Optical properties of a stack of cholesteric liquid crystal and isotropic medium layers

Some new optical properties of a stack consisting of cholesteric liquid crystal (CLC) and isotropic medium layers are studied. The problem is solved by the modified Ambartsumyan method for the summation of layers. Bragg conditions for the photonic band gaps of the proposed system are presented. It is shown that the choice of proper sublayer parameters can be used to control the band structure of the system. In the general case, the effect of full suppression of absorption, which is observed in a finite homogeneous CLC layer, is not detected in the presence of anisotropic absorption in CLC sublayers. It is shown that this effect can be generated in the system under study if certain conditions are imposed on the isotropic sublayer thickness. Under these conditions, the maximum photonic density of states (PDS) increases significantly at the boundaries of the corresponding band. The influence of a change in the CLC sublayer thickness and the system thickness on PDS is investigated.     

Optical properties of a stack of layers of a cholesteric liquid crystal and an isotropic medium

Optical properties of a stack consisting of layers of a cholesteric liquid crystal (CLC) and an isotropic medium are investigated. The problem is solved using the modified Ambartsumyan layer-summation method. Particular features of reflectance spectra of this system are studied. It is shown that, in contrast to a single CLC layer, this system exhibits multiple photonic band gaps. There are two types of photonic band gaps: selective and nonselective with respect to polarization of the incident light. It is shown that eigenpolarizations in the system generally coincide with quasi-orthogonal quasi-circular polarizations, except for regions that are selective with respect to polarization of the diffractive reflection. It is shown that, for an even number of layers, the system under consideration is nonreciprocal and can function as an optical diode. The influence of thickness of CLC sublayers, angle of incidence, local dielectric anisotropy of CLC layers, refractive indices and thicknesses of layers of an isotropic media on reflectance spectra and other optical characteristics of the system is investigated.     

Zone structure and polarization properties of the stack of a metamaterial-based cholesteric liquid crystal and isotropic medium layers

The optical properties of a stack of metamaterial-based cholesteric liquid crystal (CLC) layers and isotropic medium layers are investigated. The problem is solved by a modification of Ambartsumian’s layer addition method. CLCs with two types of chiral nihility are defined. The peculiarities of the reflection spectra of this system are investigated and it is shown that the reflection spectra of the stacks of CLC layers of these two types differ from each other. Besides, in contrast to the single CLC layer case, these systems have multiple photonic band gaps. There are two types of such gaps: those selective with respect to polarization of the incident light and nonselective ones. It is shown that the system eigenpolarizations mainly coincide with the quasi-orthogonal, quasi-circular polarizations for normally incident light, except the regions of diffraction reflection selective with respect to the polarization of incident light. The influence of the CLC sublayer thick-nesses, the incidence angle, the local dielectric (magnetic) anisotropy of the CLC layers, and the refractive indices and thicknesses of the isotropic media layers on the reflection spectra and other optical characteristics of the system is investigated.     

Optical properties of a stack of right- and left-handed layers of a cholesteric liquid crystal

We have studied the optical properties of a stack that consists of right- and left-handed layers of a cholesteric liquid crystal (CLC). The problem has been solved using the modified Ambartsumyan layer-summation method. We have examined particular features of the reflection spectra of this system and have shown that, as distinct from a single CLC layer, this system has multiple photonic band gaps. Particular features of the spectra of the photonic density of states have been considered. We have shown that this system has unique polarization characteristics; thus, in particular, if the number of sublayers is even, the eigenpolarizations of the system are degenerate (both eigenpolarizations coincide) and, as distinct from ordinary gyrotropic media, the rotation of the plane of polarization can decrease with increasing thickness of the system, the sign of the rotation depends on the sign of the chirality of the first sublayer of the system, the system is very sensitive to the variation in the number of sublayers, etc. We have investigated how the sublayer thickness, the angle of incidence, the local dielectric anisotropy, and the pitch of helix of sublayers affect the reflection spectra and other optical characteristics of the system.     

复合周期的介质-液晶光子晶体研究

本文研究了掺入各向同性周期性介质层缺陷的一维复合周期胆甾型液晶光子晶体的禁带特性.发现同一般的胆甾型液晶不同,复合介质-液晶周期结构对左旋偏振入射光和右旋入射光都会出现共同禁带.当各向同性介质层占一个周期的厚度比例越小,左旋偏振光的禁带效应逐渐消失,而右旋偏振光的禁带则逐渐合并变大;且各向同性介质的折射率越大,则两种偏振光的禁带效应越明显,更容易出现共同禁带;通过调整厚度比,可以在可见光范围内出现对应红、绿、蓝色波长的禁带,从而可以用于液晶显示的反射式彩色滤光片. 关键词： 胆甾型液晶 光子晶体 禁带 缺陷     

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.     

Light transmission through a stack of right- and left-hand cholesteric liquid crystal layers with indefinite dielectric and magnetic permittivities. I. Reflection peculiarities

Optical properties of a stack of layers of right- and left-hand cholesteric liquid crystal (CLC) with locally indefinite dielectric and magnetic permittivities are investigated. The problem is solved by the modified Ambartsumian??s layer addition method. The peculiarities of reflection spectra of this system are studied and it is shown that, in contrast to a single layer of CLC, this system has multiple photonic band gaps (PBG). It is shown that the indefinite character of the dielectric and magnetic permittivities essentially affects the PBG map of the system. These multiple PBG can find wide application, in particular, in technologies of production of displays.     

Light transmission through a stack of right- and left-hand cholesteric liquid crystal layers with indefinite dielectric and magnetic permittivities. II. Optical diode

Optical properties of a stack of layers of right- and left-hand cholesteric liquid crystal with locally indefinite dielectric and magnetic permittivities are investigated. The problem is solved by the modified Ambartsumian’s layer addition method. The peculiarities of nonreciprocity of this system are studied and it is shown that at certain values of system parameters it can work as a practically perfect optical diode.     

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.     

Reflection spectrum of a cholesteric liquid crystal with structural defects

An expression is obtained for the reflection coefficient of circularly polarized light normally incident on the film of a cholesteric liquid crystal with a variable helix pitch. It is shown that, in the case of a single defect (local change in the helix pitch), the spectrum of light reflection from a cholesteric acquires a dip corresponding to the defect mode. New qualitative features appear in the reflection spectrum of a cholesteric with two defects as the distance between them varies.     

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.     

Induced birefringence in the isotropic phase of cholesteric liquid crystals

The birefringence induced by an acoustic field in the isotropic phase of the cholesteryl myristate cholesteric liquid crystal has been measured. The measurements were performed at sound frequencies of 3.2 and 9.8 MHz in a wide temperature range. Based on the results of the measurements of the induced birefringence, the relaxation time of the orientation of the liquid crystal molecules is estimated.     

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。     

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.