Identification criterion for the mechanisms of orientation instability of cholesteric liquid crystals in ultrasonic fields

An identification criterion is proposed and experimentally substantiated for the two independent acoustic mechanisms (vortex and relaxation ones) governing the formation of a system of two-dimensional domains in a planar layer of a cholesteric liquid crystal in the frequency range where the sound wavelength is greater than the step of the cholesteric circuit. The results of measuring the critical compression amplitude in mesophase layers with thicknesses from 40 to 240 μm are presented for frequencies lower and higher than the frequency equal to the inverse relaxation time of the orientation order parameter in a cholesteric liquid crystal. The data are obtained for diluted solutions of cholesterilchloride in a nematic liquid crystal with a circuit step varied from 2 to 30 μm. The concept of the binary nature of the acoustic mechanism that causes destabilization of the planar texture of a cholesteric liquid crystal in the frequency range under consideration is experimentally verified.     

Chiral photonic crystals with an electrically tunable anisotropic defect. Experiment and theory

The possibility of inducing a planar defect in a plane structure of a cholesteric liquid crystal by an external electric field is demonstrated experimentally for the first time. The results of experimental and theoretical study of the behavior of the coefficient of reflection (transmission) of light from a layer of a cholesteric liquid crystal with a planar defect induced in it are considered. Possible applications of such a system are considered.     

胆甾相和蓝相液晶的透射和反射特性

针对蓝相液晶显示器中的暗态漏光特性, 采用时域有限差分方法模拟计算了胆甾相液晶的平面态和焦锥态以及蓝相II态液晶的透射和反射特性, 得到了它们在正交和平行偏振片之间的漏光, 并提出了计算等效旋光能力的公式以比较它们的旋光能力. 通过对比焦锥态和蓝相液晶的旋光能力和漏光特性发现, 采用焦锥态替代蓝相液晶而得到良好的暗态特性是一种可行的方法. 通过研究它们的布拉格反射特性, 得知蓝相液晶的布拉格反射与平面态相似, 但反射强度小, 焦锥态无明显的布拉格反射特性.     

新型驱动原理及低电压反射式双稳胆甾型液晶显示器的实验实现

通过对液晶材料参量、液晶盒结构的优化,本文研制了一种低电压驱动反射式双稳胆甾型液晶显示器.在此基础上提出了一种新型驱动方法,通过首先将液晶显示器所有像素从平面态刷新到焦锥态,可用普通的被动式矩阵驱动器件实现1ms快速驱动,新的驱动方法不存在光学延迟.     

横向电场对平面织构胆甾相液晶螺距及反射特性的影响

阐述一种理论模型,讨论共面转换（IPS）模式下横向电场对平面织构胆甾相液晶螺距及扭曲角的影响．利用琼斯矩阵法计算了光正入射情况下未加电场时胆甾相液晶的反射谱．在上述模型基础上,计算了施加电场后胆甾相液晶的反射峰值波长以及反射带宽随场强的变化关系．探讨了忽略锚定与强锚定两种边界条件下横向电场对平面织构胆甾相液晶螺距及反射特性的影响．所得结论在理论上证实：共面转换模式下电场可以调谐胆甾相液晶的反射光颜色,从而为基于电控螺距原理的胆甾相液晶反射式彩色显示方案提供了理论上的依据． 关键词： 胆甾相液晶 电控螺距 共面转换 琼斯矩阵法     

On the nature of the acoustic mechanism of domain formation in a cholesteric liquid crystal

The dual nature of the acoustic mechanism of destabilizing the planar texture of a cholesteric liquid crystal is confirmed experimentally. This destabilization manifests itself in the formation of a system of two-dimensional domains in the frequency range where the wavelength exceeds the cholesteric-helix pitch. A criterion for identifying two independent acoustic mechanisms (vortex and relaxation ones) is suggested and experimentally verified by analyzing the measured dependence of the critical compression amplitude on the cholesteric-helix pitch in the 40-, 120-, and 240-μm-thick mesophase layers for frequencies of 2.95 and 14.5 MHz, which are respectively lower and higher than the reciprocal relaxation time of the orientational order parameter of the cholesteric liquid crystal.     

复指数矩阵法研究螺旋液晶光学传播特性

提出一种研究螺旋液晶光学特性的数学处理方法.螺旋型液晶可当作N层双折射材料晶片叠合而成,而相邻晶片的光轴依次旋转一个小角度.在光学转换矩阵中引入复数来研究螺旋型液晶光学性能,给出了相关的数学处理方法.研究了线性偏振光在螺旋液晶中的传播特性,并给出了光经过螺旋液晶后电场强度的矩阵表达式.研究发现,如果入射光波长等于螺旋液晶螺距,线偏振光经过右螺旋液晶后是左旋圆偏振光.理论上分析了螺旋液晶对可见光选择性反射机理.     

Localized optical states in a liquid-crystal structure adjacent to a metal

The spectrum of light transmission through a structure consisting of a metallic layer and a cholesteric liquid crystal with an induced planar defect is calculated. The possibility of existence of localized states at the metal–dielectric interface of this system is demonstrated. The transmission spectra at different positions of the defect in the structure are studied.     

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。     

螺状相液晶旋光特性理论模拟与实验研究

利用旋光理论模拟了一种用于螺状相液晶显示器(CH-LCD)的螺状相液晶的旋光率与入射波长、螺距的关系,并从理论上给予定量分析;用磁光调制倍频法测试了这种螺状相液晶旋光率随温度变化的规律,并利用所测的旋光率的值计算得出这种螺状相液晶的螺距、折射率、及双折射率随温度变化的规律;对模拟结果给予了实验验证。     

基于矩阵理论的螺旋液晶透射光偏振态研究

将螺旋型液品看作是N层双折射材料晶片叠合而成,而每层光轴相对于相邻的晶片有一个小的旋转.本文基于矩阵理论,研究了光在螺旋液晶中传输时偏振态的变化.经过详细的数学推导,理论研究表明.线性偏振光经过螺旋液晶后偏振态会发生改变.当满足一定的条件时,线偏振光的偏振方向旋转90°,左旋圆偏振光经过左旋螺旋液晶后转变为右旋圆偏振光.     

楔形盒染料掺杂胆甾相液晶激光器研究

设计制作了楔形盒掺杂激光染料PM580的胆甾相液晶器件, 研究了激光辐射行为. 在楔形液晶盒中出现了一系列与楔棱平行的向错线和不同规则形状的晶畴, 胆甾相液晶形成了平面态排列. 采用固体Nd:YAG倍频532 nm 波长激光作为抽运光, 获得调谐精度约1 nm, 调谐范围约17 nm的一维波长可调谐激光器. 楔形盒中, 液晶扭曲力与取向膜表面锚定力相互平衡的过程中胆甾相液晶螺距伸张, 光子禁带位置移动, 从而调谐光子禁带边沿出射激光波长. 关键词： 胆甾相液晶 楔形盒 激光辐射     

用于电响应红外反射窗的液晶聚合物薄膜特性研究

提出了一种红外反射器件,通过载体液晶在电场下的转向控制胆甾型液晶聚合物粒子在器件中的排布方向,实现红外反射与透射之间切换.其中,胆甾型液晶聚合物粒子是实现红外反射关键.介绍了胆甾型液晶聚合物薄膜的制备方法,并采用超声波破碎的方法制备液晶粒子.研究了液晶混合物中不同比例的交联剂液晶1对液晶聚合物薄膜脆性和反射波段的影响,发现当液晶1占100%时,液晶聚合物薄膜的脆性最大,且其清亮点最高,并且薄膜的反射波段随着液晶1比例的增加而向短波方向偏移.同时,研究了薄膜厚度对液晶聚合物粒子的影响,发现液晶薄膜厚度越小,制备的液晶聚合物粒子越小且越均匀,其制成的器件电驱动性更好.该研究有利于帮助电响应红外反射窗的性能优化.     

Polymer-stabilized cholesteric liquid crystals as switchable photonic broad bandgaps

A cholesteric liquid crystal can be considered as a one-dimensional photonic crystal with a refractive index that is regularly modulated along the helix axis because of the particular arrangement of the molecules. The result is that the propagation of light is suppressed for a particular range of wavelengths (bandgap). A polymer-stabilized cholesteric liquid crystal (PSCLC), which is obtained by in situ photopolymerization of reactive liquid-crystal molecules in the presence of non-reactive liquid-crystal molecules in an oriented Bragg planar texture, is elaborated by combining the UV-curing with a thermally induced pitch variation. As a consequence, it is shown here that memory effects are introduced into the characteristics of the reflection band of the material at room temperature. In the visible spectrum, the reflection bandwidth can be tuned in agreement with the thermal ramp and broadened. In addition, the bandgap filters can be switched between broadband reflective, scattering and transparent states by subjecting them to an electric field. Related application fields of these functional materials are switchable smart windows for the control of the solar-light spectrum and white-or-black polarizer-free reflective displays.     

Thermally switchable flexible liquid crystal devices in prepolymer-doped cholesteric liquid crystals

This work describes an approach for fabricating thermally switchable flexible liquid crystal devices in prepolymer-doped cholesteric liquid crystals (CLCs). The roughness of the UV-cured polymer film eliminates the stability of planar CLCs, allowing the textures in the UV-cured regions to be changed from planar to focal conic. Impurities associated with doping with prepolymers cause the clearing temperature of LCs in the UV-cured regions to differ from that in the uncured regions as the prepolymers are polymerized. Therefore, the textures in these two regions can be switched by controlling the temperature. Thermally switchable flexible LC devices, such as optically addressed smart cards, light valves, and others, can be realized using this approach.     

胆甾相液晶可见光布拉格反射实验

胆甾相液晶可见光反射行为，在某种意义上与晶体粉末样品Ｘ光衍射相似，本主要提供一个巧妙而又直观的布拉格反射实验方法，并测量胆甾相结构周期－螺距与温度的关系，进而揭示胆甾相液晶热色效应的机理。     

Bistable surface anchoring and hysteresis of pitch jumps in a planar cholesteric liquid crystal

The relationship between bistable surface anchoring and the pitch jump process is examined for a planar cholesteric liquid crystal. Introducing a high-order, azimuthal surface anchoring potential into a simple model to describe a cholesteric, we derive an expression for the director twist as the natural pitch of the liquid crystal is allowed to vary. Writing the energy in terms of the surface twist, we are able to determine the twists which minimize the total energy of the system. We demonstrate how a pitch jump is related to an energy exchange from one branch of metastable states to another. We then discuss how the co-existence of energy minima and their associated solution branches may help explain the thermal hysteresis observed experimentally in cholesterics in the neighbourhood of a pitch jump. The presence of a higher-order surface energy term can expand the range of anchoring strengths in which pitch jumps are possible. We also investigate the influence of bidirectional surface anchoring on the behaviour of the total energy. Intermediate quarter-turn pitch jumps can occur, depending on the relative strength of the high-order anchoring term, and these can have a significant effect on the system hysteresis.     

Does the electric Lehmann effect exist in cholesteric liquid crystals?

Experiments have shown that cholesteric droplets or cholesteric fingers may be put into motion by the action of an electric field. The former rotate whereas the latter drift perpendicularly to their axes. In all cases, the texture moves without visible material transport. The electric Lehmann effect was initially used to interpret these observations but, recently, alternative explanations were found, based on electrohydrodynamics. Another experiment in this area was that of Padmini and Madhusudana (Liq. Cryst. 14, 497 (1993)). Performed in 1993 with a compensated cholesteric liquid crystal under fixed planar boundary conditions, it was also explained in terms of electric Lehmann effect. We conducted the same experiment and extended it to a π -twisted planar geometry. Although our experimental results agree with those of Padmini and Madhusudana, we demonstrate that they are incompatible with an electric Lehmann effect. By contrast, an explanation based on flexoelectricity allows us to interpret the whole data set obtained in both geometries. The consequence is that there is at the moment no clear experimental evidence of the electric Lehmann effect.     

Enhanced optical nonlinearity near the photonic bandgap edges of a cholesteric liquid crystal

The third-order Kerr nonlinear optical effect of a one-dimensional photonic bandgap structure of a cholesteric liquid crystal is investigated. In a femtosecond nonlinear transmission measurement, nonlinear optical changes in the bandgap edges are observed. From analysis of the dispersion relation, Kerr nonlinear coefficients of nematics, forming the cholesteric liquid crystal, are found to be enhanced by 1-2 orders of magnitude through the photonic bandgap structure.