Electromagnetic analysis of cylindrical cloaks of an arbitrary cross section

We extend the design of radially symmetric invisibility cloaks through transformation optics as proposed by Pendry et al. [Science 312, 1780 (2006)] to coated cylinders of an arbitrary cross section. The validity of our Fourier-based approach is confirmed by both analytical and numerical results for a cloak displaying a non-convex cross section of varying thickness. In the former case, we evaluate the Green's function of a line source in the transformed coordinates. In the latter case, we implement a full-wave finite-element model for a cylindrical antenna radiating a p-polarized electric field in the presence of a F-shaped lossy object surrounded by the cloak.     

Finite elements modelling of scattering problems for flexural waves in thin plates: Application to elliptic invisibility cloaks,rotators and the mirage effect

We propose a finite elements algorithm to solve a fourth order partial differential equation governing the propagation of time-harmonic bending waves in thin elastic plates. Specially designed perfectly matched layers are implemented to deal with the infinite extent of the plates. These are deduced from a geometric transform in the biharmonic equation. To numerically illustrate the power of elastodynamic transformations, we analyze the elastic response of an elliptic invisibility cloak surrounding a clamped obstacle in the presence of a cylindrical excitation i.e. a concentrated point force. Elliptic cloaking for flexural waves involves a density and an orthotropic Young’s modulus which depend on the radial and azimuthal positions, as deduced from a coordinates transformation for circular cloaks in the spirit of Pendry et al. [Science 312, 1780 (2006)], but with a further stretch of a coordinate axis. We find that a wave radiated by a concentrated point force located a couple of wavelengths away from the cloak is almost unperturbed in magnitude and in phase. However, when the point force lies within the coating, it seems to radiate from a shifted location. Finally, we emphasize the versatility of transformation elastodynamics with the design of an elliptic cloak which rotates the wavevector of a flexural wave within its core.     

Physical bounds on electromagnetic invisibility and the potential of superconducting cloaks

The possibility of suppressing the scattering cross section of an object is subject to fundamental physical bounds imposed by causality and passivity. Global cloaking limitations have been recently derived, which imply that any linear, causal and passive cloak necessarily increases the global scattering, integrated over the whole electromagnetic spectrum, compared to the uncloaked object. Here, we expand on this topic, discussing in detail an interesting exception to this limit represented by cloaks with static diamagnetism. In this context, we explore the potential of superconducting materials to realize global and local reduction of the scattering cross section. The concepts of plasmonic and mantle cloaking are extended to superconductors, realizing strong and tunable invisibility, with some unique properties stemming from the peculiar electrodynamics of superconductors. We conclude by qualitatively discussing a possible method to derive more stringent local bounds on cloaking.     

Design of two-dimensional elliptically cylindrical invisible cloaks with multiple regions

Two-dimensional(2D) elliptically cylindrical invisible cloaks with multiple regions are designed based on the transformation optics and the complementary media theory. Multiple invisible cloak regions can be obtained by properly using the compressed or folded transformation in each space layer. The constitutive parameter tensor expressions for each region have been obtained. The results of full wave simulations by using finite element software confirm the validity of the constitutive parameter tensor expressions. In addition, the parameters are relatively easier to realize.     

Developing a time-domain finite-element method for modeling of electromagnetic cylindrical cloaks

In this paper we propose a time-domain finite element method for modeling of electromagnetic cloaks. The permittivity and permeability of the cloak model are described by the Drude dispersion model. The model to be solved is quite challenging in that we have to solve a coupled problem with different partial differential equations given in different regions. Our method is based on a mixed finite element method using edge elements with different types of meshes in different regions. Numerical results demonstrate that our algorithm is quite effective for simulating cloaks in time-domain. To our knowledge, this is the first cloak simulation carried out by the time-domain finite element method.     

正N边形柱的隐身条件的严格推导及其隐身特性验证

基于坐标变换理论,提出并推导了正N边形柱的隐身条件,并得到了相应隐身罩材料本构参数张量的通解表达式. 根据导出的本构参数张量,利用电磁仿真软件分别对N取不同值时的三个典型算例进行仿真验证. 仿真结果证实了所得到的本构参数张量的正确性. 考虑到损耗对于隐身效果的影响分析,这些分析结果为隐身物理机理的进一步理解,以及降低对称度隐身罩的设计奠定了理论基础.     

Photoelastic effect and mirage deflection in anisotropic materials

The collinear mirage technique is widely used to measure the thermal diffusivity of semi-transparent materials. However, in a recent paper [A. Salazar, M. Gateshki and A. Sánchez-Lavega: Appl. Phys. Lett. 76, 2665 (2000)], it was shown that for isotropic materials, because of the influence of photoelastic effect, the method was sensitive to the polarization state of the probe beam. The present paper extends the previous work to include anisotropic materials. In particular, we focus on the experimental conditions under which the thermal diffusivity of each crystal system can be measured using the phase method. Our theoretical model indicates that while the thermal diffusivity of isotropic materials can be measured using an unpolarized probe beam, for anisotropic materials, even the use of an unpolarized probe beam does not guarantee the validity of the method in all crystal systems. Experimental measurements performed on cubic, hexagonal and monoclinic crystals confirm the validity of the model. Received: 17 March 2001 / Accepted: 17 March 2001 / Published online: 25 July 2001     

Projecting the Kondo effect: theory of the quantum mirage

A microscopic theory is developed for the projection (quantum mirage) of the Kondo resonance from one focus of an elliptic quantum corral to the other focus. The quantum mirage is shown to be independent of the size and the shape of the ellipse, and experiences lambdaF/4 oscillations ( lambdaF is the surface-band Fermi wavelength) with an increasing semimajor axis length. We predict an oscillatory behavior of the mirage as a function of a weak magnetic field applied perpendicular to the sample.     

正N边形柱的隐身条件的严格推导及其隐身特性验证

基于坐标变换理论,提出并推导了正N边形柱的隐身条件,并得到了相应隐身罩材料本构参数张量的通解表达式. 根据导出的本构参数张量,利用电磁仿真软件分别对N取不同值时的三个典型算例进行仿真验证. 仿真结果证实了所得到的本构参数张量的正确性. 考虑到损耗对于隐身效果的影响分析,这些分析结果为隐身物理机理的进一步理解,以及降低对称度隐身罩的设计奠定了理论基础. 关键词： 坐标变换 非均匀各向异性介质 本构参数张量 隐身     

Extraordinary surface voltage effect in the invisibility cloak with an active device inside

The electromagnetic field solution for a spherical invisibility cloak with an active device inside is established. Extraordinary electric and magnetic surface voltages are induced at the inner boundary of a spherical cloak, which prevent electromagnetic waves from going out. The phase and handness of polarized waves obliquely incident on such boundaries are kept in the reflected waves. The surface voltages due to an electric dipole inside the concealed region are found equal to the auxiliary scalar potentials at the inner boundary, which consequently gain physical counterparts in this case.     

Improving the performance of acoustic invisibility with multilayer structure based on scattering analysis

In this paper, acoustic scattering from the system comprised of a cloaked object and the multilayer cloak with only one single pair of isotropic media is analyzed with a recursive numerical method. The designed acoustic parameters of the isotropic cloak media are assumed to be single-negative, and the resulting cloak can reduce acoustic scattering from an acoustic sensor while allowing it to receive external information. Several factors that may influence the performance of the cloak, including the number of layers and the acoustic dissipation of the medium are fully analyzed. Furthermore, the possibility of achieving acoustic invisibility with positive acoustic parameters is proposed by searching the optimum value in the parameter space and minimizing the scattering cross-section.     

任意多面体隐身罩材料参数的推导及验证

基于坐标变换理论推导了任意多面体隐身罩的隐身条件,并得到了相应隐身罩材料参数的张量表达式.根据导出的材料参数的张量表达式,通过全波仿真分别对正四面体和十四面体隐身罩进行仿真验证,仿真结果证实了所得材料参数张量表达式的正确性,研究结果为三维复杂形状隐身罩的设计奠定了理论研究基础. 关键词： 坐标变换 多面体隐身罩 张量表达式 全波仿真     

Electromagnetic radiation from filamentary sources in the presence of axially magnetized cylindrical plasma scatterers

Electromagnetic radiation from filamentary electric-dipole and magnetic-current sources of infinite length in the presence of gyrotropic cylindrical scatterers in the surrounding free space is studied. The scatterers are assumed to be infinitely long, axially magnetized circular plasma columns parallel to the axis of the filamentary source. The field and the radiation pattern of each source are calculated in the case where the source frequency is equal to one of the surface plasmon resonance frequencies of the cylindrical scatterers. It is shown that the presence of even a single resonant magnetized plasma scatterer of small electrical radius or a few such scatterers significantly affects the total fields of the filamentary sources, so that their radiation patterns become essentially different from those in the absence of scatterers or the presence of isotropic scatterers of the same shape and size. It is concluded that the radiation characteristics of the considered sources can efficiently be controlled using their resonance interaction with the neighboring gyrotropic scatterers.     

三维椭球隐身条件的严格推导及其隐身特性验证

基于坐标变换理论,提出并推导了任意轴比下三维椭球的隐身条件,得到了相应隐身罩材料本构参数张量的通解表达式.根据导出的本构参数张量,利用电磁仿真软件分别对不同轴比的三个典型算例进行仿真验证.仿真结果证实了所导出的本构参数张量的正确性.这些分析结果为隐身物理机理的进一步理解,以及三维隐身罩的设计奠定了更充分的理论基础.     

筒内高功率脉冲磁控放电的电磁控制与优化

高功率脉冲磁控溅射(Hi PIMS)技术被提出以来就受到广泛关注,其较高的溅射材料离化率结合适当的电磁控制,可产生高致密度、高结合力和高综合性能的涂层,但其沉积速率低、放电不稳定、溅射材料离化率差异较大.我们设计了一种筒形溅射源,通过对结构的设计优化,利用类空心阴极放电效应,使问题得到解决.然而其靶面切向磁场不均匀,电子逃逸严重,进而造成等离子体密度偏低,且放电不均匀.本文通过对其放电和等离子体分布进行仿真,提出电场阻挡和磁铁补偿两种方案,研究了不同电场控制条件下的放电行为和等离子体分布.结果表明:增加电子阻挡屏极可以生成势阱,从而有效抑制电子从边缘的逸出;优化后的磁铁补偿可以显著提高靶面横向磁场的均匀性及靶面利用率.两种方案同时作用时,Hi PIMS放电刻蚀环面积更大、且更加均匀.     

An arbitrary-shaped acoustic cloak with merits beyond the internal and external cloaks

Based on transformation acoustics, an arbitrary-shaped acoustic cloak capable of functioning as an information exchange-enabling internal cloak and a movement-allowing external cloak is presented. The general expressions of material parameters for the acoustic cloaks with arbitrarily conformal or non-conformal boundaries are derived, and then the performances of developed cloaks are validated by full-wave simulations. Finally, the different characteristics of the linear and nonlinear transformations-based cloaks are compared and analyzed. The proposed cloak could lead to wider applications beyond that of normal cloaks, since it effectively compensates the insufficiencies of traditional internal and external cloaks. Besides, this work also provides a new method to design bifunctional device and suggests an alternative way to make a large object invisible.     

Rainbow and blueshift effect of a dispersive spherical invisibility cloak impinged on by a nonmonochromatic plane wave

We demonstrate some interesting phenomena associated with a nonmonochromatic plane wave passing through a spherical invisibility cloak whose radial permittivity and permeability are of Drude and Lorentz types. We observe that the frequency center of a quasimonochromatic incident wave will suffer a blueshift in the forward scattering direction. Different frequency components have different depths of penetration, causing a rainbowlike effect within the cloak. The concept of group velocity at the inner boundary of the cloak needs to be revisited. Extremely low scattering can still be achieved within a narrow band.