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.     

Selective reflection of light from a layer of cholesteric liquid crystal with induced inhomogeneity of helix pitch

A possibility to induce a planar defect in a plane-layer structure of a cholesteric liquid crystal via an external electric field is shown experimentally. The results of studies of behavior of the selective reflection (transmission) coefficient of light for a layer of the cholesteric liquid crystal, where planar defects are induced, are presented     

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.     

Estimation of the relaxation parameters of a cholesteric liquid by the parameters of acoustic streaming

The possibility of determining the relaxation time of cholesteric liquid crystals with a large helix pitch from the parameters of acoustic streaming is demonstrated experimentally.     

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.     

Absolute backscatter coefficient estimates of tissue-mimicking phantoms in the 5-50 MHz frequency range

Absolute backscatter coefficients in tissue-mimicking phantoms were experimentally determined in the 5-50 MHz frequency range using a broadband technique. A focused broadband transducer from a commercial research system, the VisualSonics Vevo 770, was used with two tissue-mimicking phantoms. The phantoms differed regarding the thin layers covering their surfaces to prevent desiccation and regarding glass bead concentrations and diameter distributions. Ultrasound scanning of these phantoms was performed through the thin layer. To avoid signal saturation, the power spectra obtained from the backscattered radio frequency signals were calibrated by using the signal from a liquid planar reflector, a water-brominated hydrocarbon interface with acoustic impedance close to that of water. Experimental values of absolute backscatter coefficients were compared with those predicted by the Faran scattering model over the frequency range 5-50 MHz. The mean percent difference and standard deviation was 54%?±?45% for the phantom with a mean glass bead diameter of 5.40 μm and was 47%?±?28% for the phantom with 5.16 μm mean diameter beads.     

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.     

Piezoelectric effect in liquid crystals

A piezoelectric effect has been observed in a cholesteric liquid crystal layer (an electric field effect type, a mixture of cholesteryl oleyl carbonate, cholesteryl chloride and cholesteryl nonanoate) [1]. The layer, subjected to shear vibration, generates an alternating electric potential of the same frequency as the exciting vibration. In the present paper an extension of these investigations is reported to cases with various mixing ratios and also to one of smectic liquid crystals (lecithin with water). For a cholesteric liquid crystal of a particular mixing ratio, the strength of the electric generation is found to reach as high as 450 mV(p-p) for a vibratory shear displacement of 1 μm(p-p) under a certain molecular orientation preparation. Small electric generation is also observed for the above smectic liquid crystal but not for the nematic liquid crystals (MBBA and EBBA). A simple continuum elasticity model can qualitatively explain the behaviour of this electric potential generation.     

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.     

Acoustic study of adsorbed liquid layers

Interference measurements of small variations in the velocity and attenuation of surface acoustic waves (SAWs) are used to investigate water layers up to 15 nm thick adsorbed on the surface of a lithium niobate crystal. The frequency dependence of the relative variation of the SAW velocity with the adsorption of water vapor is measured in the range from 40 to 400 MHz. Acoustic techniques are used to experimentally estimate the frequency dependence of the dielectric constant of adsorbed water and its dipole relaxation frequency along with the dependence of the adsorption layer thickness on the water vapor pressure in the surrounding medium. A simple expression is proposed for calculating the dispersion of the SAW velocity in a solid loaded with a thin liquid layer.     

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.     

楔形光纤与半导体多量子阱平面光波光路芯片的耦合分析

采用楔形光纤(WSF)实现了与半导体多量子阱(MQW)平面光波光路(PLC)芯片的高效耦合。在多量子阱-平面光波光路前置模斑转换器(SSC)和不加模斑转换器的情况下,用阶梯串联法(SCM)数值模拟并优化设计了楔形光纤-平面光波光路间最佳耦合参量:楔形光纤楔角45°、端面圆柱透镜曲率半径2.5μm、模斑转换器-多量子阱-平面光波光路出射椭圆光斑长半轴3.5μm、纵横比5、楔形光纤-平面光波光路间垂直方向和水平方向无偏移、纵向间距5.5μm。用反向推演法(IDM)实验分析了楔形光纤样品的出射光场,与阶梯串联法(SCM)计算结果相比长轴误差为3.125%,短轴误差为0.8%。建立楔形光纤-平面光波光路-单模光纤(SMF)的耦合实验系统,在1.55μm波长处以单模光纤作为出纤的相同条件下,发现楔形光纤激励入射平面光波光路比单模光纤和锥形透镜光纤(TLF)作为入纤的耦合效率分别提高了24.827 dB和16.22 dB,为多量子阱-平面光波光路芯片尾纤封装技术提供了实验原型。     

A vortex mechanism for the formation of two-dimensional domains in cholesteric liquid crystals under the effect of ultrasound

An experimental verification is presented for the new theoretical model of a vortex mechanism that explains the formation of two-dimensional domains in planar layers of cholesteric liquid crystals under the effect of ultrasound in the range of frequencies not exceeding the inverse relaxation time of the orientational order parameter. The limits of applicability of this model are determined.     

Induction of selective reflection bands by a pulsed electric field in layers of chiral liquid crystals

The effect of the induction and “triplet” splitting of intense selective reflection bands has been experimentally revealed in chiral liquid crystals in a pulsed electric field. The effect is associated with the strong deformation of the helicoid of a chiral liquid crystal in the pulsed field and with the conservation of its step even at field amplitudes exceeding the critical field of the untwisting of the cholesteric helix. Intense polarization-sensitive light reflection bands appear in the spectral ranges where higher orders of selective reflection are forbidden for the unperturbed chiral liquid crystal.     

Specific features of photoisomerization of provitamin D 3 in a nematic liquid crystal

Photoisomerization of provitamin D ₃ (7-dehydrocholesterol) in a nematic liquid crystal (ZLI-1695, Merck) is investigated in detail by UV absorption spectroscopy. It is found that dissolution of chiral molecules of provitamin D ₃ induces the cholesteric phase in a nematic. The spectral kinetics of photoisomerization in this phase changes significantly from that in an ethanol solution. A sharp nonmonotonic dependence of the increase in accumulation of trans isomer tachysterol in a liquid crystal matrix with a decrease in the induced cholesteric pitch from 2200 to 25 μm is revealed.     

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。     

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     

Lasing in liquid crystal thin films

A lasing condition is formulated in matrix form for optically anisotropic thin films. Lasing behavior of liquid-crystal slabs is analyzed. In particular, it is shown that if the spatial extent of a liquid crystal slab is much larger than its thickness, then laser emission is feasible not only along the normal to the slab, but also in the entire angular sector. The generated laser light can be observed experimentally as a spot or as concentric rings on a screen. The lowest lasing threshold corresponds to in-plane sliding modes leaking into the substrate. The feedback required for lasing is provided by reflection from the interfaces, rather than edges, of the liquid-crystal slab operating as a planar Fabry-Perot cavity. For cholesteric liquid crystals, it is shown that energy loss to the sliding modes leaking into the substrates and escaping through their edges is a key factor that limits the efficiency of band-edge emission along the normal to the slab.