电偶极子激发的电场及其MATLAB软件的模拟仿真

分析偶极子产生的场,对于研究介质的极化、介质中的场以及电磁波的辐射等具有重要作用。关于电偶极子的电场分布,一般教科书上只给出某个特殊方向的电场表达式,本文详细介绍了电偶极子在空间任意一点激发的电场的计算公式,并用M ATLAB模拟出电偶极子的场分布图形,同时给出了一些典型图形以供参考。     

水平层状介质并矢Green函数的递推矩阵方法

采用递推矩阵方法计算任意数目水平层状介质的并矢Green函数.根据层界面处电场和磁场的连续性条件得到3个确定Sommerfeld积分待定系数的矩阵方程组,分别对应于垂向单位电偶极子产生的TM波、水平方向单位电偶极子产生的TE波和TM波,这些方程组均可通过递推方法快速求解.只需改变3个方程组中源项元素的位置,就可以方便地得到当源点和场点在任意层时的并矢Green函数.本文给出的并矢Green函数表达式形式简洁且不含指数增加项,计算时不会出现溢出现象.     

任意方向电偶极子在水平分层受限空间中的远区辐射场求解

任意方向电偶极子在分层受限空间中的远区辐射场求解对于分析云闪回击、对潜通信、地波超视距雷达等领域中电磁特性问题具有重要意义.本文基于镜像法和偶极子在自由空间远区辐射场,建立了三层水平受限空间模型,对其间任意取向电偶极子产生的远区辐射场表达式进行了推导,综合考虑了从源点到观测点的直达波及上、下界面的一次反射波影响.在此基础上,比较分析了频率为100 kHz, 6 MHz和10 MHz的电偶极子处于地-电离层模型中不同位置时传播的辐射特性.结果显示:对于同一位置辐射源,电偶极子的频率越高,辐射波瓣数目越多;当偶极子源的频率相同时,源点距离下界面越远,辐射波瓣数目亦越多.     

偶极子电场任意P点场强E的计算

电偶极子的物理模型在电介质的极化,电磁波从天线的发射中都要用到.一般普物教材[1～4]在求解电偶极子的场强E时,只求解偶极子轴线的延长线和中垂线上点的场强.有的教材[5]处理偶极子电场任意点场强E时,把场强E矢量分解成沿R方向的矢量和垂直R方向的矢量,然后再合成的方法.     

电偶极子与单摆

电偶极子是电磁学基本模型之一,其电场强度分布颇有特点.通常教材要么给出电势然后做梯度计算,要么通过矢量叠加直接给出场强,推导比较数学化,学生难以对偶极子场强特点和物理图像获得直观理解.本文把熟知的力学单摆和偶极子联系起来,通过力电类比给出偶极子电场的两种形式,特别是不依赖坐标的场强形式,清楚阐明了各分量特点,还给出类比方法的应用实例.这种方法有助于初学者从不同角度体会知识点联系,深入理解偶极子物理图像,强化类比思维.     

互易定理计算分层半空间上方任意取向偶极子的远区场

利用互易定理推导了考虑直达波和界面反射波时半空间分层界面上任意取向偶极子在上半空间远区辐射场的解析表达式. 进一步考虑电偶极子距离分界面为某一高度情形, 在考虑直达波和反射波程差基础上导出相应辐射场公式. 辐射场解析表达式中分层半空间TM和TE波反射系数可以通过连分数方法或传播矩阵法计算获得. 本文推导过程物理概念清晰, 所得解析结果适于计算观察点远离界面情形下偶极子的远区辐射场. 数值计算结果表明, 利用本文结果可以快速分析半空间上方任意取向偶极子的远区辐射场.     

匀速运动的电偶极子的电场和磁场分布

本文通过对电偶极子电磁势的分析和相对论下电磁场在不同惯性系中的协变分布，计算了作匀速直线运动的电偶极子的电场和磁场的空间分布．     

略论电偶极线

本文首先仿照电偶极子模型的定义给出了电偶极线模型的定义,然后导出了电偶极线的电势公式和电场公式,最后利用电偶极线模型求出了沿垂直于轴方向均匀极化的长圆柱体周围的电场.     

二次等效法求三层媒质中静态电偶极子的场分布

在唯一性原理的基础上，提出采用二次等效的方法，求解三层平行媒质模型下，位于中间层的静态水平电偶极子在其他层媒质中产生的电场分布的解析表达式．也就是先经过一次等效求出场源同层时的场分布，然后利用二次等效将三层模型等效为两层模型，进而利用镜像法求出场分布．所得标量电位分布的表达式被证明是满足边界条件的，由此说明推导结果的有效性．并在实验室中模拟空气一海水一海床三层模型，对位于海水中的静态水平电偶极子在海水和海床中的标量电位进行了实测，并利用实验参数进行了仿真分析．实测结果和仿真分析相符合，表明推导结果的正确性．研究所得场分布表达式为以静态电偶极子为基本模拟体的电场分布的数学模拟、场分布特征分析等应用奠定了理论基础．     

匀速转动电偶极子的辐射公式

极性分子可以等效为一个电偶极子。极性分子在转动过程中由于正负电荷作向心运动,即正负电荷均作加速运动,因此转动的电偶极子会产生辐射。所以转动极性分子的电磁辐射是电磁学及电磁场研究中的重要内容。基于振荡电偶极子的电磁辐射,把旋转电偶极子等效成二个正交放置振荡电偶极子。在振荡偶极子产生磁势矢与变化电偶极矩关系式的基础上,推导出匀速转动电偶极子辐射公式与转动角速度和电偶极矩间的关系式。     

Transfer matrices of dipoles with bending radius variation

With the increasing demand of high brightness in light source, the uniform dipole can not meet the needs of low emittance, and thus the dipole with bending radius variation is introduced in this paper. The transfer matrix of a non-uniform dipole whose bending radius is linearly changed is chosen as an example and a very simple calculation formula of non-uniform dipole transfer matrices is given. The transfer matrices of some common profile non-uniform dipoles are also listed. The comparison of these transfer matrices and the matrices calculated with slices method verifies the numerical accuracy of this formula. This method can make the non-uniform beam dynamic problem simpler, very helpful for emittance research and lattice design with non-uniform dipoles.     

真实头模型中的多电流偶极子脑磁源定位

给出了真实头模型下多电流偶极子定位的通用公式,并对多种源的情况进行了计算机模拟,逆问题的求解采用全局优化与局域优化相结合的优化方法,在较强的噪声背景下得到了较好的定位结果.其中,在偶极子源较少时,采用了球对称导体模型与真实头模型相结合的混合模型,进一步提高了求解速度 关键词： 脑磁技术 真实头模型 多偶极子脑磁源 优化方法     

Mechanisms of dendrites occurrence during crystallization: Features of the ice crystals formation

Dendrites formation in the course of crystallization presents very general phenomenon, which is analyzed in details via the example of ice crystals growth in deionized water. Neutral molecules of water on the surface are combined into the double electric layer (DEL) of oriented dipoles; its field reorients approaching dipoles with observable radio-emission in the range of 150 kHz. The predominant attraction of oriented dipoles to points of gradients of this field induces dendrites growth from them, e.g., formation of characteristic form of snowflakes at free movement of clusters through saturated vapor in atmosphere. The constant electric field strengthens DELs' field and the growth of dendrites. Described phenomena should appear at crystallization of various substances with dipole molecules, features of radio-emission can allow the monitoring of certain processes in atmosphere and in technological processes. Crystallization of particles without constant moments can be stimulated by DELs of another nature with attraction of virtual moments of particles to gradients of fields and corresponding dendrites formation.     

Das Lorentzfeld und die Dipolkopplung

The field at the center of a Lorentz-sphere due to all dipoles within this sphere has been calculated, assuming inhomogeneous orientations of the dipoles in the sphere, as one must expect in real crystals. This field has very often been neglected. The calculation shows the existence of a coupling field which stiffens the dipole directions against localized torques due to “distortion fields”. In the case of ferromagnetism, one can calculate the coupling field of an expanded Lorentz volume. This is analogous to the quantummechanical exchange field, which is due to the excess exchange energy. In many cases the above dipole coupling field is more important than the wellknown exchange field because the “dipole coupling constants” exceed the exchange constant by orders of magnitude.     

平面波照射下无限大导体板上周期孔阵屏蔽效能的解析研究

本文针对平面电磁波对无限大导体平板上周期孔阵的透射问题,首先用Bethe小孔理论将孔阵表示成偶极子阵列,然后用平均化处理得到均匀的等效磁化/极化强度,进而引入等效面源导出透射电磁波表达式,最终给出了孔阵对平面电磁波屏蔽效能的解析公式.该公式分别针对横电和横磁两种极化方式,将屏蔽效能表示成孔阵周期面积、孔的极化系数、波长和入射角的简单函数,其计算结果与全波仿真结果一致性好.结果表明,透射场强的幅值与孔极化系数和波频率成正比,与周期面积成反比;在横电极化方式,波频率和周期面积不变的情况下,透射场强的幅值与入射角的余弦值成正比,入射角越大屏蔽效能越高;在横磁极化方式下,透射场强幅值与入射角的关系相对复杂,但在入射角较小时与入射角的余弦值近似成反比,总体上入射角越大屏蔽效果越低.     

亚波长间距理想导体球阵列近区时间反演电磁场的快速求解

为快速求解亚波长间距分布的理想导体球阵列近区的时间反演电磁场,提出一种基于等效偶极子模型的解析分析方法.首先,通过分析球面波照射理想导体小球的散射场解析解发现,散射场可以近似等效为电磁偶极子辐射场的叠加.等效偶极子的强度与初始激励源的幅度成正比关系.通过建立不同小球等效偶极子矢量间的耦合方程组可以直接求解得到相应矢量的大小.然后,结合时间反演腔理论得到相应的时间反演并矢格林函数,继而得到小球阵列近区的时间反演场分布.最后,通过与数值仿真软件的计算结果进行对比,验证了方法的正确性及高效性.研究表明,时间反演技术结合近场亚波长间距小散射体加载能够实现超分辨率的场聚焦.     

Inhomogeneous states and the mechanism of magnetization reversal of a chain of classical dipoles

The inhomogeneous states (solitons) in a single chain of classical dipoles are studied numerically and analytically. An analytical solution to the problem is based on the long-wave approximation for dipole sums which holds for high magnetic fields perpendicular to the dipole chain. The analytical and numerical solutions are in reasonable agreement. The magnetization reversal is investigated by numerical simulation based on the Landau-Lifshitz stochastic equations. It is demonstrated that the magnetization reversal of a dipole chain at a finite temperature has a thermal activation nature and occurs through the formation of a stable phase nucleus (a soliton at the edge of the chain) and its growth (the motion of the soliton along the chain).     

金属纳米椭球结构表面的局域电场和吸附分子分布

过渡金属纳米结构表面吸附CO分子时会出现异常红外效应,这一现象可以用纳米结构表面吸附分子在外电场作用下产生局部凝聚从而相互作用能增加来解释.在前期研究的基础上,给出金属基底表面生长出的纳米颗粒为椭球状颗粒的理论计算结果.基于均匀外电场中金属纳米椭球颗粒按一定对称性排列的表面结构模型,用经典电磁学理论计算了纳米椭球颗粒表面附近的局域电场.在此基础上,将吸附的CO分子等效为偶极子,在考虑了偶极子与局域电场、偶极子之间以及偶极子与金属基底三种相互作用的情况下,用Monte-Carlo方法进行数值模拟,最后给出纳 关键词： 金属纳米结构表面 纳米椭球 吸附分子 局域电场     

Effect of local field fluctuations on orientational ordering in random-site dipole systems

Some peculiarities of dipole ordering in systems with uniaxial or cubic anisotropy with an arbitrary degree of dilution are analyzed in terms of random local field theory. The approach takes into account the effect of thermal and spatial fluctuations of the local fields acting on each particle from its neighbors with an accuracy corresponding to that of the Bethe-Paierls pair clusters approach. We show that ferromagnetic (ferroelectric) structure for uniaxial Ising dipoles distributed on a simple cubic lattice is intrinsically unstable against the fluctuations of the local fields for any concentration of the dipoles. This result is quite different from the prediction of the mean-field theory which implies the possibility of ferromagnetic ordering as a metastable state in field-cooled experiments. The local field fluctuations do not exclude, however, antiferromagnetic ordering above a certain critical concentration. Ferromagnetic ordering is possible for other types of lattice geometries and for an amorphous-like dipole distribution above a certain critical concentration. A simple physical explanation of such behavior is given based on the specific angular dependence of the dipole-dipole interaction that results in a relatively high value of the local field second moment for simple cubic lattice.     

Position-dependent property of resonant dipole—dipole interaction mediated by localized surface plasmon of an Ag nanosphere

We use the photon Green-function method to study the quantum resonant dipole-dipole interaction(RDDI) induced by an Ag nanosphere(ANP).As the distance between the two dipoles increases,the RDDI becomes weaker,which is accompanied by the influence of the higher-order mode of the ANP on RDDI declining more quickly than that of the dipole mode.Across a broad frequency range(above 0.05 eV),the transfer rate of the RDDI is nearly constant since the two dipoles are fixed at the proper position.In addition,this phenomenon still exists for slightly different radius of the ANPs.We find that the frequency corresponding to the maximum transfer rate of RDDI exhibits a monotonic decrease by moving away one dipole as the other dipole and the ANP are kept fixed.In addition,the radius of ANP has little effect on this.When the two dipoles are far from the ANP,the maximum transfer rate of the RDDI takes place at the frequency of the dipole mode.In contrast,when the two dipoles are close to the ANP,the higher-order modes come into effect and they will play a leading role in the RDDI if they match the transition frequency of the dipole.Our results may be used in a biological detector and have a certain guiding significance for further application.