Scattering properties of an individual metallic nano-spheroid by the incident polarized light wave

The scattering properties of a metallic nano-spheroid under the illumination of different polarized light waves are investigated using 3D boundary element method. The influences of different geometrical sizes of the nano-spheroid and incident directions of the illuminating light wave on the scattering spectrum are studied for different incident polarized light waves. The results show that the metallic nano-spheroid has two intrinsic resonant modes, corresponding to different polarization states and resonant wavelengths. The scattering enhancement, the resonant wavelength, and the location of the enhanced optical field are strongly dependent on the polarization properties of the illuminating light waves, and they can be modulated by appropriately choosing the polarization directions of the incident light wave.     

Polarization properties of subwavelength metallic gratings in visible light band

Optical polarization properties of aluminum, gold and aluminum/gold subwavelength metallic gratings (SWMGs) are investigated numerically in the visible light band. Their performance as polarizing beam splitters are simulated with RCWA for varied metallic materials, grating depth and incident angle. Metallic waveguide theory is applied to explain the performance difference between Au and Al grating. A new type of Al/Au bimetallic is also investigated numerically for potential applications. PACS 42.79.Dj; 42.81.Gs; 73.20.Mf     

On the modeling of narrow gaps using the standard boundary element method

Numerical methods based on the Helmholtz integral equation are well suited for solving acoustic scattering and diffraction problems at relatively low frequencies. However, it is well known that the standard method becomes degenerate if the objects that disturb the sound field are very thin. This paper makes use of a standard axisymmetric Helmholtz integral equation formulation and its boundary element method (BEM) implementation to study the behavior of the method on two test cases: a thin rigid disk of variable thickness and two rigid cylinders separated by a gap of variable width. Both problems give rise to the same kind of degeneracy in the method, and modified formulations have been proposed to overcome this difficulty. However, such techniques are better suited for the so-called thin-body problem than for the reciprocal narrow-gap problem, and only the first is usually dealt with in the literature. A simple integration technique that can extend the range of thicknesses/widths tractable by the otherwise unmodified standard formulation is presented and tested. This technique is valid for both cases. The modeling of acoustic transducers like sound intensity probes and condenser microphones has motivated this work, although the proposed technique has a wider range of applications.     

Polarization converter for higher-order laser beams using a single binary diffractive optical element as beam splitter

We propose a new approach to generating a pair of initial beams for a polarization converter that operates by summing up two opposite-sign circularly polarized beams. The conjugated pairs of vortex beams matched with laser modes are generated using binary diffractive optical elements (DOEs). The same binary element simultaneously serves two functions: a beam shaper and a beam splitter. Two proposed optical arrangements are compared in terms of alignment complexity and energy efficiency. The DOEs in question have been designed and fabricated. Natural experiments that demonstrate the generation of vector higher-order cylindrical beams have been conducted.     

基于边界元方法的超导核磁共振成像设备高阶轴向匀场线圈优化算法

在磁共振成像设备中,为了消除目标区域内的高阶谐波磁场分量,传统方法采用无源匀场,但该方法匀场精度较低,针对性较差,适用于全局匀场,而有源匀场则可以通过优化线圈分布来产生所需要的特定的磁场分布.但是,由于匀场线圈线型的复杂度会随着线圈阶数的增加而增加,难以满足设计需要,因此本文提出了一种用于磁共振成像超导匀场线圈系统的多变量非线性优化设计方法.该方法基于边界元方法,将匀场线圈所产生的磁场与目标磁场之间的偏差作为目标函数,线匝间距、线圈半径等作为约束条件,通过非线性优化算法,得到满足设计要求的线圈分布.通过一个中心磁场为0.5 T的开放式双平面磁共振成像超导轴向匀场线圈的设计案例,说明本方法具有计算效率高、灵活性好的特点.     

回旋管双阳极磁控注入枪的边界元法计算

根据边界元法建立物理与数学模型,编制了计算磁控注入枪的程序。使用该程序设计和模拟一个工作在35 GHz,70 kV,10 A基波回旋行波管放大器的双阳极磁控注入枪,获得较好的模拟结果。并对电子注在不同工作电压和磁场下的质量进行了分析计算。计算表明,边界元法在分析回旋器件电子光学方面是一种非常有效的方法。     

回旋管双阳极磁控注入枪的边界元法计算

 根据边界元法建立物理与数学模型,编制了计算磁控注入枪的程序。使用该程序设计和模拟一个工作在35 GHz,70 kV,10 A基波回旋行波管放大器的双阳极磁控注入枪,获得较好的模拟结果。并对电子注在不同工作电压和磁场下的质量进行了分析计算。计算表明,边界元法在分析回旋器件电子光学方面是一种非常有效的方法。     

Polarization imaging for 3D inspection of highly reflective metallic objects

In the field of industrial vision, the 3D inspection of highly reflective metallic objects is still a delicate task. The specular reflections prevent the use of a 3D laser scanner, whereas the phase shifting-based 3D systems are more adapted to inspect surfaces with low curvature. This paper deals with a new automated 3D inspection system based on polarization analysis. Studying the state of polarization of the reflected light is very useful for obtaining information on the normals of the surface. An extension of the shape from polarization method from dielectric to metallic surfaces is demonstrated. Then, an improved relaxation algorithm is provided in order to reconstruct the shape from the normal field given by the polarization analysis. Finally, applications to shape defect detection are discussed and the efficiency of the system in discriminating defects on highly reflective metallic objects made by stamping and polishing is presented. The text was submitted by the authors in English.     

Polarization tomography of metallic nanohole arrays

We report polarization tomography experiments on metallic nanohole arrays with square and hexagonal symmetry. As a main result we find that a fully polarized input beam is partly depolarized after transmission through a nanohole array. This loss of polarization coherence is found to be anisotropic; i.e., it depends on the polarization state of the input beam. The depolarization is ascribed to a combination of two factors: (i) the nonlocal response of the array as a result of surface-plasmon propagation and (ii) the non-plane-wave nature of a practical input beam.     

用边界元法分析非均匀介质中的传输线

 从静电场边值问题的积分解出发,推导出用边界元法求解分区均匀介质填充传输线问题的矩阵表达式,给出传输线电容参数的计算公式,介绍用边界元法求解分区均匀介质填充传输线问题的基本原理和求解过程。对两类传输线的计算结果表明：用边界元法求解分区均匀介质填充传输线问题,不仅具有较高的计算精度,而且可以很方便地应用于各类复杂截面分区均匀介质填充传输线问题的工程设计与计算,边界元法是求解分区均匀介质填充传输线问题的一种有效方法。     

Profile measurement of a one-dimensional phase boundary sample using a single shot phase-step method

Phase shifting interferometry is a preferred technique for high-resolution phase profile measurement, but the difficulty in generating the requested shifted pattern has limited the use of the technique to low-noise environment and in case accurate calibration of the phase shifting device is available. In the present experiment, a sample having one-dimensional straight phase boundary is mounted in one arm of an interferometer. One single image of the fringe pattern is recorded, a simple image process is applied generating phase shifted patterns from the original image. Using the appropriate phase shift algorithms, a phase map of the sample is obtained which gives a quantitative measurement of the topographical structure with the resolution of the phase shift method but a single shot recorded pattern.     

A high-sensitivity terahertz sensing method using a metallic mesh with unique transmission properties

Metallic meshes work as band-pass filters in the terahertz (THz) region, with their transmission spectra acutely affected by the refractive index of the material inside and above the metallic-mesh openings. We used a metallic mesh for high-sensitivity observations by focusing on the “dip,” that is, a sudden change in transmittance that only appeared when the THz wave was obliquely incident onto the metallic mesh. Here we consider the interference between apertures of the metallic mesh in the diagonal direction and explain the dip using finite-difference time-domain (FDTD) electromagnetic simulations.     

Accelerated boundary element method for diffuse optical imaging

The boundary element method (BEM) is a useful tool in diffuse optical imaging (DOI) when modelling large optical regions whose parameters are piecewise constant, but are computationally expensive. We present here an acceleration technique, the single-level fast multipole method, for a highly lossy medium. The enhanced practicability of the BEM in DOI is demonstrated through test examples on single-layer problems, where order of magnitude reduction factors on solution time are achieved and on a realistic three-layer model of the neonatal head. Our experimental results agree very closely with theoretical predictions of computational complexity.     

基于点阵编码的三维主动视觉标定

提出一种基于位错点阵编码三维成像系统的标定方法.首先在物空间建立三维数据基准,然后通过基准传递的概念标定摄像子系统,再通过建立摄像子系统坐标系与投影子系统坐标系之间的约束关系,将标定后的摄像子系统的准确度传递到投影子系统坐标系.摄像子系统坐标系与投影子系统都具有标定的准确度之后,可以根据位错点阵编码三维成像技术的解码算法获得深度图像空间坐标的计算值,然后将其与物体空间的三维标定数据基准进行比较,建立目标函数为误差平方和最小的非线性优化方程.通过迭代求解这个优化方程,最终获得三维系统的结构参量.实验结果表明,经过三维标定的位错点阵编码三维成像系统,对300×300×80 mm3的测量体积内,可以获得X方向的标准差为0.29mm和Y方向的标准差为0.24mm,Z方向的标准差为0.29mm的测量准确度.     

消声器传递损失预测的边界元模态展开混合方法

将边界元法和解析方法结合形成一种混合方法用于计算消声器的传递损失,消声器被划分成若干个子结构,解析方法和边界元方法被分别用于计算规则结构和不规则结构的阻抗矩阵,不同子结构之间通过阻抗矩阵连接起来。为减少计算时间,采用一种基于模态配点法的简化方法。对单级膨胀腔、双级膨胀腔和穿孔管阻性消声器的传递损失进行了计算,混合方法计算结果与解析方法和三维数值方法计算结果吻合良好。分析了混合方法的计算效率并与传统子结构方法进行了比较,混合方法能明显节省计算时间。     

消声器传递损失预测的边界元模态展开混合方法

将边界元法和解析方法结合形成一种混合方法用于计算消声器的传递损失,消声器被划分成若干个子结构,解析方法和边界元方法被分别用于计算规则结构和不规则结构的阻抗矩阵,不同子结构之间通过阻抗矩阵连接起来。为减少计算时间,采用一种基于模态配点法的简化方法。对单级膨胀腔、双级膨胀腔和穿孔管阻性消声器的传递损失进行了计算,混合方法计算结果与解析方法和三维数值方法计算结果吻合良好。分析了混合方法的计算效率并与传统子结构方法进行了比较,混合方法能明显节省计算时间。     

三维声学多层快速多极子边界元及其应用

快速多极子边界元算法可以加速矩阵和向量乘法运算, 将传统边界元算法的计算量和内存占用量分别降为O(N log²N)和O(N), 适用于大型声学模型模拟计算. 本文发展了一种基于Burton-Miller方程的三维多层声学快速多极子边界元算法. 将新的自适应树状算法应用到对角形式的快速多极子边界元算法, 并使用最新提出的解析式源点矩计算公式, 进一步提高了快速多极子边界元的计算效率. 绝对软球体在内部共振频率处的散射声场计算, 验证了所发展算法在共振频率处求解的正确性. 与Bapat所提供的程序在多脉动球体辐射声场计算精度的比较, 验证了算法及程序在大型模型声学计算中的准确性, 同时显示了其求解的高效性. 最后, 将该算法用于车内声场及水下声学探测的分析计算.     

Sound source reconstruction using inverse boundary element calculations

Whereas standard boundary element calculations focus on the forward problem of computing the radiated acoustic field from a vibrating structure, the aim in this work is to reverse the process, i.e., to determine vibration from acoustic field data. This inverse problem is brought on a form suited for solution by means of an inverse boundary element method. Since the numerical treatment of the inverse source reconstruction results in a discrete ill-posed problem, regularization is imposed to avoid unstable solutions dominated by errors. In the present work the emphasis is on Tikhonov regularization and parameter-choice methods not requiring an error-norm estimate for choosing the right amount of regularization. Several parameter-choice strategies have been presented lately, but it still remains to be seen how well these can handle industrial applications with real measurement data. In the present work it is demonstrated that the L-curve criterion is robust with respect to the errors in a real measurement situation. In particular, it is shown that the L-curve criterion is superior to the more conventional generalized cross-validation (GCV) approach for the present tire noise studies.     

快速多极边界元法用于扩散光学断层成像研究

在求解扩散光学断层成像中的正向问题时, 目前普遍采用有限元法, 但是随着实际模型规模的增大, 有限元法的计算量问题日益显著, 而边界元法则由于可以降低计算维度使计算量减少而备受关注. 本文以均匀的高散射介质为模型, 研究了将快速多极边界元法用于扩散光学断层成像的正向问题. 快速多极边界元法利用核函数的多极展开, 将常规边界元法中系数矩阵和迭代矢量的乘积项等价为相应四叉树结构的一次递归, 再结合广义最小残量法进行迭代求解. 将计算结果和蒙特卡罗法的模拟结果进行了比较, 表明利用快速多极边界元法的模拟结果和蒙特卡罗法的结果有很好的一致性. 研究结果验证了快速多极边界元法可以用于扩散光学断层成像, 为其大规模和实时成像带来可观的前景. 关键词： 扩散光学断层成像 边界元法 快速多极边界元法