Reflection and transmission of light by cholesteric liquid crystal-glass-cholesteric liquid crystal and cholesteric liquid crystal(1)-cholesteric crystal(2) systems

Reflection and transmission of light by cholesteric liquid crystal-glass-cholesteric liquid crystal and cholesteric liquid crystal(1)-cholesteric liquid crystal(2) systems were studied. The classical Ambartsumyan method of adding layers and the concept of a sewing function were used. This approach was developed earlier in astrophysics for the theory of radiation transfer. Here, we used a version of this method adapted to wave optics. The Jones matrices are constructed for these systems. The features of the reflection and transmission spectra, optical rotation and ellipticity of polarization were studied for these systems. It is proposed to use these systems as tunable narrow-band filters and mirrors. These systems can be used, for example, to develop a variety of optical elements for lasers and of polarimetric elements in ellipsometry. The specific features of eigenpolarization are also discussed. It is shown that optical rotation of the two layers of cholesteric liquid crystals, which differ from each other only by the sign of the helix, is nonzero, and it becomes substantial in the diffraction reflection region. A unique property of these systems is the degeneracy (coincidence) of eigenpolarizations.     

Conoscopic patterns in photonic band gap of cholesteric liquid crystal cells with twist defects

We investigate theoretically the effects of the angle of incidence on light transmission through cholesteric liquid crystals. The systems are two-layer sandwich structures with a twist defect created by rotation of the one layer about the helical axis. The conoscopic images and polarization-resolved patterns are obtained for thick layers by computing the intensity and the polarization parameters as a function of the incidence angles. In addition to the defect angle-induced rotation of the pictures as a whole, the rings associated with the defect mode resonances are found to shrink to a central point and disappear, as the defect twist angle varies from zero to its limiting value π/2 and beyond.     

Fullerene solar cells with cholesteric liquid crystal doping

This paper reports the doping effect of cholesteric liquid crystal 3β-Hydroxy-5-cholestene 3-oleate on polymer solar cells composed of the poly 3-hexyl thiophene and the fullerene derivative.With a doping ratio of 0.3 wt%,the device achieves an ideal improvement on the shunt resistor and the fill factor.Compared with the reference cell,the power conversion efficiency of the doped cell is improved 24%.The photoelectric measurement and the active layer characterization indicate that the self-assembly liquid crystal can improve the film crystallization and reduce the membrane defect.     

Reflection properties of distorted cholesteric liquid crystals

The optical reflection properties of distorted cholesteric liquid crystals are theoretically investigated. It is shown that higher-order reflection bands occur with distorted helical structures even for light propagation parallel to the twist axis. For aperiodic structures, the positions of the main reflection bands differ from that of periodic ones. Ferroelectric helical structures could be detected by reflection spectroscopy when the liquid crystal is deformed by an electric field.     

Instability of a cholesteric liquid crystal in an ultrasonic field

A new approach to the analysis of orientation instability of a cholesteric liquid crystal in an ultrasonic field is substantiated experimentally. The approach employs the ideas of nonequilibrium hydrodynamics and takes account of the relaxational properties of the mesophase. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 11, 866–870 (10 June 1996)     

Optical reflection from cholesteric liquid crystal films

The interpretation of the reflection spectrum from the measurements of Fergason and Adams et al. by Subramanyam envisaging absorption of light by the material is shown to be incorrect. Some refined measurements of circular dichroism on the mixtures of the cholesteric substances as used by the above authors are reported which conclusively invalidate the explanation of Subramanyam.     

Optical vortices generated by dislocations in a cholesteric liquid crystal

We report the observation of optical vortices in a laser beam propagating through the stripe pattern of a cholesteric liquid crystal. The liquid crystal is confined in a cell with homogeneous boundary conditions and forms a diffraction phase grating. Optical vortices are produced by edge dislocations of the cholesteric grating. The vortices show up as spots of zero light intensity in the diffraction maxima. There is one spot in each +1 and -1 diffraction maximum and two spots in diffraction maxima +2 and -2.     

胆甾相液晶激光器的调谐特性研究

基于胆甾相液晶螺距及折射率随温度与电场变化的特性, 研究了温度与电场对液晶染料可调谐激光器发射特性的影响. 首先探讨了手性剂浓度、温度与液晶螺距的关系, 制作了液晶染料可调谐激光器, 在温度23–35℃变化时, 其发射波长调谐范围为618.90–594.76 nm, 达到24.14 nm, 在电压0–9 V 变化时, 其发射波长调谐范围为617.40–608.11 nm, 共9.29 nm. 关键词： 可调谐激光器 胆甾相液晶 激光染料     

胆甾相液晶温控旋光色散特性的研究

胆甾相液晶的螺距易受温度和电压的影响,螺距变化时,胆甾相液晶的旋光色散特性发生相应的变化。实验证明:温度对胆甾相液晶旋光色散特性的影响与所对应的波长范围有关,在不同的波长范围内,温度对其旋光色散特性的影响规律不同;温度对胆甾相液晶旋光色散特性的影响还与所测试的波长离该温度下选择反射波带的远近有关,离选择反射波带越近,温度对其旋光色散特性的影响越大。实验表明:温度变化时,胆甾相液晶的旋光色散特性的变化是偏振透射谱移动的一个重要原因。     

Time-dependent behaviour of mechanical shear deformations in a cholesteric liquid crystal

Summary In this paper we present the experimental results of time-resolved variations of the apparent pitch of a cholesteric mixture after a steplike shearing deformation is applied to it. We show that the relative variation of the wave-length of the maximum reflection ΔP/P _o varies from initial positive values (P>P ₀) to negative ones (P<P ₀ after a few seconds and finally it tends to a saturation value with ΔP/P ₀<0 in a long time. A qualitative explanation based on the mechanism of the unwinding of the twisted helix structure and a possible tilt of the molecular is also given.

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Riassunto In questo lavoro sono presentati i risultati sperimentali della variazione risolta nel tempo del passo apparente di un CLC sottoposto ad una deformazione di taglio a gradino. Si mostra che la variazione relativa della lunghezza d’onda del massimo di riflessione ΔP/P ₀ varia da un valore iniziale positivo (P>P ₀) ad un valore negativo (P<P ₀) dopo pochi secondi, e tende ad un valore di stturazione con ΔP/P ₀<0 all’infinito. Noi diamo una spiegazione qualitativa basata sul meccanismo dello srotolamento della struttura elicoidale e sul possibile tilt degli strati molecolari.

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Резюме В этой статье приводятся экспериментальные результаты для разрешенных во времени изменений видимого шага холестеричиой смеси после приложения ступенчатой деформации сдвига. Мы показываем, что относительное изменение длины волны максимального отражения ΔP/P ₀ изменяется от начальных ппложительных знаьений (P<P ₀) до отрицательных значениь (P<P ₀ после нескольких секунд и окончательно стремится к величине насьшения с ΔP/P ₀<0 в течение длительного времени. Предлагается качественное объяснение, основанное на механизме структуры закрученной спирали и возможного изменния наклона молекул.

     

Observation of dislocation line motion in a stressed cholesteric liquid crystal

Summary By means of an optical microscope we observed the instantaneous displacement and motion of dislocation lines in a cholesteric liquid crystal, when the sample was subjected to a mechanical normal strain. It was found that in the linear response regime the equilibrium is reached in a quasi-periodic way. We explain our results just in a phenomenological way. A preliminary account of this work was presented at the IX International Conference on Liquid Crystals, Bangalore (1982).     

Texture evolution of cholesteric liquid crystal driven by a thermal process

Cholesteric liquid crystal displays (Ch-LCDs) have potential as screens in portable readers. In the present study, it is demonstrated that a Ch-LCD can be driven by a thermal process at a temperature of 90 °C. At a suitable temperature, the texture of the cholesteric liquid crystal will be transfrred from planar to focal conic. The experimental results show that the reflectance of a Ch-LCD is influenced by texture after the thermal process. After thermal treatment at 90 °C, the periodicity was reduced from 63 nm to 55 nm. A chain tilting angle of ∼29° was estimated.     

Photonic defect modes in cholesteric liquid crystal films

We discuss the properties of photonic defect modes in cholesteric liquid crystals. Twist defects, isotropic defect layers, and combinations of both are considered. After deriving the reflection and transmission properties of the defects, we study the effect of a finite sample thickness on the defect modes amplitude and on the required polarization of incident light to excite the defect mode.Received: 18 August 2003, Published online: 5 February 2004PACS: 42.70.Qs Photonic bandgap materials - 61.30.-v Liquid crystals     

Reflection chromaticity of cholesteric liquid crystals with sandwiched periodical isotropic defect layers

We study one-dimensional photonic crystals made of cholesteric liquid crystals with sandwiched isotropic defect layers. Based on the Berreman Fast 4 × 4 matrix method, the dispersion relation of one-dimensional photonic crystals is calculated and the corresponding reflection chromaticity is obtained. It is found that the color shift could be controlled by adjusting the thickness and refractive index of the isotropic defect layers. Compared with conventional structures, the reflection chromaticity of this structure is insensitive to the incident angle, if the thickness ratio of the cholesteric liquid crystals to that of the isotropic defect layers and the refractive index of periodical isotropic defect layers are properly set. Furthermore, the common forbidden bands for both left and right circular polarizations can be obtained, and we also take the wavelength-dependent refractive indices into consideration and obtain the reflected light chromaticity with the incident angle increasing. The proposed device can be used as a reflective color filter in the display industry.     

Electro-optical properties of polymer stabilized cholesteric liquid crystal film

Liquid crystals (LCs) and polymers are extensively used in various electro-optical applications.In this paper,normal mode polymer stabilized cholesteric LC film is prepared and studied.The effects of chiral dopant and monomer concentrations on the electro-optical properties,such as contrast ratio,driving voltage,hysteresis width and response time,are investigated.The reasons of electro-optical properties influenced by the concentrations of the materials are discussed.Through the proper material recipe,the electro-optical properties of polymer stabilized cholesteric LC film can be optimized.     

Sharply directed emission in microcavity organic light-emitting diodes with a cholesteric liquid crystal film

A flexible microcavity organic light-emitting diode (OLED) was fabricated and the emitting characteristics were examined. A pair of right- and left-handed cholesteric liquid crystal (CLC) films were attached to the microcavity OLED between aluminum (Al) and silver (Ag). Sharply directed spontaneous emission was observed from microcavity OLEDs, in which a typical luminescent material, 8-hydroxyquinoline aluminum (Alq₃), with a broad emission spectrum was used for emitting layer. The introduction of the CLC film improved both the emission bandwidth and directionality, preserving the turn-on voltage and maximum brightness.