Tunable properties of light propagation in photonic liquid crystal fibers

Tunable properties of light propagation in photonic crystal fibers filled with liquid crystals, called photonic liquid crystal fibers (PLCFs) are presented. The propagation properties of PLCFs strongly depend on contrast between refractive indices of the solid core (pure silica glass) and liquid crystals (LCs) filing the holes of the fiber. Due to relatively strong thermo-optical effect, we can change the refractive index of the LC by changing its temperature. Numerical analysis of light propagation in PLCF, based on two simulation methods, such as finite difference (FD) and multipole method (MM) is presented. The numerical results obtained are in good agreement with our earlier experimental results presented elsewhere [1].     

Ultra-short pulse propagation in 3D GaAs photonic crystals

We have investigated ultra-short pulse propagation through 3D GaAs photonic crystals with a complete photonic band gap in the optical wavelength region. The pulse propagation was calculated by using the finite-differential time-domain (FDTD) method. This is the first time pulse shape measurements have been made using femtosecond pulses. From the experimental results, the shapes of the ultra-short pulses were found to change when the frequency was above the photonic band-gap after the propagation through the photonic crystal, corresponding to the simulation results.     

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.     

高功率激光装置中局部波前畸变的非线性传输

 根据高功率激光装置中强激光束传输的特点,建立了描述光束通过光学元件时引入的局部波前畸变模型,并利用纹波传输的线性化处理方法,研究了带有局部波前畸变的高强度光束的传输规律。以一个连续的非增益激光介质为例,用该模型进行数值模拟,给出了不同空间尺度的局部波前畸变的频谱分布、不同空间频率的纹波引起的振幅非线性增长曲线、不同B积分的非线性增益随光束传输距离的变化曲线,光束振幅非线性增益达到最大时光束的传输距离和标准传输距离不同时由局部波前畸变引入的振幅分布。研究表明为了防止光学元件损坏,应避免光学元件表面出现半径为0.5~2.0 mm的局部瑕疵。     

胶体晶体微结构光纤传输特性研究

介绍了胶体晶体微结构光纤的结构, 采用时域有限差分法研究了微结构胶体晶体光纤的传输特性. 建立了该结构的FDTD模型, 计算了其透射谱和不同波长的传输场分布. 搭建了光学系统, 测量了实验制备的胶体晶体微结构光纤的透射谱, 并将其与模拟所得透射谱进行比较, 具有良好的一致性. 胶体晶体微结构光纤的传输场分布结果表明, 胶体晶体在不同波长下改变光纤传输场分布.     

胶体晶体微结构光纤传输特性研究

介绍了胶体晶体微结构光纤的结构, 采用时域有限差分法研究了微结构胶体晶体光纤的传输特性. 建立了该结构的FDTD模型, 计算了其透射谱和不同波长的传输场分布. 搭建了光学系统, 测量了实验制备的胶体晶体微结构光纤的透射谱, 并将其与模拟所得透射谱进行比较, 具有良好的一致性. 胶体晶体微结构光纤的传输场分布结果表明, 胶体晶体在不同波长下改变光纤传输场分布.     

Lateral stress-induced propagation characteristics in photonic crystal fibres

Using the finite element method, this paper investigates lateral stress-induced propagation characteristics in a photonic crystal fibre of hexagonal symmetry. The results of simulation show the strong stress dependence of effective index of the fundamental guided mode, phase modal birefringence and confinement loss. It also finds that the contribution of the geometrical effect that is related only to deformation of the photonic crystal fibre and the stress-related contribution to phase modal birefringence and confinement loss are entirely different. Furthermore, polarization-dependent stress sensitivity of confinement loss is proposed in this paper.     

Proposal for a simple integrated optical ion-exchange waveguide polarizer with a liquid crystal overlay

A simple, compact electro-optic polarizer based on an ion-exchanged glass channel waveguide covered with a nematic liquid crystal (LC) is proposed. A full-vectorial beam propagation method is employed to simulate this device for the first time. For the cases of zero and strong LC surface anchoring, the performance of the proposed polarizer under different applied voltages is analyzed numerically. Analysis indicates that surface anchoring of the liquid crystal is a key issue influencing the performance for the proposed optical polarizer device.     

Free wave propagation in two-dimensional periodic plates

The propagation of flexural waves in a two-dimensional periodic plate which rests on an orthogonal array of equi-spaced simple line supports has been investigated. A type of plane wave motion has been considered. An energy method has been developed to predict the frequency of wave propagation in terms of the propagation constants. A Galerkin type of analysis has been used, incorporating assumed complex modes of wave motion for the identical rectangular elements of the periodic plate. Expressions for the frequency have been obtained firstly by using simple polynomial modes for the plate displacements, and then (alternatively) by using characteristics beam function modes. The use of these different modes has first been demonstrated by applying them to the analysis of wave propagation in periodic beams. A single polynomial mode which satisfies the geometric and wave-boundary conditions of the periodic plate element leads to an elegant expression relating the frequency and the wave propagation constants in the first propagation band. The frequencies so obtained compare well with those found from a multi-mode, characteristic beam function analysis. The latter involves much more algebra, is solved as an eigenvalue problem, and yields the frequencies in as many propagation bands as are desired. The bounding frequencies and corresponding wave motions in the first and higher propagation bands have been identified, and it has been shown that the propagation bands can overlap. Consideration has been given to one-dimensional “strip” structures which are equivalent to the two-dimensional plate when a plane wave in a general direction is propagating. Furthermore, it is shown that the natural frequencies of finite rectangular periodic plates can be obtained very simply from the results of the wave propagation analysis.     

Observations of defect propagation in [100]-oriented opal-type photonic crystals

Charged colloidal suspensions have been used as experimental models for the study of crystal nucleation. Here we propose that the technique of template-assisted colloidal self-assembly can be used to visualize the effects of defect propagation in atomic crystal films produced using epitaxial growth. Templates with periodic line defects were used to grow [100]-oriented three-dimensional photonic crystals by means of the template-assisted colloidal self-assembly method, aided by capillary and gravitational forces. The defect propagation in the [100]-oriented photonic crystal was observed using scanning electron microscopy, both at the surface of the crystal and on cleaved facets. This method is useful in the understanding of defect propagation in the growth of colloidal films on templates - and the same approach may also prove useful for the understanding of atomic crystal growth on substrates with defects. Additionally, the deliberate incorporation of line defects may prove valuable as a way of introducing waveguide channels into three-dimensional photonic crystals.     

Computational simulation of wave propagation problems in infinite domains

This paper deals with the computational simulation of both scalar wave and vector wave propagation problems in infinite domains. Due to its advantages in simulating complicated geometry and complex material properties, the finite element method is used to simulate the near field of a wave propagation problem involving an infinite domain. To avoid wave reflection and refraction at the common boundary between the near field and the far field of an infinite domain, we have to use some special treatments to this boundary. For a wave radiation problem, a wave absorbing boundary can be applied to the common boundary between the near field and the far field of an infinite domain, while for a wave scattering problem, the dynamic infinite element can be used to propagate the incident wave from the near field to the far field of the infinite domain. For the sake of illustrating how these two different approaches are used to simulate the effect of the far field, a mathematical expression for a wave absorbing boundary of high-order accuracy is derived from a two-dimensional scalar wave radiation problem in an infinite domain, while the detailed mathematical formulation of the dynamic infinite element is derived from a two-dimensional vector wave scattering problem in an infinite domain. Finally, the coupled method of finite elements and dynamic infinite elements is used to investigate the effects of topographical conditions on the free field motion along the surface of a canyon.     

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.     

晶体相场法研究应力状态及晶体取向对微裂纹尖端扩展行为的影响

采用晶体相场模拟研究了单向拉伸作用下初始应力状态、晶体取向角度对单晶材料内部微裂纹尖端扩展行为的影响, 以(111)晶面上的预制中心裂纹为研究对象探讨了微裂纹尖端扩展行为的纳观机理, 结果表明: 微裂纹的扩展行为主要发生在<011>(111)滑移系上, 扩展行为与扩展方向与材料所处的初始应力状态及晶体取向紧密相关. 预拉伸应力状态将首先诱发微裂纹尖端生成滑移位错, 进而导致晶面解理而实现微裂纹尖端沿[011]晶向扩展, 扩展到一定程度后由于位错塞积, 应力集中, 使裂纹扩展方向沿另一滑移方向[101], 并形成锯齿形边缘; 预剪切应力状态下, 微裂纹尖端首先在[101]晶向解理扩展, 并诱发位错产生, 形成空洞聚集型长大的二次裂纹, 形成了明显的剪切带; 预偏变形状态下微裂纹尖端则直接以晶面解理形式[101]在上进行扩展, 直至断裂失效; 微裂纹尖端扩展行为随晶体取向不同而不同, 较小的取向角度会在裂纹尖端形成滑移位错, 诱发空位而形成二次裂纹, 而较大的取向角下的裂纹尖端则以直接解理扩展为主, 扩展方向与拉伸方向几近垂直.     

Electrically controlled waveguide mode in LC layer for fiber optic applications

A new electrically controlled waveguide mode for optical beams propagating in the plane of a liquid crystal (LC) layer is described. The mode is based on the total internal reflection of light beams from two parallel boundaries, which separates the regions of a liquid crystal cell with different director orientations and refractive indices. In experiments electric field was applied to the initially planar layer to produce the channel with a homeotropic orientation surrounded by the planar regions. It results in focusing and waveguide propagation of a light beam emitted by an optical fiber inserted in a liquid crystal cell. Scattering of light by thermal LC director fluctuations is used to visualize light beams propagation. The obtained waveguide mode can be used for an elaboration of electrically controlled LC switches with low operating voltages. The prototype of such device made with usage of a photolithography and a photo alignment technique is described. The possible ways of optimization of LC-switch are considered.     

Resolution improvement of surface plasmon-enhanced, liquid crystal spatial light modulator: Simulation studies

Spatial resolution is an important performance characteristic of spatial light modulators (SLM). One of the key factors affecting the spatial resolution of liquid crystal (LC)-based SLM is the fringing field effect. This effect can be reduced in thin LC cells with corresponding reduction in the electro-optical response. A strong electro-optic response in thin LC layer can be attained using the surface plasmon resonance (SPR) phenomenon. While SPR-based LC SLMs were already demonstrated about 15 years ago, their development has been hampered in part by low resolution, due to the finite propagation length of the surface plasmons (SPs). A fine patterning of the metal layer supporting the propagation of SPs is studied as a possible solution for reducing the spatial blurring associated with the long propagation length of SPs. The results of detailed computer simulations showing improved resolution SPR-LC-SLM are presented.     

Steering the propagation of photovoltaic solitons by spatial phase modulation

A simple scheme is presented to steer the propagation of a spatial photovoltaic soliton in a photovoltaic crystal. The basic idea is to impose a sinusoidal phase modulation on the light beam before entering the crystal. It is found that the self-deflection, soliton-like propagation, annihilation and splitting of this spatial-phase-modulated (SPM) beam can be realized by choosing appropriate modulation parameters; besides, a SPM beam can produce a waveguide with strong stability to steer a non-phase-modulated beam. These unique properties would have potential applications in the optical switch, splitter, modulator, and waveguide, etc.     

Improved method for studying the propagation dynamics of ultrafast electron pulses based on mean-field models

We have studied the propagation dynamics of ultrafast electron pulses by using an improved mean-field model, in which the propagation of the electron pulses across the boundary of the acceleration region is explicitly considered. A large decrease in the speed spread of the electron pulses(we called "boundary kick") is observed and properly treated leading to a significant improvement in the simulation accuracy, particularly when the density of electrons is very large. We show that our method is consistent with the simulation by the N-particle method, while others can introduce factorial error.     

Real-time quantitative optical method to study temperature dependence of crack propagation process in colloidal photonic crystal film

A real-time quantitative optical method to characterize crack propagation in colloidal photonic crystal film(CPCF)is developed based on particle deformation models and previous real-time crack observations. The crack propagation process and temperature dependence of the crack propagation rate in CPCF are investigated. By this method, the crack propagation rate is found to slow down gradually to zero when cracks become more numerous and dense. Meanwhile, with the temperature increasing, the crack propagation rate constant decreases. The negative temperature dependence of the crack propagation rate is due to the increase of van der Waals attraction, which finally results in the decrease of resultant force. The findings provide new insight into the crack propagation process in CPCF.     

JEMS-FDTD软件在电磁脉冲区域传播数值模拟中的应用

电磁脉冲区域传播数值模拟是电磁环境效应分析的重要环节, 面临空间尺度巨大、多辐射源、复杂地貌等技术挑战。本文介绍了三维时域全波电磁模拟并行软件JEMS-FDTD的研制进展。在大规模并行计算FDTD方法的基础上结合自适应网格技术, 研制了一种适应于大区域电磁脉冲传播的高效时域全波计算方法。并基于该技术实现了千km2级城市电磁脉冲区域传播的数值模拟, 获取了全空间的时域电磁场信息, 验证了软件在电磁脉冲区域传播仿真应用中的可行性。     

双轴棱锥产生长距离近似无衍射光的新技术

提出了一种利用会聚透镜、轴棱锥等简单光学元件产生长距离近似无衍射光的新技术. 分别利用几何光学和衍射理论分析了该方法产生长距离近似无衍射光束的原理, 通过软件模拟了长距离近似无衍射光束的形成过程,得出了该光束在不同距离处的横向光强分布. 模拟结果显示该光束在较长距离处的横向光强分布满足Bessel分布. 从实验上获得了传播距离长达80 m、中心光斑发散角约为0.12 mrad的近似无衍射光束, 相比于国外学者最近的研究成果(Belyi et al. 2010 Opt. Exp. 18 1966)将传播距离延长了50多米,而光束发散角压缩了22倍.实验中, 对光束沿光轴传播时在不同距离处的光斑进行了拍摄,所得实验结果与理论分析基本符合.