液晶电控双折射偏光显微镜观测实验

介绍液晶电控双折射知识,给出在偏光显微镜下观测液晶盒的颜色随外加电压变化的实验。该实验对丰富大学物理实验内容,拓宽学生知识具有一定的意义。     

液晶全漏导模透射率的实验研究

实验研究了由棱镜、折射率匹配液及扭曲排列和垂直排列向列相液晶盒构成的液晶全漏导模的透射率。随着施加在液晶盒上的电压不同,得到了依赖于角度变化的透射率（包括偏振保存和偏振转换）的实验曲线。这些曲线由液晶指向矢在液晶盒中的分布直接控制,由此可以证实液晶全漏导模透射率同样可以用来探索液晶盒内部的细微变化。     

气体脉冲放电等离子体阻抗的参量研究

在纯净氦气条件下，对高频纵向脉冲放电的等离子体动态阻抗进行了实验研究.测量了放电等离子体阻抗随放电电压、频率以及气体压强之间的变化关系，表明等离子体阻抗随频率和电压的增加而下降，随气压的升高而增大.定性分析了这些宏观参量与等离子体阻抗变化之间的关系，拟合出等离子体阻抗随电压、频率、气压变化的经验公式，并对数值模拟与实验结果进行了比较研究. 关键词： 等离子体阻抗 气体脉冲放电     

液晶磁控偏光特性的研究

利用偏光干涉理论,通过对BL-009型向列相液晶透射比的测试,分析了液晶透射比随磁场的变化情况,并对液晶的磁控双折射效应进行了研究.实验在室温20℃下用JG-3型连续可调磁场仪对液晶盒施加垂直于其表面的磁场,用CT5A型特斯拉计准确读出磁场强度数值,使液晶盒光轴方向与起偏镜和检偏镜偏振方向成45°,分别测出了起偏镜和检偏镜偏振方向平行和垂直时的透射光强度.通过数学函数拟合,得出了液晶的双折射率随磁场的变化规律,即：当磁场强度大于液晶的阈值磁场时,拟合函数能很好地描述液晶磁控双折射率的变化规律.     

连续变焦的聚合物分散液晶电控透镜

介绍了一种基于聚合物分散液晶材料的连续调焦电控透镜.在聚合物分散液晶盒的上表面电极上刻蚀圆孔,形成一个非对称电极,在液晶盒上下极板之间,诱发一个非均匀电场,从而引起聚合物分散液晶材料的折射率非均匀分布,形成电控变焦透镜.阐述了聚合物分散液晶可调焦透镜的基本原理,分析了透镜孔径对聚合物分散液晶透镜焦距的影响,在直径3mm和6mm的圆孔条件下,分别测量了透镜焦距随电压的变化关系.结果表明:电压从50V加到170V的过程中,透镜焦距逐渐减短,刻蚀3mm圆孔的聚合物分散液晶盒焦距从1.361 63m到0.429 21m,刻蚀6mm圆孔的聚合物分散液晶盒焦距从1.769 92m到0.548 43m.     

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。     

全漏光导波技术确定液晶材料的挠曲电系数

利用全漏光导波技术,测量负性向列相液晶材料MS-N01300-000的展曲和弯曲挠曲电系数之和.根据液晶弹性和多层光学理论得到同一电压下不同挠曲电系数的反射率随内角(入射到液晶层角度)的理论变化曲线.实验中,将液晶材料灌注到混合排列向列相液晶盒中,在液晶盒上分别加交流电和直流电,采用全漏光导波技术得到反映液晶指向矢分布的液晶波导反射率随内角变化曲线.实验发现,由于液晶挠曲电作用,同一电压值下反射率随内角变化曲线发生微小移动.对比理论与实验曲线,由曲线移动距离得到MS-N01300-000液晶材料展曲和弯曲挠曲电系数之和为2.5×10-11 C/m.     

胆甾相液晶电控螺旋畸变导致的布拉格反射特性

研究了平面织构胆甾相液晶在垂直于螺旋轴方向施加电场后,引起螺旋畸变导致布拉格反射光强度变化特性,利用液晶矩阵光学,琼斯矩阵法与Matlab矩阵计算软件对有螺旋畸变情况下的布拉格反射光谱特性进行了分析和数值计算.从数值计算和实验结果可知,胆甾相液晶在其垂直螺旋轴方向施加不同电压电场时,布拉格反射光中心波长不随电压改变,而布拉格反射光强会随电压的增大而减弱.提出一种电致螺旋畸变模型,对实验结果和数值计算结果给出了合理的解释,认为利用电场改变胆甾相液晶螺距进而设计电控颜色变化液晶器件的原理是不可靠的.     

电控聚合物分散液晶变焦全息透镜制作

介绍了相位型全息聚合物分散液晶(PDLC)材料全息透镜,在电场作用下液晶微滴折射率逐渐与聚合物折射率匹配,实现透镜电控变焦。研究了微米尺寸和纳米尺寸液晶微滴聚合物分散液晶材料配方特性和微观结构。采用优化纳米尺寸材料配方制作5~6μm聚合物分散液晶盒,采用离轴式平面波和球面波干涉全息写入光路,成功制作电控变焦聚合物分散液晶全息透镜样品。该透镜样品焦距为20 mm,能够正一级衍射放大成像。实现“0”,“1”变焦的驱动电压阈值为60 V。并进一步提出了基于聚合物分散液晶电控变焦元件集成叠加技术实现电控变焦光学成像系统的技术思路。     

光诱导液晶中偶氮聚合物形成相光栅的研究

报道了一种新型偶氮聚合物掺杂液晶的复合体系.该体系中的偶氮聚合物在光诱导下发生顺反式异构形成与液晶分离的相光栅.通过建立多指数模型研究该相光栅在He-Ne光的辐照下折射率的变化情况,并且通过对聚合物掺杂液晶的样品加上连续变化的电压来研究其对电场的响应过程.同时研究了聚合物的单体掺杂液晶后随光场和电场的变化情况.结果表明,光诱导下聚合物掺杂液晶的样品折射率改变值经45s达到饱和,弛豫时间为10min;单体掺杂时,饱和时间与弛豫时间分别为0.17s和0.9s.外加电场在1.0V、1.4V、2.6V和4.0V时光栅的衍射效率会突然下降,而单体在变化的外电场下衍射无明显变化.     

太赫兹液晶材料与器件研究进展

液晶是一种性能优异的可调控光电功能材料,基于液晶的太赫兹器件有着广泛的应用前景,但高性能太赫兹功能器件的研发仍处于初级阶段.本文综述了太赫兹领域液晶材料与器件的研究现状,探讨了液晶技术与太赫兹技术相结合的发展趋势.     

New regions of diffraction reflection in apodized one-dimensional photonic crystals with a large modulation depth

The specific features of light transmission in a cholesteric liquid crystal (LC) cell with a director rotated by 90° have been investigated. In this structure, where a light wave is incident at a large angle with respect to the LC surface, the light is reflected (refracted) in the LC layer near the opposite boundary. It is shown that the application of an electric field changes the character of extraordinary wave refraction, as a result of which light starts passing through a cell. The transmission threshold voltage is determined, and its dependence on the angle of incidence of light is obtained. The dependence of the transmitted-light intensity on the voltage across the cell is obtained as well. The same dependences are also derived by numerical calculations with allowance for the turning points and extinction.     

Dielectric and electrooptical properties of hockey-stick-shaped liquid crystal with a negative dielectric anisotropy

We investigated dielectric properties of a hockey-stick-shaped liquid crystal (HLC). Two dielectric relaxation modes were observed at 0.91 kHz and 4.51 MHz. The low frequency relaxation modes in isotropic, smectic, and nematic phases are related to the motion of ions, collective tilt fluctuation, and rotation around the long molecular axis, respectively. Meanwhile, the high frequency relaxation modes in nematic and smectic phases was due to the rotation around the short axis of the molecules and hindered by the resistance of the ITO layers. We also examined the electrooptical response of the 5.0 wt% HLC-doped commercial nematic liquid crystal (LC) mixtures. The birefringence of the LC mixture was slightly increased, while the falling time and the rotational viscosity was decreased in the nematic phase.     

频率驱动液晶光阀的电光特性研究

讨论了液晶光阀频率驱动方式的原理, 并分析了液晶光阀在频率驱动方式下的电光特性。在特定的交流电压下改变外加电场频率, 测量了TB3639型液晶光阀在频率驱动方式下的电光特性关系曲线。测量结果表明,TB3639液晶光阀在可见光区域具有电光显示灰度变化特性,不仅能实现黑白显示,也能实现不同灰度级显示。频率驱动方式在可见光的范围内对不同波长的透射率变化趋势影响大致相同,在应用中可避免出现色差较大的现象。这种新型频率驱动方式对液晶器件的显示有改善作用,将开拓液晶显示器件的新应用, 具有实际应用价值。     

Electrically controlled switching of light beams in the plane of liquid crystal layer

A simple experimental method and a set-up to control light beams in the plane of liquid crystal (LC) cell is developed. The main idea of the method is the reflection and the refraction of light beams propagating in a plane of LC layer from the boundary, which separates the regions of a liquid crystal cell with different director orientations and refractive indices. This boundary can be produced by electric field applied to the LC layer with a planar homogeneous orientation. The disclination lines that separate the extremely sharp changes of LC orientation can be also used for this purpose. The possibility of discrete deflection of light beam from the initial direction is shown. A computer simulation confirmed that the proposed experimental geometry was most suitable to control intensity of extraordinary rays propagating in the plane of liquid crystal layer.     

Mode splitting in an optical slab waveguide filled with nematic liquid crystals

Mode splitting was observed when a He-Ne laser beam was reflected through a prism-coupled liquid-crystal slab waveguide under an applied electric field. The splitting manifests itself as the imposed voltage reaches a critical level, and diminishes when the voltage increases above a critical high value. If the applied voltage increases even further, mode splitting vanishes, attributing to the result that almost all the directors of the liquid crystal turn upright to the surface. The multi-layer matrix simulation can satisfactorily account for this phenomenon by exploiting the property of the anisotropic optical birefringence of a liquid crystal under applied voltages. Received: 2 December 2002 / Revised version: 24 January 2003 / Published online: 23 May 2003 RID="*" ID="*"Corresponding author. Fax: +886/3572-3052, E-mail: jtlue@phys.nthu.edu.tw     

Electro- and magneto-optical switching of defect modes in one- dimensional photonic crystals

The transmission spectra of polarized light waves in a photonic crystal/liquid crystal (PC/LC) cell placed between crossed polarizers and controlled by an electric or magnetic field have been studied experimentally and theoretically. Electro- and magneto-optical switching based on the interference of polarized defect modes has been demonstrated. The transmission spectra of the PC/LC cell have been calculated as a function of the voltage applied to the LC layer and the magnetic field strength. The results of the calculations agree well with the experimental data.     

Effective frequency-dependent anchoring coefficient and rotational viscosity of nematic liquid crystals

The effective rotational viscosity and polar anchoring coefficient of a nematic liquid crystal were determined using electro-optical response by changing the frequency of the electric field in the kilohertz range. Both quantities monotonically decreased with an increase in the frequency and converged to a certain value at several tens of kilohertz. The effective rotational viscosity changes more than the order of magnitude in experimental range. On the other hand, the effective value of anchoring coefficient changes several times. We tried to explain the trends with the effect of an electric field induced by the motion of ionic impurity in liquid crystal.     

Electric field effect on refraction of light in a layer of chiral liquid crystal with large pitch

The effect of an alternating electric field on the trajectory of an extraordinary light wave in a layer of a chiral liquid crystal with a 180° turn of the director is studied. In this structure, in the absence of the field and at a large angle of incidence of the light wave on the liquid crystal layer, the light undergoes refraction inside the layer. It is shown that the deformation of the director that arises under the action of the electric field changes the character of refraction of the extraordinary wave and the layer begins to transmit the light. The threshold voltage of this effect is determined. The dynamics of the effect is studied. At large voltages, in addition to the extraordinary wave, an ordinary light wave is observed in the light passed through the cell. The ordinary wave intensity is modulated by the initial frequency of the control signal, whereas the extraordinary wave intensity is modulated by the double initial frequency.