Characteristics analysis of hybrid plasmonic waveguide for low-loss lightwave guiding

It has been experimentally demonstrated that a low-loss guided hybrid mode is supported if a metal strip is embedded in a low index polymer layer surrounded by two high index slabs. In this paper, further numerical analyses on the guided hybrid modes are reported to fully elucidate the characteristics of the hybrid plasmonic waveguide. For a one-dimensional slab structure with a metal film of infinite width, simulation results exhibit that low-loss guided hybrid modes are associated with surface plasmon modes and dual dielectric slab modes. The optical properties of the guided modes are improved by increasing the field intensity which is confined into lossless dielectric layers by decreasing the metal film thickness and increasing the refractive index and thickness of the high-index slabs. The finite element method is used to investigate the lateral mode confinement of the optical guided modes by the corresponding metal strip. By reducing the metal film width, the guided modes are confined in the plane transverse to the direction of propagation and the characteristics are significantly improved. The hybrid plasmonic waveguide can be exploited for long-range propagation-based application such as optical interconnection.     

New developments in the multiscale hybrid energy density computational method

Further developments in the hybrid multiscale energy density method are proposed on the basis of our previous papers. The key points are as follows.(i) The theoretical method for the determination of the weight parameter in the energy coupling equation of transition region in multiscale model is given via constructing underdetermined equations.(ii)By applying the developed mathematical method, the weight parameters have been given and used to treat some problems in homogeneous charge density systems, which are directly related with multiscale science.(iii) A theoretical algorithm has also been presented for treating non-homogeneous systems of charge density. The key to the theoretical computational methods is the decomposition of the electrostatic energy in the total energy of density functional theory for probing the spanning characteristic at atomic scale, layer by layer, by which the choice of chemical elements and the defect complex effect can be understood deeply.(iv) The numerical computational program and design have also been presented.     

A uniform approximation for the eigenmodes of a planar dielectric waveguide

This paper derives a new approximation for the eigenmodes of a planar waveguide. The approximation is uniformly valid at both the high and low frequency regions of the dispersion relation. It is shown that a Pade approximation of the frequency equation leads to very accurate solutions. The new approximate solution is used to compute the frequency spectrum and the results compared with the exact analytical solution. The solutions presented here are ideal for analytically studying transient wave fields by means of modal summation.     

微扰实验法测试螺旋线行波管耦合阻抗及模拟仿真

 应用有限元仿真软件HFSS,建立了准确的3维螺旋线行波管耦合阻抗仿真模型。应用该模型,对微扰实验法在微扰杆内电场所做出的4个假设和近似,进行了定量的分析和讨论：电场在轴向是均匀的,忽略掉轴向高次空间谐波的存在;电场在角向是均匀的,忽略掉角向高次空间谐波的存在;忽略掉电场中TE波部分,认为电场角向分量为零;假设微扰前后的电场是相等的。同时,对微扰杆的尺寸和介电常数与耦合阻抗的依赖关系进行了模拟分析。结果表明：当微扰法所引入的假设和近似逐步消失后,微扰法的结果最后收敛于定义法。     

A hybrid Godunov method for radiation hydrodynamics

From a mathematical perspective, radiation hydrodynamics can be thought of as a system of hyperbolic balance laws with dual multiscale behavior (multiscale behavior associated with the hyperbolic wave speeds as well as multiscale behavior associated with source term relaxation). With this outlook in mind, this paper presents a hybrid Godunov method for one-dimensional radiation hydrodynamics that is uniformly well behaved from the photon free streaming (hyperbolic) limit through the weak equilibrium diffusion (parabolic) limit and to the strong equilibrium diffusion (hyperbolic) limit. Moreover, one finds that the technique preserves certain asymptotic limits. The method incorporates a backward Euler upwinding scheme for the radiation energy density E_r and flux F_r as well as a modified Godunov scheme for the material density ρ, momentum density m, and energy density E.     

A simulation based method to assess inversion algorithms for transverse relaxation data

NMR relaxometry is a very useful tool for understanding various chemical and physical phenomena in complex multiphase systems. A Carr-Purcell-Meiboom-Gill (CPMG) [P.T. Callaghan, Principles of Nuclear Magnetic Resonance Microscopy, Clarendon Press, Oxford, 1991] experiment is an easy and quick way to obtain transverse relaxation constant (T₂) in low field. Most of the samples usually have a distribution of T₂ values. Extraction of this distribution of T₂s from the noisy decay data is essentially an ill-posed inverse problem. Various inversion approaches have been used to solve this problem, to date. A major issue in using an inversion algorithm is determining how accurate the computed distribution is. A systematic analysis of an inversion algorithm, UPEN [G.C. Borgia, R.J.S. Brown, P. Fantazzini, Uniform-penalty inversion of multiexponential decay data, Journal of Magnetic Resonance 132 (1998) 65–77; G.C. Borgia, R.J.S. Brown, P. Fantazzini, Uniform-penalty inversion of multiexponential decay data II. Data spacing, T₂ data, systematic data errors, and diagnostics, Journal of Magnetic Resonance 147 (2000) 273–285] was performed by means of simulated CPMG data generation. Through our simulation technique and statistical analyses, the effects of various experimental parameters on the computed distribution were evaluated. We converged to the true distribution by matching up the inversion results from a series of true decay data and a noisy simulated data. In addition to simulation studies, the same approach was also applied on real experimental data to support the simulation results.     

A kernel-free boundary integral method for elliptic boundary value problems

This paper presents a class of kernel-free boundary integral (KFBI) methods for general elliptic boundary value problems (BVPs). The boundary integral equations reformulated from the BVPs are solved iteratively with the GMRES method. During the iteration, the boundary and volume integrals involving Green’s functions are approximated by structured grid-based numerical solutions, which avoids the need to know the analytical expressions of Green’s functions. The KFBI method assumes that the larger regular domain, which embeds the original complex domain, can be easily partitioned into a hierarchy of structured grids so that fast elliptic solvers such as the fast Fourier transform (FFT) based Poisson/Helmholtz solvers or those based on geometric multigrid iterations are applicable. The structured grid-based solutions are obtained with standard finite difference method (FDM) or finite element method (FEM), where the right hand side of the resulting linear system is appropriately modified at irregular grid nodes to recover the formal accuracy of the underlying numerical scheme. Numerical results demonstrating the efficiency and accuracy of the KFBI methods are presented. It is observed that the number of GMRES iterations used by the method for solving isotropic and moderately anisotropic BVPs is independent of the sizes of the grids that are employed to approximate the boundary and volume integrals. With the standard second-order FEMs and FDMs, the KFBI method shows a second-order convergence rate in accuracy for all of the tested Dirichlet/Neumann BVPs when the anisotropy of the diffusion tensor is not too strong.     

模式匹配与积分方程在HPM模拟中的应用

针对具有电大尺寸、厚介质罩且馈源采用特定模式激励的高功率微波(HMP)传输及辐射结构研究了一种新型的电磁建模技术。将模式匹配方法与积分方程方法进行混合,构建了电磁模型的方程组,采用多层快速多极子技术、预条件器等进行求解加速,最终形成了可对电大尺寸HPM传输、辐射系统进行高效全波电磁仿真的技术。以电大变张角喇叭馈源、功率合成天线、波束波导及波束波导天线作为实例,构建了几何及电磁模型,并进行了包括远区方向图、近区功率密度分布在内的数值模拟,对该技术的正确性和通用性进行了验证。结果表明,该技术边界拟合准确,内存消耗低,且对馈电模式能予以准确反映,适用于HPM传输发射的高效高精度模拟。     

模式匹配与积分方程在HPM模拟中的应用

针对具有电大尺寸、厚介质罩且馈源采用特定模式激励的高功率微波(HMP)传输及辐射结构研究了一种新型的电磁建模技术。将模式匹配方法与积分方程方法进行混合,构建了电磁模型的方程组,采用多层快速多极子技术、预条件器等进行求解加速,最终形成了可对电大尺寸HPM传输、辐射系统进行高效全波电磁仿真的技术。以电大变张角喇叭馈源、功率合成天线、波束波导及波束波导天线作为实例,构建了几何及电磁模型,并进行了包括远区方向图、近区功率密度分布在内的数值模拟,对该技术的正确性和通用性进行了验证。结果表明,该技术边界拟合准确,内存消耗低,且对馈电模式能予以准确反映,适用于HPM传输发射的高效高精度模拟。     

A proposed method for generating and receiving narrow beams of ultrasound in the Fast Reactor liquid sodium environment

In the liquid sodium coolant of the Fast Reactor there are several measurements which may be performed using ultrasonic techniques. This paper gives a brief outline of one such measurement and then describes the design of an ultrasonic waveguide for the generation and detection of ultrasound in this hostile situation. The successful laboratory tests of the device are also reported.     

Numerical simulation of laser-generated ultrasound in non-metallic material by the finite element method

The use of a pulsed laser for the generation of the elastic waves in non-metallic materials in the thermoelastic regime is investigated by using finite element method (FEM), taking into account not only thermal diffusion and the finite spatial and temporal shape of the laser pulse, but also optical penetration and the temperature dependence of material properties. The optimum finite element model is established based on analysis of two important parameters, meshing size and time step, and the stability of solution. Temperature distributions and temperature gradient fields in non-metallic material for different time steps are obtained, this temperature field is equivalent to a bulk force source to generate ultrasonic wave. The laser-generated ultrasound waveforms at the epicenter and surface acoustic waveforms (SAWs) are obtained and the influence of optical penetration into the material on the temperature field and the ultrasound waveforms are analyzed. The numerical results indicate that the heat penetration into non-metallic material is caused mainly by the optical penetration, and the ultrasound waveforms, especially the shape of the precursor, are strongly dependent on the optical penetration depth into non-metallic material.     

Numerical simulation of laser-generated surface acoustic waves in the transparent coating on a substrate by the finite element method

The optimum finite element model in the system consisting of a transparent coating and an opaque substrate is established based on the analysis of two important parameters: meshing size and time step, and the stability of solution. Taking into account the temperature dependence of material properties, the transient temperature and temperature gradient field are obtained. According to the thermoelastic theory, this temperature gradient field can be taken as a buried bulk source to generate ultrasonic wave. The surface acoustic waves (SAWs) are obtained. The influence of the coating thickness on the SAWs is analyzed. The model provides a useful tool for the determination of modes which are generated by a laser source in transparent coating on opaque substrate. The surface skimming longitudinal wave exists for the multiple oscillations and it charges from unipolar waveforms to dipolar.     

数字散斑相关方法亚像素求解的一种混合方法

亚像素求解是数字散斑相关方法(DSCM)中最重要的技术。亚像素求解算法的精度和效率直接影响DSCM的精度和计算速度。目前广泛应用的两种亚像素求解方法在处理高质量散斑图时都存在一定的缺陷:基于灰度插值的散斑场亚像素恢复方法,其计算精度较高,但计算消耗太大;基于相关系数分布的拟合方法,其计算消耗小,但计算精度较低。在分析两种亚像素求解方法各自优缺点的基础上,提出了一种结合两种算法优势的混合算法。混合算法在保证亚像素求解精度的基础上,可以大大降低整个算法的计算消耗。     

单线法测量横向阻抗系统设计与模拟计算

 介绍了单线法测量横向阻抗的基本原理、系统设计和初步模拟计算结果。单线法横向阻抗测量系统由待测元件/参考元件、内置导线、导线偏移调节机构、阻抗匹配段和测量仪器构成。调节机构可以精确调整内置导线的横向位置以激励偶极模式。利用微波工作室软件模拟计算待测元件（2 856 MHz“凸型”腔）的偶极模。“凸型”腔横向阻抗的幅值和偶极模频率的计算结果与ABCI程序数值计算的结果基本相符,说明了单线法横向阻抗测量系统理论上的可行性。     

单线法测量横向阻抗系统设计与模拟计算

介绍了单线法测量横向阻抗的基本原理、系统设计和初步模拟计算结果。单线法横向阻抗测量系统由待测元件/参考元件、内置导线、导线偏移调节机构、阻抗匹配段和测量仪器构成。调节机构可以精确调整内置导线的横向位置以激励偶极模式。利用微波工作室软件模拟计算待测元件（2 856 MHz“凸型”腔）的偶极模。“凸型”腔横向阻抗的幅值和偶极模频率的计算结果与ABCI程序数值计算的结果基本相符,说明了单线法横向阻抗测量系统理论上的可行性。     

A new ultrasonic technique for the measurement of liquid level

A new design of pulse-echo ultrasonic liquid level gauge is described in which the ultrasonic pulse (here a Rayleigh or a Lamb wave) travels down a metal bar or strip towards the liquid surface, where, through a double mode-conversion process and a special reflecting structure, a strong echo is generated which travels back up the bar to the receiving transducer.The geometry of the special reflecting structure is outlined, and it is shown how this affects the basic accuracy of measurement. Inaccuracies arising from other effects like temperature variation, viscosity and poor signal-to-noise ratio are also discussed. Some mechanical variants, like using the wall of the tank in place of the metal bar carrying the ultrasonic wave and the use of corner reflectors as part of the reflecting structure, are also considered.A note in the Appendix briefly describes one method which has been used successfully for the generation of Rayleigh and Lamb waves.     

A hybrid method for hydrodynamic-kinetic flow Part I: A particle-grid method for reducing stochastic noise in kinetic regimes

In this work we present a hybrid particle-grid Monte Carlo method for the Boltzmann equation, which is characterized by a significant reduction of the stochastic noise in the kinetic regime.     

A three-dimensional Eulerian method for the numerical simulation of high-velocity impact problems

In the present paper, a three-dimensional （3D） Eulerian technique for the 3D numerical simulation of high-velocity impact problems is proposed. In the Eulerian framework, a complete 3D conservation element and solution element scheme for conservative hyperbolic governing equations with source terms is given. A modified ghost fluid method is proposed for the treatment of the boundary conditions. Numerical simulations of the Taylor bar problem and the ricochet phenomenon of a sphere impacting a plate target at an angle of 60~ are carried out. The numerical results are in good agreement with the corresponding experimental observations. It is proved that our computational technique is feasible for analyzing 3D high-velocity impact problems.