高斯波束在双负媒质中的传播特性分析

 提出了一种基于Drude色散媒质模型的2维TM模时域有限差分法，数值模拟了高斯波束从自由空间垂直入射和斜入射到双负媒质(介电常数和磁导率同时为负)平板后的折射现象，并给出了平板内外的电场强度分布。模拟结果显示：垂直入射时，双负媒质平板对高斯波束具有汇聚作用，通过改变媒质的折射率，可以控制高斯波束通过组合平板的传播时间而不发生衰减；斜入射时，折射波束与入射波束均位于法线的同一侧，高斯波束在双负媒质与空气交界面处发生了负折射。     

STUDY OF MILLIMETER-WAVE WAVEGUIDES USING A 2-D ORDER-MARCHING TIME-DOMAIN METHOD

In this paper, the application of the order-marching time-domain (OMTD) method to the solution of the cutoff frequencies of TE and TM modes in millimeter-wave waveguides is described. Starting from Maxwell’s two-dimensional (2-D) differential equations for TE or TM case, the OMTD method uses the orthonormality of weighted Laguerre polynomials and Galerkin’s testing procedure to eliminate the temporal variables and results in an order-marching scheme. To verify it’s accuracy and efficiency, the numerical results for millimeter-wave guided systems are compared with those of finite-difference time-domain (FDTD) method and analytical solutions. The OMTD method improves computational efficiency notably, especially for fine grid division problems restricted by stability constraints in the FDTD method. This work was supported by the National Natural Science Foundation of China (No. 60371008) and the CRT Program of UESTC.     

X波段五腔渡越管振荡器的高频特性研究

 从麦克斯韦方程出发，采用时域有限差分的方法，对X波段五腔渡越管振荡器的谐振腔进行了高频特性研究，给出了该谐振腔中的TM₀₁模的非π模式的本征频率、Q值以及对应的场分布，并用实验冷测法测出了谐振腔的谐振频率与Q值。数值计算与实验结果比较一致，说明了该高频特性研究结果的可靠性。     

FDTD model of non-magnetized plasma coatings on metals based on collocated surface impedance boundary conditions

A finite-difference time-domain (FDTD) model is developed for non-magnetized plasma thin layers on metal surfaces illuminated by vertical polarization (TE) and parallel polarization (TM) plane waves at oblique incidence. The model is based on collocated first-order surface impedance boundary conditions (SIBCs), permitting one to remove the plasma coating and the substrate from the computational space. The reflection of TE and TM plane waves at varying oblique incident angles from the plasma coating surfaces is simulated. The results are numerically verified by the comparison with the exact results in the one-dimensional situation. Magnitude and phase error of the calculated reflection coefficients are studied to illustrate that the proposed model has greatly improved accuracy over the original SIBCs implementation. Finally, we calculate the radar cross-section (RCS) of a perfectly conducting cube covered with plasma coatings utilizing the proposed collocated SIBCs, which further demonstrates the practical application of the model.     

Field enhancement effect of metal probe in evanescent field

Field enhancement effect of metal probe in evanescent field, induced by using a multi-layers structure for exciting surface plasmon resonance （SPR）, is analyzed numerically by utilizing two-dimensional （2D） TM- wave finite difference time-domain （FDTD） method. In this letter, we used a flmdamental mode Gaussian beam to induce evanescent field, and calculated the electric intensity. The results show that compared with the nonmetal probe, the metal probe has a larger field enhancement effect, and its scattering wave induced by field enhancement has a bigger decay coefficient. The field enhancement effect should conclude that the metal probe has an important application in nanolithography.     

有限厚度金属狭缝对太赫兹脉冲的整形和滤波

用时域有限差分算法（FDTD）分析了有限厚度金属狭缝对太赫兹（THz）脉冲进行时空整形的效应和特性.分析表明：这种基于衍射方法的准光学技术手段对横向磁场（TM）模式THz脉冲有很好的整形和滤波作用,而对横向电场（TE）模式THz脉冲几乎没有整形和滤波作用,并运用平面光波导理论对所获得的数据模拟结果进行了定性说明.这些分析结果可在相关实验中予以考虑和采用. 关键词： 时空整形 FDTD THz脉冲 狭缝滤波器     

用二维时域有限差分法分析条形多层波导

为精确分析条形多层波导和多量子阱波导，本文提出了一种以二维时 限差分法为基础的全波数值方法，给出了场分布的色散特性的数值结果，并与矢量有限元法已有的结果作了比较；文中还研究了以二维时域有限差分法为基础的标量近似技术，提出了它的有效性和精度的局限性。     

PML-FDTD法在分析负折射率材料中的应用

将时域有限差分(FDTD)法引入了对负折射率材料物理现象的仿真研究.给出了二维TM波在负折射率材料中的时域差分方程,并且在吸收边界处使用了理想匹配层(PML).为了避免在迭代过程中出现的不稳定现象,在差分方程的推导中引入了Drude模型, 并对Pendry提出的由负折射率材料构成的平板透镜具有的完美成像现象进行了数值仿真验证.由仿真结果发现,完美成像现象只在平板透镜的折射率n=－1时出现,当n≠－1时则会出现近轴聚焦效应.     

一种计算分层半空间上方导线瞬态响应的新方法

提出了一种将时域积分方程(time domain integral equation, TDIE)方法和时域有限差分(finite differnce time domain, FDTD)方法相结合计算分层有耗半空间上方导线瞬态电磁响应的新方法.其中,一维FDTD方法用于计算入射电磁波经分层半空间反射的时域波形.TDIE用于求解细导线在加入两个激励源(直接入射电磁波和经分层半空间反射的电磁波)时的瞬态响应.相关计算理论和数值模拟结果说明了本文方法是一种解决了分层有耗介质上方水平放置导线瞬态响应的高效解决方案. 关键词： 时域积分方程 时域有限差分 细导线 分层半空间     

Positive-negative birefraction phenomenon for TM polarization in annular photonic crystal

We propose a two-dimensional (2D) annular photonic crystal (APC) with dual equi-frequency contours (EFCs) in one band. The refractive behaviors of a Gaussian beam incident from air to the APC are analyzed by the EFC analysis and finite-diflerence time-domain (FDTD) method. The results show the positive-negative birefraction phenomenon for the transverse magnetic (TM) polarization in the same band occurs at the interface between air and the APC, and the surface termination of the APC has a large effect on the strength of the negatively refracted beam.     

一种适用于任意阶空间差分时域有限差分方法的色散介质通用吸收边界条件算法

基于伸展坐标完全匹配层方程和辅助微分方程 方法, 给出了一种在时域有限差分(FDTD) 计算中适用于常见色散介质的通用边界条件算法. 该算法适用于任意阶的FDTD空间差分, 并且由于所采用的D-H方程独立于计算区域, 所以可以被用于截断任意电介质. 数值试验结果表明, 与卷积完全匹配层 算法相比较, 所提出的吸收边界条件算法不仅通用性强、 计算复杂度低、 计算时间短, 并且吸收效果有明显的提高.     

一种新型的光子晶体偏振光分束器的设计

基于光波在直波导和复合结构光子晶体中的传播特性,结合平面波展开法和时域有限差分法,提出并讨论了一种新型的超紧凑的光子晶体偏振光分束器. 它是由输入波导,分束结构和输出波导三部分组成. 对这种结构的三角晶格光子晶体光分束器的数值计算与模拟结果表明,该结构可以实现TE模和TM模的高效大角度分离,并且在通信波段设计尺寸小,这些特性使其在未来的集成光回路中有着重要的应用前景. 关键词： 偏振光分束器 能带结构 平面波展开法 时域有限差分法     

磁化铁氧体覆盖二维导体目标FDTD分析的移位算子方法

采用移位算子方法把时域有限差分法推广应用于二维磁各向异性色散介质—磁化铁氧体中.证明了电磁波横向入射二维轴向磁化铁氧体目标情形下,电磁波可按目标的轴向分解为横电波(TE波)和横磁波(TM波),且TE波的散射特性与铁氧体介质无关,而TM波的散射特性与介质电磁参量密切相关,同时对其物理原因进行了分析.通过采用移位算子方法处理磁化铁氧体频域本构关系,得到该情形下目标电磁散射的移位算子时域有限差的迭代计算公式,同时解决了电磁波在各向异性和频率色散介质中传播的问题.计算了轴向磁化铁氧体涂敷VonKarman型导体柱的TM波双站雷达散射截面,分析了铁氧体参量对目标双站雷达散射截面的影响.结果表明:恰当地选择铁氧体参量能有效地减少目标的雷达散射截面,本文时谐因子取exp〔jωt〕.     

The finite element method for micro-scale modeling of ultrasound propagation in cancellous bone

Quantitative ultrasound for bone assessment is based on the correlations between ultrasonic parameters and the properties (mechanical and physical) of cancellous bone. To elucidate the correlations, understanding the physics of ultrasound in cancellous bone is demanded. Micro-scale modeling of ultrasound propagation in cancellous bone using the finite-difference time-domain (FDTD) method has been so far utilized as one of the approaches in this regard. However, the FDTD method accompanies two disadvantages: staircase sampling of cancellous bone by finite difference grids leads to generation of wave artifacts at the solid–fluid interface inside the bone; additionally, this method cannot explicitly satisfy the needed perfect-slip conditions at the interface. To overcome these disadvantages, the finite element method (FEM) is proposed in this study. Three-dimensional finite element models of six water-saturated cancellous bone samples with different bone volume were created. The values of speed of sound (SOS) and broadband ultrasound attenuation (BUA) were calculated through the finite element simulations of ultrasound propagation in each sample. Comparing the results with other experimental and simulation studies demonstrated the capabilities of the FEM for micro-scale modeling of ultrasound in water-saturated cancellous bone.     

分布式负载垂直极化有界波电磁脉冲模拟器外泄场的规律分析

将基于MPI平台的并行时域有限差分(FDTD)方法与基于完全磁导体(PMC)镜像法相结合,并结合CST模拟软件,模拟给出分布式负载垂直极化有界波电磁脉冲(EMP)的外泄场(包括侧泄场和后泄场)的分布规律。模拟结果与实验结果符合得很好。研究表明:在高度方向上,地面附近的外泄场峰值最大,但远离模拟器时,在1.5 m高的高度范围内,外泄场的峰值差别不大;不管采用何种双指数脉冲源,距离模拟器边缘位置比较近的测点在传输线段的侧泄场的幅值大于分布式负载段侧泄场的幅值,且两者都大于分布式负载末端的后泄场幅值,但随着测点与模拟器边缘的垂直距离的增加,分布式负载段的后泄场可能会比侧泄场大;对于电压峰值相同的双指数激励源而言,所含的高频分量越多,在一定范围内,从其分布式负载末端外泄的后泄场更大;模拟器下方大地的电导率增加,模拟器的外泄场增加。     

Analysis of Millimeter Wave Microstrip Circulator with a Magnetized Ferrite Sphere by FDTD Method with Modified Matrix Pencil Method

In this paper, the modified matrix pencil (MMP) method is used in the finite difference time domain (FDTD) method to eliminate the late time instability of time domain responses when analyzing the novel microstrip circulator with a magnetized ferrite sphere. The frequency dependent scattering parameters: reflection, isolation and insertion parameters of the microstrip circulator calculated over a wide band of frequencies with this hybrid method are compared with that obtained by direct FDTD method which agree well with the experimental results.     

Photonic bandgap properties of 8-fold symmetric photonic quasicrystals

The bandgap properties of 2D 8-fold symmetric photonic quasicrystals (PQCs) composed of a set of rods are numerically investigated for various fill factors, dielectric constants and propagation angles using the finite difference time domain (FDTD) method. Some interesting properties are found besides the known results: (i) the central frequency of the bandgap shifts to low frequency as r/a increases; (ii) an optimum fill factor to obtain the largest gap width exists, which decreases with increasing dielectric constant; (iii) compared with the behavior for a given fill factor, the variation of bandgap width for the optimum fill factor increases much more significantly with the dielectric constant. Selection guidelines for 8-fold symmetric PQC designs are provided for optimum fill factors and variation of the relative bandgap width at different dielectric constants.     

影响高功率光导开关临界频率热因素的数值分析

基于时域有限差分法(FDTD)对非线性模式面结构砷化镓光导开关的热传导过程进行了数值模拟。研究了光导开关临界频率与电流丝位置、半径、数量、芯片尺寸、环境温度等参数的关系。研究表明:临界频率随电流丝半径、数量的增加,呈指数上升趋势;临界频率随着电流丝深度、芯片厚度的增加,呈指数下降趋势;临界频率在一定范围内随环境温度的增加呈线性下降趋势。     

Effect of transition-metal substitution in iron-based superconductors

We study theoretically the effect of transition-metal (TM) substitution in iron-based superconductors through treating all of the TM ions as randomly distributed impurities. The extra electrons from TM elements are localized at the impurity sites. In the meantime the chemical potential shifts upon substitution. The phase diagram is mapped out and it seems that the TM elements can act as effective dopants. The local density of states (LDOS) is calculated and the bottom becomes V-shaped as the impurity concentration increases. The LDOS at the Fermi energy ρ(ω = 0) is finite and reaches the minimum at the optimal doping level. Our results are in good agreement with scanning tunneling microscopy experiments.     

Antireflection subwavelength structures analyzed by using the finite difference time domain method

Antireflection subwavelength structures (ASSs) are analyzed by using the finite difference time domain (FDTD) method in the visible light spectrum. Low reflectance can be obtained by both the conical and pyramidal shapes over a broadband range. Comparing the reflectance of different structure shapes and aspect ratios by the FDTD method, it shows that the antireflection efficiencies of the pyramidal structures are better than that of the conical structures when the aspect ratio is up to 0.8. It is found that, for the conical structure surface, the average transmittance increases gradually with the aspect ratio and the average transmittance is about 99.6% with the aspect ratio of 2.0. However, for the pyramidal structure with the aspect ratio ranging from 1.0 to 2.0, the average transmittance is up to 99.7%.