures (PSCTs) has been investigated. The scanning electron microscopy indicates that the shape of polymer networks transforms from honeycomb-like to fiber-like due to the increased solubility of the monomer at higher curing frequencies. The PSCTs cured at lower frequencies with sufficiently large network voids show a two-stage reorientation process that correlates with two kinds of cholesteric liquid crystal (LC) under different environments. The threshold voltage decreases as the curing frequency increases from 1 Hz to 10,000 Hz, while the field-off response time increases.     

Chiral SmC* liquid crystals obtained by mixing of SmC and cholesteric phase

The binary mixtures of the substance providing cholesteric phase exhibit a chiral SmC* phase for certain concentration and temperature range. This phase was verified to be ferroelectric. The temperature dependences of the spontaneous polarization and the coercive field were determined for three concentrations. The pitch of the helical structure is approximately indirectly proportional to the molar concentration of the cholesteric substance and varies from 3 μm to about 25 μm in the concentration range from 20 mol% to 5 mol% of the cholesteric substance. For the lower concentrations the sample is unwound in planar samples due to the surface anchoring. Two possible unwound planar configurations can be switched by electric field.     

Nematic and Cholesteric PDLC Elaborated under Shear Stress

Abstract

The properties of nematic and cholesteric Polymer Dispersed Liquid Crystal (PDLC) with ellipsoidal droplets are reported. The shape of the droplets are modified by a shear stress during the preparation. Different values of the shear stress were used which permit an ellipticity ratio modification by a factor 10. The electro-optical properties are measured for different types of samples and compared with theoretical previsions: transmission or reflection under voltage, threshold voltage, response times. We also describe the preparation of the sample, the experimental apparatus used for the shear stress, and the expected theoretical quantities like response times. When the liquid crystals are nematics, we obtain an increase of the threshold voltages, as predicted. The relaxation time decreases with increasing deformation but the diminution is not as great as predicted. When the liquid crystals are cholesterics, the high deformations induce polygonal fields, which strongly modify the electro-optical properties of the micro-composite. The threshold voltages and response time at applied voltage decrease whereas the relaxation time is increasing.     

Phase transition and thermal stability of reentrant smectic phase in mixture of liquid crystalline materials

In the present work, our investigation is to study the optical and thermal properties of the binary mixture of cholesteric and nematic compounds namely, decyloxy benzoic acid (DBA), and cholesteryl chloride (ChCl), which exhibits different liquid crystalline phases with reentrant smectic-A phase. The reentrant smectic-A phase has been observed at different concentrations and at different temperatures. The existence of reentrant smectic-A phase has been observed by optical microscopic studies. The temperature variation of optical anisotropy, X-ray and helical pitch of the cholesteric (N^*) phase has also been discussed.     

Optical Rotatory Power of a Cholesteric Liquid Crystal for Obliquely Incident Light Using Multiple Scaling

The axis of polarization of light propagating through a cholesteric liquid crystal rotates with the angular rotation per distance traversed termed rotatory power. The rotation is due to the helical array of the molecules rather than due to the individual molecules as is found, for instance, with light being propagated through a sugar solution. The De Vries model for rotatory power of a cholesteric liquid crystal is generalized using multiple scaling theory to include light traveling at an oblique angle to the pitch axis. The resulting expression is tested experimentally using a mixture of cholesteryl oleyl carbonate and cholesteryl chloride.     

Light-controlled change in the helical pitch and broadband tunable cholesteric liquid-crystal lasers

This paper presents an overview of the results obtained in recent investigations of the control over the helical pitch under irradiation and the use of these data for the design of compact broadband tunable lasers based on dye-doped cholesteric liquid crystals. It is demonstrated that a reversible change in the lasing frequency can be achieved upon exposure to two low-power light-emitting diodes. Another alternative approach to the generation of tunable laser radiation in the visible and ultraviolet spectral regions (370–680 nm) in a specially designed cell with a cholesteric liquid crystal that is doped with several dyes and possesses a helical pitch gradient is considered.     

“Bands” and “Torsads” Textures in Films and Threads of Hydroxypropyl Cellulose

The present paper deals with films and threads of a mesomorphic cholesteric polymer: the hydroxypropyl cellulose. The steady flow behaviour and the shear induced textures (observed by optical microscopy under crossed polars) of films of lyotropic solutions of this polymer have been studied. A detailed analysis of the banded structures, of either the sheared solutions or the dried films obtained by solvent evaporation, is reported. We point out the existence of transverse striated patterns—“torsads”—superposed on the main banded structures, which are associated with the rewinding of the cholesteric structure following the cessation of the shear stress. The torsads, also observed in some threads, originate from the same relaxation phenomena and are discussed in the last part of the paper.     

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.     

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.     

Electrical and Optical Properties of Ferroelectric Liquid Crystals and Influence of Applied Pressure

Much larger dielectric constants and spontaneous polarizations are evaluated in the smectic-C phase of p-alkoxybenzylidene-p'-amino-2-chloropropyl-cinnamate (HOBACPC, OOBACPC and DOBACPC) which have dipole moments at the chiral part compared with those of p-alkoxybenzylidene-p'-amino-2-methylbutyl-cinnamate. In the mixture of D-DOBAMBC and L-DOBACPC with inverse chirality, the pitch of the helical structure is longer compared with those of original compounds. On the other hand, the dielectric constant and the spontaneous polarization of the compounds are nearly linearly dependent on the concentration of L-DOBACPC.

The phase transition temperature from smectic-A to chiral smectic-C phases in DOBAMBC decreases remarkably with the mixing of organic molecules (PAA, eicosane, etc.) but increases with applied pressure.

The threshold field of electro-optical effects (transmission increase, colour switching, memory effects) in DOBACPC and HOBACPC etc. is lower than that in DOBAMBC etc. in accordance with the increase of the spontaneous polarization in the former.     

On the Transition between Cholesteric and Nematic Phases in Magnetic Field

Singularities at the critical lines bounding the cholesteric and nematic phases on the magnetic field versus temperature plane are investigated. Critical exponents, α and γ, of the specific heat and magnetic susceptibility are found both unities. By regarding the inverse of pitch of the cholesteric structure as an order parameter, it is found that the exponents β and δ are zero and infinity, respectively, and these exponents satisfy the scaling relations, α + 2β + γ = 2 and α + β (δ + 1) = 2. In relation to the discommensurate charge density wave in layer structure materials, it is noted that the transition from the nematic to cholesteric phase occurring with the change of magnetic field and/or temperature is interpreted as a condensation process of solitons.     

New chiral imines based on S-α-phenylethylamine and S-α-benzylethylamine in induced cholesteric and smectic mesophases: II. Behavior of chiral dopants in ester liquid-crystal systems

The temperature dependences of the pitch of induced helical structures in the ester liquid-crystal systems containing hydroxyphenyl benzoate derivatives and chiral N-aroyloxybenzylidene-S-α-phenylethylamines or N-aroyloxybenzylidene-S-α-benzylethylamines have been measured. The twisting power of chiral dopants is calculated and the temperature dependence of the helical pitch in the induced cholesteric (N ^*) and smectic C ^* mesophases is quantitatively characterized. The parameters A = dT _is/dC that characterize the effect of dopants on the thermal stability of the N ^* phases are determined from the concentration dependences of the cholesteric-isotropic transition temperature T _ts for the n-hexyloxyphenyl-n-butylbenzoate-chiral dopant systems. The results obtained are compared with the data for the 4-n-pentyl-4′-cyanobiphenyl (5CB)-based cholesteric systems induced by the chiral dopants under consideration.     

Steroid motor: dynamics of cholesteric helix induction in the nematic droplet

A nematic liquid crystal can be converted into a cholesteric phase by a chiral dopant, and the cholesteric pitch can be changed by its photochemical transformations [1]. For the first time we investigate the dynamics of the cholesteric phase induction using seven steroids (vitamin D isomers and related compounds) as chiral dopants. Here we report the new effect of rotation of a rod-like steroid crystal (0.1-1 mm length) when it is placed at the surface of a nematic drop and its dissolution course was followed with a polarizing microscope. For all the compounds univocal correspondence was noticed between the crystal rotation direction, the helicity of the molecular steroid ring system [2] and the sign of the cholesteric macrohelix determined by the Cano-Grandjean method [3]. No rotation was observed in the isotropic phase.     

Temperature Dependence of Critical Sample Thickness for Spontaneous Cholesteric-Nematic Phase Transition Between Surfaces with Homeotropic Anchoring

Temperature dependence of the critical sample thickness at which a cholesteric structure unwinds spontaneously between surfaces with homeotropic anchoring are presented for some dielectrically negative chiral nematic mixtures. The results obtained are pertinent to the application of the nematic-cholesteric phase change-effect in liquid crystal devices.     

Crosslinked Cholesteric Network from the Acrylic Acid Ester of (Hydroxypropyl)Cellulose

The acrylic acid ester of (hydroxypropyl)cellulose was prepared from (hydroxypropyl)cellulose and acryloyl chloride. The resultant polymer, with 2.2 ester groups per anhydroglucose unit, formed a thermotropic cholesteric mesophase with visible reflection hands at temperatures between ambient and 60°C. By exposing a thin layer of the mesophase to UV light, the mesophase structure was stabilized to give a crosslinked cholesteric film.