Surface currents on a perfect conductor, induced by a magnetic dipole

An oscillating magnetic dipole located near a perfect conductor induces a current density on the surface of the metal. We have derived an expression for this current density, and studied its field line patterns for various orientations of the dipole moment. When the dipole moment is perpendicular to the surface, the field lines are circles which run clockwise and counterclockwise. For a linear dipole oriented parallel to the surface, the field line pattern is much more complex, and it contains singular points. When the dipole moment rotates in a plane parallel to the surface, the field lines are spirals. A field line spirals inward from infinity to some given point, after which it spirals outward back to infinity. We have also considered the Poynting vector of the electromagnetic field near the surface, and we found that its field lines can have singular points or exhibit a vortex.     

理想腔中具有正交偶极矩的级联型三能级原子发射谱

假设原子具有两正交跃迁电偶极矩，运用全量子论研究了与单模电磁场共振相互作用的等距三能级原子的发射谱. 对不依赖于场强耦合和依赖强度耦合两种情况分别进行了发射谱的计算，并比较了原子偶极矩正交和平行条件下发射谱的异同. 结果表明，一般情况下，具有平行偶极矩的原子的发射谱谱线少于具有正交偶极矩的原子的谱线，一些谱线由于干涉而消失. 关键词： 量子光学 发射谱 电偶极矩 级联型三能级原子     

Diffraction of sound from a dipole source near to a barrier or an impedance discontinuity

Pierce's formulation for the diffraction of spherical waves by a hard wedge has been extended to the case of the sound field due to a dipole source. The same approach is also used to extend a semiempirical model for sound propagation above an impedance discontinuity due to a dipole source. The resulting formulas have been validated by comparing their numerical solutions with that computed by summing the sound fields due to two closely spaced monopole sources of equal magnitude but opposite in phase. These new formulations are then used to develop a simple model for calculating the dipole sound field diffracted by a barrier above an impedance ground. Applications of these models relate to transportation noise prediction, particularly railway noise abatement, for which dipole sources are commonly used. The numerical predictions have been found to compare reasonably well with indoor measurements using piezoceramic transducers as dipole sources.     

Excitation of oscillations in a hemispherical dielectric resonator by a radial magnetic dipole

Forced oscillations excited by a radial magnetic dipole in a dielectric hemisphere (resonator) placed on a perfectly conducting plane are studied. It is shown that the dipole excites H modes. When the dipole radiation frequency equals the eigenfrequency of the resonator, an amplitude resonance is observed in the spectrum. The excitation efficiency is high when the magnetic dipole is placed at the maximum of the radial field component of the resonator’s eigenmode.     

Elastic behaviour of a wedge disclination dipole near a sharp crack emanating from a semi-elliptical blunt crack

The interaction between a wedge disclination dipole and a crack emanating from a semi-elliptic hole is investigated. Utilising the complex variable method, the closed form solutions are derived for complex potentials and stress fields. The stress intensity factor at the tip of the crack and the image force acting on the disclination dipole center are also calculated. The influence of the morphology of the blunt crack and the position of the disclination dipole on the shielding effect to the crack and the image force is examined in detail. The results indicate that the shielding or anti-shielding effect to the stress intensity factor increases when the wedge disclination dipole approaches the tip of the crack. The effects of the morphology of the blunt crack on the stress intensity factor of the crack and the image force are very significant.     

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...     

Dipole capture of a slow electron by a water cluster

It is shown that the anomalously large cross section for attachment of a slow electron to a water cluster (H₂O) _n≥50 that is observed in molecular-beam experiments may be explained by the capture of the electron by a long-range field of the permanent electric dipole moment of the cluster. The cross section values are used to estimate the dipole moment of the cluster as a function of its diameter n. The values obtained significantly exceed the random dipole moments in the case of the proton-disordered cluster structure and indicate the ferroelectric ordering of the orientations of dipole moments of the molecules included in the cluster.     

Physical model and the associative memory of a cytoskeleton microtubule as a system of dipoles

A physical model of the conformational degrees of freedom of a cytoskeleton microtubule represented as a system of interacting dipoles is elaborated, characteristic physical quantities are estimated, and a phase diagram of the ferroelectric state of the microtubule at T = 0 is constructed. The presence of frustrated couplings J _ij between the dipoles appears to be the most important feature of the disordered dipole system of the microtubule, owing to which the ground state of the dipole system splits into a large number of lower energy states. The dynamics of the dipole system is determined by the relaxation of the dipole-dipole interaction energy. After the dipole system has evolved to its final state, the input image is associated with one of reference images stored earlier; hence, the dipole system of a microtubule can be viewed as a distributed system with associative memory.     

Conservation issue of pairwise quantum discord and entanglement of two coupled qubits in a two-mode vacuum cavity

The conservation issues of pairwise quantum discord and entanglement of two qubits coupled to a two-mode vacuum cavity are investigated by considering the dipole-dipole interaction between two qubits.It is found that the sum of the square of the pairwise quantum discords and the sum of the square of the pairwise concurrences are both conserved in the strong dipole-dipole interaction limit.However,in the middle dipole-dipole and weak dipole-dipole interaction limits,the sum of the square of the pairwise concurrences is still conserved while the sum of the square of the pairwise discords is not.The crucial reason for this is that the quantum discords are not equivalent if the measurements are performed on different subsystems in a general situation.So it is very important for quantum computation depending on the quantum discord to select the target performed by the measurements.     

Setup of a dipole trap for all-optical trapping

Micromotion induced by the radio-frequency field contributes greatly to the systematic frequency shifts of optical frequency standards. Although different strategies for mitigating this effect have been proposed, trapping ions optically has the potential to provide a generic solution to the elimination of micromotion. This could be achieved by trapping a single ion in the dipole trap composed of a highpower laser field. Here, we present the setup of the dipole trap composed of a 532 nm laser at a power of 10 W aiming to optically trap a single ~(40)Ca~+ and we observe an AC-Stark shift of the fluorescence spectrum line of ～22 MHz caused by the 532 nm dipole beam. The beam waist of the dipole laser is several microns, which would provide a dipole potential strong enough for all-optical trapping of a single ~(40)Ca~+ ion.     

Redistribution of energy flow in a material due to damping

The field lines of energy flow of the radiation emitted by a linear dipole in free space are straight lines, running radially outward from the source. When the dipole is embedded in a medium, the field lines are curves when the imaginary part of the relative permittivity is finite. It is shown that due to the damping in the material all radiation is emitted in directions perpendicular to the dipole axis, whereas for a dipole in free space the radiation is emitted in all directions except along the dipole axis. It is also shown that some field lines in the near field form semiloops. Energy flowing along these semiloops is absorbed by the material and does not contribute to the radiative power in the far field.     

The correspondence between a current-carrying coil and a magnetic dipole

The difficulties associated with the correspondence between the electric vectors E and D and the magnetic vectors B and H, when a current-carrying coil is considered as a magnetic dipole, are eliminated if it is considered, not as a simple dipole, but as a ‘dipotential’ or dipole with a constant potential difference.     

Multipole resonance in the interaction of a spherical Ag nanoparticle with an emitting dipole

The effect of multipole resonance in the interaction between a spherical metallic nanoparticle （MNP） and an emitting dipole is studied with the Mie theory. The results show that the absorption peak of the MNP with respect to the field of the emitting dipole is blue-shifted with the decrease of the spacing between MNP and emitting dipole due to the enhanced multipole resonance. At a short distance, the enhanced multipole terms of scattering are not obvious compared with the dipole term. For the decay rate of the emitting dipole, multipole resonance brings about the enhancement of it largely at short spacing. For the radiative decay rate, the behavior is quite different. The dipole term is dominant at a short spacing, and the multipole term is dominant at a larger spacing.     

The added masses of a monopole and a dipole in a narrow pipe

The added masses of a monopole and a dipole placed in a narrow pipe are studied experimentally. It is shown that, when a monopole passes from a free space into a narrow pipe, its added mass decreases, whereas the added mass of a dipole in a similar situation increases. Inside the pipe, for both monopole and dipole radiators, the value of the added mass depends on the radiator position with respect to the pipe wall. If the radiator is at the pipe axis, its added mass is at a minimum; when the radiator moves toward the wall, the added mass increases. This dependence is qualitatively explained using a simple example.     

Dipole resonance muffler at the exit of a narrow pipe

Locking of sound by a dipole resonator installed at the exit of a narrow pipe is studied theoretically and experimentally. The efficiencies of locking by dipole and monopole resonators are compared, and the dipole resonator is found to be more efficient than the monopole one.     

偶极子位置及偏振对激发光子晶体H1微腔的影响

光子晶体微腔和量子点的集成是实现量子信息处理非常具有潜力的平台之一,利用微腔和量子点的耦合可以制备纠缠光子对,实现对量子态的操控.因为光子晶体微腔具有品质因子高、模场体积小等优点,可以极大地增强光与物质之间的相互作用,从而易于实现量子态在不同物理体系之间的转换.通过单量子点和光子晶体H1微腔的耦合可以产生纠缠光子对,因为H1微腔具有简并的、模式偏振正交的基态模式.通常微腔模式的激发随着量子点在微腔中的位置变化而改变,本文用时域有限差分方法研究了偶极子光源的位置及偏振对激发光子晶体H1微腔模式的影响.结果表明:通过改变偶极子光源位置可以选择性地激发H1微腔简并模式中的一个;具有某一偏振的偶极子光源只能激发相应偏振的微腔模式;模式激发强度的大小也是由偶极子光源在微腔中的位置决定的.鉴于目前量子点在微腔中的位置尚不能精确控制,所以微腔模式受激发光源位置的影响的研究具有重要意义.     

Strong enhancement of magnetic dipole emission in a multilevel electronic system

The Purcell effect is commonly used to increase light emission by enhancing the radiative decay of electric dipole transitions. In this Letter, we demonstrate that the opposite effect, namely, the inhibition of electric dipole transitions, can be used to strongly enhance light emission via magnetic dipole transitions. Specifically, by exploiting the differing symmetries of competitive electric and magnetic dipole transitions in trivalent europium, we demonstrate a fourfold enhancement of the far-field emission from the (5)D(0)→(7)F(1) magnetic dipole transition in trivalent europium. We show that this strong enhancement is well predicted by a three-level model that couples the individual Purcell enhancement factors of competitive transitions from the same excited state.     

The scattering cross section of a resonator in a multimode waveguide

The scattering of sound by a single monopole-dipole resonator in a multimode pipe is investigated. The resonator has the form of a Helmholtz resonator connected through a small bar to the pipe wall. In fact, this resonator is a combination of a monopole resonator and a dipole one positioned at the same point. The scattered fields of these resonators are orthogonal to each other. The scattering cross sections of the monopole and dipole resonators in a multimode pipe are calculated. The scattering cross section of the monopole-dipole resonator is determined as the sum of the scattering cross sections of the monopole and the dipole resonators. The friction in a resonator (the monopole or dipole resonator) reduces its scattering cross section by a factor of (1 + β)², where β is the ratio between the friction resistance and the radiation resistance of this resonator.     

Quantum bound states embedded in a continuum and their classical analogs

We show that both a rigid and a nonrigid dipole can be trapped by an external uniform magnetic field in classical mechanics. The trapped states of a dipole present a nontrivial example of classical bound states embedded in a continuum (BSEC) that can be treated as analogs of quantum BSECs. For example, the classical motion of a dipole is confined to a finite region in space, though there are no classical turning points. We also examine the quantum motion of a dipole in a magnetic field and show that, for the most natural choices of the parameters (the rigid rotating dipole or the one bound by oscillator potential, uniform time-independent magnetic field, etc.), there are no quantum BSEC solutions.     

Force and torque on a sphere with electric dipole moment moving in a dielectric fluid in the presence of a uniform magnetic field

The torque exerted by a magnetic field on a sphere with permanent electric dipole moment moving steadily in a dielectric fluid is calculated to lowest order in the dipole moment. Hence the force exerted on a steadily rotating sphere with electric dipole moment is found by Onsager symmetry. The modification from the vacuum values of torque and force depends only on the slip parameter and the static dielectric constant of the fluid. It is suggested that for a macroscopic ferro electric sphere the calculated effects could be measured experimentally without great difficulty.