负性向列相液晶电致缺陷的产生与湮灭过程

取向有序的液晶材料具有丰富的物理各向异性、外场响应性、物理效应,催生了新一代的光电应用.利用电场可在液晶中产生拓扑缺陷.缺陷动态过程受材料自身特性和外界条件的影响尚未明晰.本文选用介电各向异性△ε在-1.1到-11.5之间的7种向列相液晶材料,通过施加线性增加的交流电场,研究了负性向列相液晶电致脐点缺陷产生到湮灭过程中材料特性(△ε)和外界条件(温度、外加电场参数)对脐点缺陷的标度规律及湮灭快慢的影响.结果表明:在不同的△ε、温度和电场频率下,缺陷产生过程均满足Kibble-Zurek机制,即缺陷密度与电场变化率之间存在标度关系,且标度指数约为1/2;温度越高,产生缺陷密度越大;△ε越强或电场变化越快,缺陷湮灭速度越快.本文的研究厘清了拓扑缺陷产生湮灭与材料特性和外界条件的依赖关系,有利于对软物质中拓扑缺陷动态过程的认识和理解.     

向列相液晶中强非局域空间光孤子传输的理论研究

对向列相液晶中非局域空间孤子的传输进行了理论研究.基于非线性液晶孤子传输方程，采用Gauss形式的试探解，不仅得到了空间孤子的解析解,而且还在临界功率附近得到了呼吸子的解析解.通过数值模拟证明我们的结果比Conti和Assanto等人的结果更合理.同时，对液晶中的非局域孤子模型和Snyder等提出的强非局域孤子模型进行了全面的比较. 关键词： 向列相液晶 空间孤子 非局域非线性 呼吸子     

竞争非局域非线性向列相液晶中亮孤子

针对热效应的非局域程度和分子取向效应的非局域程度不等的情况,利用JUNG P S等提出的模型研究了具有竞争非局域非线性效应的向列相液晶中的亮孤子特性.通过变分法得到光束振幅、宽度、啁啾和位相等参数随传输距离的变化规律,即光束振幅和宽度与入射光的功率息息相关,光束宽度随热非局域程度增加单调递减,随热非线性系数增加单调递增.在此基础上给出和入射光束功率密切相关的势能函数.利用势能函数预言了入射光束功率越大亮孤子宽度越小.通过解变分方程组可得,入射光功率为4或8时,光束宽度均随传输距离周期性震荡.这两个震荡趋势有明显的区别,功率为4时光束传输过程中宽度始终大于或等于初始宽度,功率为8时光束传输过程中宽度始终小于或等于初始宽度.该变化规律与数值仿真结论一致.     

各向异性介质中的椭圆空间光孤子特性

对矩形铅玻璃中椭圆孤子的形成进行了理论研究,在理论模型中引入各向异性衍射效应.采用变分法,得到了强非局域线性各向异性椭圆孤子的变分解.结果表明,各向异性衍射效应对椭圆孤子的形成有很大的影响.为了验证变分解的正确性,采用牛顿迭代法算出强非局域线性各向异性椭圆孤子的数值解,变分解和数值解符合得很好.     

负性向列相液晶中1+1维空间光孤子:微扰法

利用微扰法对负性向列相液晶中的1+1维空间光孤子进行了系统性的研究, 得到了近似解析孤子解.通过数值迭代, 发现只有当非局域程度大于某一临界值时孤子才能存在.近似解析解即使在一般非局域程度下和数值解也符合得较好. 为讨论孤子的稳定性, 进行了线性稳定性分析, 发现孤子均是稳定的, 数值模拟进一步证实了线性稳定性分析的结果.     

向列相液晶中强非局域空间光孤子的实验观察

通过实验对向列相液晶中的非局域空间光孤子的传输进行了研究。理论上基于非线性的液晶孤子传输方程,采用高斯形式的试探解,得到了孤子传输的解析解,以及对液晶中的非局域孤子和Snyder等提出的强非局域孤子模型的结果进行了比较。实验上,观察了空间光孤子在向列相液晶中的传输,找到了不同束宽下空间光孤子的临界功率。比如束宽为2μm时临界功率为2.0mW。观察了向列相液晶中空间光孤子的弛豫过程,并注意到液晶分子的响应时间长达几秒钟,液晶中的孤子形成速度较慢。     

非局域程度对向列相液晶中空间光孤子的影响

理论分析了非局域程度对向列相液晶中(1+2)维空间光孤子的影响.得到了非线性系数以及特征长度和预倾角的关系,并且给出了强非局域性的非线性薛定谔方程,最终得到了单孤子和临界功率的解析解.实验上,通过改变偏压从而间接测量了向列相液晶中的临界功率,并与数值模拟结果相结合与理论结果进行了比较,发现基本相符. 关键词： 空间光孤子 非线性系数 非局域程度 临界功率     

空间各向异性两体势向列相液晶形变研究

基于分子两体势研究向列相液晶的形变.该两体势是空间各向异性的并且依赖于液晶的弹性常数.理论处理中假定具有理想向列序，这意味着分子长轴取向方向与液晶指向矢是重合的，而总自由能等于总相互作用能.以解析形式研究了三种基本的Fréedericksz 转变并对混合排列向列相液晶盒中的指向矢分布进行了数值计算.检查了文献中最近提出的两种从弹性能到两体作用势的映射方案，发现只有一种方案给出的结果与连续体理论一致. 关键词： 空间各向异性两体势 理想向列序 液晶形变 Fréedericksz 转变     

向列相液晶中强非局域空间光孤子的相互作用

在数值和实验上研究了向列相液晶中空间光孤子的相互作用.向列相液晶中的响应函数为指数衰减函数，这不同于Snyder-Mitchell模型.数值模拟发现向列相液晶中的孤子相互作用仍然满足交叉点位置与输入功率的平方根成反比，强非局域下的Snyder-Mitchell模型交叉点位置与两束光的间距无关，而在向列相液晶中交叉点位置与两束光的间距有关；在实验上发现，向列相液晶中两束光交叉点的位置与输入功率平方根成反比，还和形成孤子的临界功率以及两束光之间的间距有关.     

向列相液晶中强非局域空间光孤子的相互作用

在数值和实验上研究了向列相液晶中空间光孤子的相互作用.向列相液晶中的响应函数为指数衰减函数,这不同于Snyder-Mitchell模型.数值模拟发现向列相液晶中的孤子相互作用仍然满足交叉点位置与输入功率的平方根成反比,强非局域下的Snyder-Mitchell模型交叉点位置与两束光的间距无关,而在向列相液晶中交叉点位置与两束光的间距有关;在实验上发现,向列相液晶中两束光交叉点的位置与输入功率平方根成反比,还和形成孤子的临界功率以及两束光之间的间距有关. 关键词： 强非局域非线性 向列相液晶 孤子相互作用 交叉点     

Temperature dependence of ratio between dielectric anisotropy and order parameter in fluorinated nematic liquid crystals

Temperature dependence of ratio between dielectric anisotropy and order parameter of fluorinated nematic liquid crystal is investigated by using a semi-empirical molecular orbital package that can accurately calculate an angle between molecular dipole moment and long axis. We optimize the molecular conformations with three semi-empirical Hamiltonians AM1, PM3 and PM5, and then make a comparison between computational results and experimental measurements. It is shown that the results obtained from AM1 method are in good agreement with the measurements. The present study offers an applicable method to predict the dielectric properties of liquid crystal material.     

Temperature dependence of ratio between dielectric anisotropy and order parameter in fluorinated nematic liquid crystals

Temperature dependence of ratio between dielectric anisotropy and order parameter of fluorinated nematic liquid crystal is investigated by using a semi-empirical molecular orbital package that can accurately calculate an angle between molecular dipole moment and long axis. We optimize the molecular conformations with three semi-empirical Hamiltonians AM1, PM3 and PM5, and then make a comparison between computational results and experimental measurements. It is shown that the results obtained from AM1 method are in good agreement with the measurements. The present study offers an applicable method to predict the dielectric properties of liquid crystal material.     

Calculating the dielectric anisotropy of nematic liquid crystals: a reinvestigation of the Maier——Meier theory

This paper investigates the average dielectric permittivity (\overline ε ) in the Maier--Meier theory for calculating the dielectric anisotropy (Δε) of nematic liquid crystals. For the reason that \overline ε of nematics has the same expression as the dielectric permittivity of the isotropic state, the Onsager equation for isotropic dielectric was used to calculate it. The computed \overline ε shows reasonable agreement with the results of the numerical methods used in the literature. Molecular parameters, such as the polarizability and its anisotropy, the dipole moment and its angle with the molecular long axis, were taken from semi-empirical quantum chemistry (MOCPAC/AM1) modeling. The calculated values of Δε according to the Maier--Meier equation are in good agreement with the experimental results for the investigated compounds having different core structures and polar substituents.     

向列相液晶中空间光孤子的观测

分别以He-Ne激光器、半导体激光器和Ar⁺激光器作为光源照射液晶材料，在低光强入射的情况下均观察到了光束的自聚焦现象以及空间光孤子的形成，观察到了长度l>5mm的孤子波和多模空间光孤子，并对空间孤子的产生条件及特性进行了讨论. 关键词： 自聚焦 空间光孤子 向列相液晶     

Discrete propagation and spatial solitons in nematic liquid crystals

We investigate, for the first time to our knowledge, the discrete propagation of near-infrared light in a voltage-controlled array of channel waveguides in undoped nematic liquid crystals under planar anchoring conditions. This novel geometry enables us to drive the system from one-dimensional bulk diffraction to discrete propagation and, for larger excitations, to discrete spatial solitons, or nematicons. The observed phenomena are adequately described by a scalar model that encompasses the voltage-dependent reorientational response of the material.     

Spectral investigation of nematic liquid crystals with high optical anisotropy at THz frequency range

Liquid crystals (LCs) with high optical anisotropy are very desirable for different applications in devices, such as filters, phase shifters, or phase gratings [T. Göbel, P. Meissner, A. Gaebler, M. Koeberle, S. Mueller, and R. Jakoby, Dual-Frequency Switching Liquid Crystal Based Tunable THz Filter, CLEO, Baltimore, MD, 2009; C.-Y. Chen, T.-R. Tsai, C.-L. Pan, and R.-P. Pan, Room temperature terahertz phase shifter based on magnetically controlled birefringence in liquid crystals, Appl. Phys. Lett. 83 (2003), pp. 4497–4499; and C.-J. Lin, C.-H. Lin, Y.-T. Li, R.-P. Pan, and C.-L. Pan, Electrically controlled liquid crystal phase grating for terahertz waves, IEEE Photon. Technol. Lett. 21 (2009), pp. 730–732]. We present spectral studies of LCs with large optical anisotropy in the range from 0.3 to 3?THz. Nematic LC mixtures which have Δn?>?0.30 for visible frequency range, i.e., 1825 (Δn?=?0.42 at 633?nm) were measured. Properties of LC materials like birefringence, absorption coefficients, and refractive indices for ordinary and extraordinary polarization in THz range were obtained. Orientation of LC was done by a high electric field. Measurements were performed using a TDS spectra 3000 spectrometer.