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.     

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.     

A room temperature solution-phase process to synthesize pure phase single-crystalline hexagonal cobalt hydroxide nanoplates

A simple room temperature solution-phase method has been developed to synthesize pure phase single-crystalline hexagonal β-Co(OH)₂ nanoplates. The chemical composition and morphology of the as-prepared β-Co(OH)₂ nanoplates were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and selected area electron diffraction (SAED). The results indicated that the as-prepared β-Co(OH)₂ nanoplates were composed of pure brucite-like β-Co(OH)₂ phase with single-crystalline feature. The effect of polyethylene glycol (PEG), NH₄OH, and NaOH on the morphology and size of β-Co(OH)₂ nanocrystals were discussed in detail. The growth mechanism of the as-synthesized nanoplates was also discussed in detail based on the experimental results.     

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.     

Phenomenological theory of phase transition from isotropic to uniaxial, biaxial, and reentrant nematic phases in achiral liquid crystals

The topological properties of phase diagrams of liquid crystals with achiral molecules (mesogens) were studied for transitions from the isotropic to uniaxial and biaxial phases by using the phenomenological Landau-de Gennes theory. An isostructural phase transition between two uniaxial nematic phases was investigated.     

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.     

Increasing the lasing efficiency in cholesteric liquid-crystal photonic structures

Two types of lasing in cholesteric liquid crystals (LCs) in the range of luminescence of laser dye molecules have been investigated. The first type belongs to the Bragg modes at the photonic band edge, which propagate along the normal to the LC layer. The second type of lasing is related to the modes leaking into the substrate and propagating at small angles to the LC layer. It is shown that the Bragg lasing efficiency can be significantly increased under wide-aperture optical pumping. The method proposed for increasing the lasing efficiency is based on suppressing the excitation of leaky laser modes using partially absorbing thin films as the coatings for LC-orienting substrates. Both experimental results and the theoretical model of the effect using the numerical simulation data are discussed.     

Twin-plane reentrant edge growth of rhombohedra boron suboxide platelets

Large quantities of rhombohedra and elongated rhombohedra boron suboxide platelets with flat (0 0 1) surface have been synthesized through conventional solid state reaction. Detailed structural investigations by selected area electron diffraction (SAED) and high-resolution electron microscopy (HRTEM) of these platelets are presented. We present the direct experimental observation of extensive lateral (0 0 1) microtwins in rhombohedra platelets and they give rise to the fractional diffractions spots. It is believed that the growth of these rhombohedra platelets is prompted by the twin-plane reentrant edge (TPRE) mechanism. The transition from rhombohedra platelets to elongated rhombohedra platelets in morphology is probably the result of catalytic growth at the apexes of the platelets. This proposed growth model can be representative of various platelets with low defects formation energy, especially in twinned crystals having a rhombohedra structure. Besides, the presence of extensive microtwins will yield interesting physical properties and probably results in the broadening of photoluminescence (PL) spectra from the rhombohedra and elongated rhombohedra platelets.     

Topological defects of cholesteric liquid crystals for volumes with spherical shape

The existence of the monopoles both with one x (+2) line and two x (+1) lines in cholesteric volumes with spherical shape is shown experimentally by microscopic observations with polarized light. The effect of disapperarnce of x lines near the cholesteric-smectic A transition is discovered.     

Investigations of surface of glasses by nematic and cholesteric liquid crystals

The relatively strong interaction between the surface of the glass and the nematic liquid crystals is demonstrated. The same texture of liquid crystals is observed after every renewed cycle of warming and cooling. Defects, which are invisible without an electrical field, become visible by the dynamic scattering effect. The thermografic results, which are received by cholesteric liquid crystals, are able to demonstrate only strong defects.     

Reversible LED controlled optical activity of a cholesteric liquid crystal layer

Abstract

A thin layer of the photosensitive cholesteric liquid crystal possessing a high value of the optical activity is studied. Reversible change in the optical activity controlled by two light emission diodes (LEDs) with different emission wavelengths is applied for information recording. The behavior of this structure under exposure to UV and blue light is investigated. A smooth decrease in optical activity caused by the UV light resulting in a change in the color of the structure between crossed polarizers is obtained. The reverse process occurs under exposure to the blue light. On the basis of the obtained results, two methods of optical information recording in such a layer have been demonstrated.     

The investigation of the kinking process with help of cholesteric liquid crystals

Process of kinking in CsI crystals has been studied with help of CLC (cholesteric liquid crystals) sensitive to deformation and temperature. It is shown that kinking formation is accompanied by local heat in the kink band. The velocity of deformation is the more the larger the quantity of heat in the kink band.     

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.     

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.     

A new-type composite film of cholesteric liquid crystal doped with PCBM for laser-damage prevention in both visible and near-infrared region

Nowadays, with the increasing use of high-power laser, research and development of laser-damage prevention materials with multifunctional features and wideband wavelengths are urgent and indispensable. In the paper, cholestric liquid crystal (ChLC) is first proposed to be doped with a high-efficiency optical limiting material, 6, 6-phenyl C61-butyric acid methyl ester (PCBM), to fabricate a new-type laser-damage-prevention film. The ChLCs have the desired reflection wavelengths covering 750–2300 nm, which is an effective and potential candidate for the prevention of continuous or tuneable laser in near infrared (NIR) region. Meanwhile, the solubility of PCBM in LC and its nonlinear absorption property of are discussed, and it was proved that the good solubility of PCBM in LC contributed to the excellent nonlinear absorption of mixture (LC doped with PCBM). Taking advantages of this phenomenon, a novel composite film of ChLCs doped with PCBM is prepared by UV polymerization. As a result, the composite film not only performs broadband reflection in NIR region, but also exhibits a strong nonlinear absorption and excellent optical limiter properties in visible region, which proves the film's potential application in laser damage prevention.