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.     

Grating coupled interferometry for optical sensing

An interferometric sensor based on gratings on a planar waveguide is introduced. The device combines the advantages of known interference-based waveguide sensors with the simplicity of grating couplers. In the presented configuration, two parallel and coherent light beams, laterally separated in the direction of mode propagation, are coupled into a planar waveguide through a grating. One of the coupled beams is phase modulated using a periodically relaxing liquid crystal modulator, resulting in a time varying intensity signal at the end face of the waveguide. Refractive index changes within the waveguide section between the two coupling regions are monitored by observing characteristic changes in the intensity signal.     

Light propagation in periodic photonic structures formed by photo-orientation and photo-polymerization of nematic liquid crystals

Propagation of linearly polarized light beams in a nematic liquid crystal cell with distinguished regions of different molecular orientation has been analyzed. Specifically, combination of the planar/homogenic and homeotropic alignment, forming thus spatially limited regions characterized by a different LC molecular orientation, has been tested, as achieved by means of the photo-orientation and photo-polymerization processes, independently. An influence of molecular orientation on the light beam propagation has been checked for different directions of the linear polarization. Thanks to the molecular reorientation induced by the low frequency external electric field and also to the reorientational nonlinearity taking place in NLCs, propagation direction of the light beam can be additionally controlled by the electric bias and/or optical power, respectively. Proposed structural solutions and techniques, related to the photo-orientation and photo-polymerization processes described in this communication, give rise to the novel LC geometries and structures. The latter act as promising candidates for new practical photonic applications as they are expected to be of a particular importance for integrated optic elements and devices.     

基于光子晶体马赫-曾德尔干涉仪的太赫兹开关及强度调制器

通过对二维正方晶格光子晶体线缺陷模色散曲线慢光特性的研究, 利用外电场对液晶分子取向的调控作用, 在填充液晶的正方晶格波导的马赫-曾德尔干涉仪结构中实现了开关和强度调制等功能. 利用平面波展开法计算了光子晶体波导的线缺陷模, 分析了液晶折射率的变化对缺陷模的影响. 计算表明, 液晶折射率仅改变了0.1, 线缺陷模有效折射率改变达0.168, 该特性可以更为有效地实现对相位的控制, 进而实现高消光比开关和强度调制功能, 这种高效的相位调节器件在集成光系统中将有很好的应用前景.     

金属包层对称平面单轴晶体波导的模式场(Ⅱ)

对于晶体光轴平行于波导界面的结构,讨论了光在金属包层对称平面单轴晶体波导(波导层是单轴晶体,两个波导界面均为金属)内的传输特性.解析地得到了这种结构下波导模式场的精确解.模式场的性质因单轴晶体的性质不同而异.对于正单轴晶体,波导的主模是横电波,任何频率的光波均可激励该模式;当频率满足一定条件时,波导内传输单模,否则,将激励起高阶模式.高阶模既非TE波,也非TM波,而是两者耦合而成的混合模.对于负单轴晶体,波导的主模是一种混合模,该模式同样可被任何频率的光波所激励;当频率满足一定条件时,波导内传输单模,否则 关键词： 平面金属波导 单轴晶体 模式场 混合模     

Modeling light propagation in liquid crystal devices with a 3-D full-vector finite-element beam propagation method

A full-vector finite-element beam propagation method in 3-D is introduced for the simulation of light propagation in liquid crystal (LC) devices. The three electric field components are expressed in terms of mixed finite elements, providing the correct enforcement of boundary conditions. Moreover, the optical dielectric tensor of the medium can have all its nine elements nonzero, thus allowing the LC director to have an arbitrary orientation. A photonic crystal fiber with a LC infiltrated core and a homeotropic to multi-domain cell are analyzed. Comparison with other existing simulation techniques is provided, in order to validate the accuracy of the proposed method.     

Electrically switchable finite energy Airy beams generated by a liquid crystal cell with patterned electrode

A pair of electrically switchable finite energy Airy beams is generated by a liquid crystal (LC) cell, where one electrode is patterned by a photomask with a binary-phase pattern. Applying voltage across the LC cell, an index modulation is produced due to the liquid crystal molecules realignment, and the finite energy Airy beams can be generated or erased corresponding to the phase difference between the regions with and without electrode. The diffraction-free and transverse acceleration dynamics of this binary-phase element based finite energy Airy beams were experimentally verified, exhibiting excellent agreement with the theory.     

对称平面单轴晶体金属波导的模式场(1)

由于平面各向异性介质金属波导的重要应用,本文讨论了光在对称平面单轴晶体金属波导(波导层是单轴晶体,两个波导界面均为金属)内的传输特性.对于晶体光轴位于波导界面法方向与传输方向构成的平面内的特殊波导结构,利用麦克斯韦方程组并结合单轴晶体的性质,精确解出了波导模式场.结果表明:1)此种特殊结构波导内存在横电波(TE)和横磁波(TM);2)TM波的坡印廷矢量与波矢方向不在一条直线上;3)波导层为负单轴晶体时波导主模是TE波主模,而波导层为正单轴晶体时波导主模是TM波主模.     

有限宽平板波导中的克尔空间孤子及全光器件的设计

利用数值模拟的方法研究了倾斜入射的傍轴光束在有限宽非线性平板波导内的传输规律.研究发现,当不同波长相同束宽的光束各以临界功率倾斜入射波导时,如果倾斜角、入射点都一样,它们将沿大致相同的周期性的Z字形路径传输;如果光束功率进一步增大,相邻反射点之间的间距随传输距离有逐渐变大的趋势,Z字形路径不再有严格的周期性;当功率相当大时,光束将沿波导z方向传输,不再在波导的两个边界之间来回反射.利用倾斜入射光束在波导内的传输路径随功率而变的特点,设计了一个功率开关和一个光时分解复用器. 关键词： 空间光孤子 有限宽非线性平板波导 全光器件     

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。     

溶胶-凝胶法制备SiO2-TiO2平板光波导工艺研究

采用溶胶-凝胶法制备了SiO₂-TiO₂平板光波导,计算了平板光波导通光条件,分析了硅/钛溶胶-凝胶材料的热性能,观测了平板光波导的结构形貌,并测试了其通光损耗。结果表明：经过200℃,30 min干燥处理的凝胶薄膜呈疏松多孔状态,对于非对称平板波导,存在芯层通光截止厚度,而且当SiO₂-TiO₂芯层厚度为0.5 μm时,SiO₂下包层厚度至少有6 μm才能防止1550 nm波长光泄露入单晶硅衬底中。制备的光波导对于1550 nm波长光传输损耗最小值为0.34 dB/cm。     

Planar waveguides in neodymium-doped calcium niobium gallium garnet crystals produced by proton implantation

In this work, the fabrication and optical properties of a planar waveguide in a neodymium-doped calcium niobium gallium garnet(Nd:CNGG) crystal are reported. The waveguide is produced by proton(H~+) implantation at 480 ke V and a fluence of 1.0×10~(17) ions/cm~2. The prism-coupling measurement is performed to obtain the dark mode of the waveguide at a wavelength of 632.8nm. The reflectivity calculation method(RCM) is used to reconstruct the refractive index profile. The finite-difference beam propagation method(FD-BPM) is employed to calculate the guided mode profile of the waveguide.The stopping and range of ions in matter 2010(SRIM 2010) code is used to simulate the damage profile induced by the ion implantation. The experimental and theoretical results indicate that the waveguide can confine the light propagation.     

Simple optical modeling of polarized fluorescent liquid crystal cells

We investigate the fluorescence emission properties of dye-doped nematic liquid crystal (LC) cells with planar and zigzag emissive layers. To explain the measured angular dependence of the emitted light (emission pattern), we introduce a simple optical model, in which both planar and zigzag emissive layers are approximated as a homogeneously oriented flat LC layer with an equivalent anisotropy. It was revealed that all the measured emission patterns under different LC molecular orientation conditions agree well with the optical model, whereas the conventional Lambertian-based distribution function does not coincide with the measured profile in homeotropically aligned LC cells.     

平面光波导光场分布的有限时域差分法计算

用有限时域差分法计算了简单三层对称平面光波导中的光场,得到了任意截面上光场的分布.这一方法的优点在于它能完全模拟波导中光场的时域特性,使波导中的光传播完全可视化,模拟横向传播,提取光在波导中的传输模式,也可用于有源波导的计算如光波导放大器.有限时域差分法直接求解依赖时间的麦克斯韦旋度方程,利用二阶精度的中心差分近似,把旋度方程中的微分符直接转换为差分形式.每一步计算都无须作矩阵求逆运算,因此,比其他数值计算方法更有效、精度更高.     

Technological aspects in the design of a planar X-ray waveguide resonator

The waveguide-resonance mechanism of X-ray beam propagation and a design for a planar waveguide resonator based on this mechanism are briefly described. Principal attention is concentrated on estimating the admissible nonparallelism of reflectors in the waveguide-resonance channel and on revealing its effects. Practical recommendations are given for adjusting the waveguide resonator used in the X-ray optical scheme of diffractometers. The parameters of beams formed by waveguide resonators and cutoff-slit systems are compared.     

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.     

Electrically controlled local Frédericksz transition in a layer of a nematic liquid crystal

A local Frédericksz transition caused by electrically controlled surface anchoring in a nematic liquid crystal has been implemented owing to the displacement of surface-active ions. In the initial case, the nematic is homeotropically aligned by monomolecular layers of cetyltrimethylammonium bromide cations adsorbed on the surface of substrates. One of the substrates in a static electric field becomes free of a cation layer and specifies planar boundary conditions; as a result, a homeoplanar orientation structure is formed. The features of the dynamics of the optical response of the cell under study that are characteristic of the effect under consideration are discussed.     

Light-controlled helical pitch and dynamic gratings

A chiral nematic liquid crystal (LC) with a photo-sensitive chiral agent is employed for a light-controlled modulation of optical activity and used as a model substance for a dynamic diffraction grating recording. The described liquid crystalline system has shown a strong nonlinear response with effective parameter of cubic nonlinearity being much greater of that characteristic of the orientational nonlinearity of LC. A simple mathematical model of light diffraction on the grating of modulated optical activity was developed. Calculated values of intensities and polarisation states of diffracting beams have shown very good agreement with the experimental data. LC systems with light-controlled chirality could be promising media for nonlinear optical applications or all-optical switching devices.     

One-dimensional photonic crystals with a planar oriented nematic layer: Temperature and angular dependence of the spectra of defect modes

Transmission spectra of a one-dimensional photonic crystal (PC) formed by two multilayer dielectric mirrors and a planar oriented layer of 5CB nematic liquid crystal (LC) that is sandwiched between these mirrors and serves as a structure defect are investigated experimentally. Specific features of the behavior of the spectrum of defect modes as a function of the angle of incidence of light on the crystal are studied for two polarizations: parallel and perpendicular to the director of the LC; the director either lies in the plane of incidence or is perpendicular to it. It is shown that, for the configurations considered, the maxima of the defect modes shift toward the short-wavelength region as the tilt angle of incidence radiation increases; this tendency is more manifest for the parallel-polarized component, when the director lies in the plane of incidence. In the latter case, the width of the photonic band gap (PBG) appreciably decreases. The temperature dependence of the polarization components of the transmission spectra of a PC is investigated in the case of normal incidence of light. The spectral shift of defect modes due to the variation of the refractive index of the LC at the nematic-isotropic liquid phase transition point is measured. It is shown that, in real PCs, the amplitude of defect modes decreases when approaching the center of the band gap, as well as when the number of layers in the dielectric mirrors increases. Theoretical transmission spectra of the PCs calculated by the method of recurrence relations with regard to the decay of defect modes are in good agreement with experimental data.     

Analytical model of light beam diffraction by one-dimensional electrically controlled non-uniform transmission photon PDLC structures

A mathematical model describing two-dimensional diffraction of quasi-monochromatic light beams by onedimensional electrically controlled non-uniform holographic photon structures in photopolymer dispersed liquid crystal (PDLC) materials is presented. The model describes light field distributions in the near and far field zones with allowance for the refractive index profile in the structure, light beam divergence, and encapsulation of liquid crystal molecules.