Electromagnetic wave interactions with a metamaterial cloak

We establish analytically the interactions of electromagnetic wave with a general class of spherical cloaks based on a full wave Mie scattering model. We show that for an ideal cloak the total scattering cross section is absolutely zero, but for a cloak with a specific type of loss, only the backscattering is exactly zero, which indicates the cloak can still be rendered invisible with a monostatic (transmitter and receiver in the same location) detection. Furthermore, we show that for a cloak with imperfect parameters the bistatic (transmitter and receiver in different locations) scattering performance is more sensitive to eta(t)=square root micro(t)/epsilon(t) than n(t)=square root micro(t)epsilon(t).     

Ideal cylindrical cloak: perfect but sensitive to tiny perturbations

A cylindrical wave expansion method is developed to obtain the scattering field for an ideal two-dimensional cylindrical invisibility cloak. A near-ideal model of the invisibility cloak is set up to solve the boundary problem at the inner boundary of the cloak shell. We confirm that a cloak with the ideal material parameters is a perfect invisibility cloak by systematically studying the change of the scattering coefficients from the near-ideal case to the ideal one. However, because of the slow convergence of the zeroth-order scattering coefficients, a tiny perturbation on the cloak would induce a noticeable field scattering and penetration.     

Three dimensional multilayered acoustic cloak with homogeneous isotropic materials

A three-dimensional (3D) spherical acoustic cloak is designed using an acoustic layered system, which can hide an object from the detection of acoustic wave in arbitrary direction. The cloak is constructed from multilayered concentric spherical shells filled with homogeneous isotropic materials. Based on spherical wave expansion method, we confirm that significant low-reflection, acoustic-shadow-reducing, and wavefront-bending effects in 3D space can be achieved by the proposed cloak. The angle distribution of the scattered wave is further evaluated by the far-field scattering pattern. In addition, the cloak is demonstrated to work efficiently in a wide bandwidth in which the cloaking efficiency decreases with increasing frequency. This study may be helpful to design high-performance 3D acoustic cloaks for broadband acoustic waves in all incidence directions.     

Invisibility of a metamaterial cloak illuminated by spherical electromagnetic wave

In this paper, the invisibility of a metamaterial cloak illuminated by spherical electromagnetic wave is analytically investigated based on the full wave Mie scattering model. It is shown that for a cloak with ideal parameters the scattered field intensity is zero, but for a cloak with a loss, only the backscattering is exactly zero. Moreover, in the loss case, the scattered field intensity increases as the loss increases, which is very different from that in the conventional stealth case, where the scattered field intensity decreases as the loss of coated material increases. In addition, it is shown that scattering cross-section of the cloak with perturbed parameters decreases as the thickness of the cloak decreases, which means that thinner cloak can exhibit more stable invisibility.     

运用有限差分时域法计算具有径向色散特性的圆柱形隐形斗篷

提出了一种具有径向色散特性有限差分时域法（FDTD法）的电磁隐形斗篷建模.隐形斗篷的介电常数和磁导率是采用了drude色散模型以及相应的色散FDTD算法.数值模拟表明,在理想的条件下,隐形斗篷中的物体对外电磁场是隐形的.对于入射平面波和点声源,波前平滑地进入隐形斗篷,并绕过中心的物体区域,离开隐形斗篷后又恢复为原来的传播状态.远场散射图表明,与一个理想导体圆柱相比隐形斗篷可以减少沿各个方向的散射.     

基于主动声学超材料的圆柱声隐身斗篷设计研究

基于多层复合材料结构的二维声隐身斗篷设计思想, 利用主动隔膜声学空腔有效密度可以任意控制这一特性, 设计了主动声学超材料下的无限长圆柱声隐身斗篷. 给出了主动隔膜声学空腔单元的声电元件类比模拟电路图和具体的有效密度控制方法. 进行了主动声学超材料声隐身斗篷的结构建模, 并对平面入射波入射下此圆柱隐身斗篷周围声压分布场进行仿真计算. 结果表明, 平面波在一定频率范围内可以毫无阻碍地透过圆柱斗篷, 似乎不存在这种障碍物, 达到声隐身效果. 同时, 计算了主动声材料斗篷下总散射截面随频率变化曲线, 研究了此斗篷隐身效果随频率的变化特性. 本文从主动控制角度探讨实验实现隐身斗篷的技术问题, 有望给声隐身斗篷实验设计提供一条新的技术途径.     

Acoustic cloak with duplex communication ability constructed by multilayered homogeneous isotropic materials

Based on the effective medium theory, we propose a practical implementation of a cylindrical acoustic cloak with a concentric alternating multilayered structure of homogeneous isotropic materials, which can perfectly mimic the ideal radius-dependent and anisotropic ordinary lens cloak. The proposal exhibits near-ideal cloaking performance such as low-scattering and shadow-reducing in a wide frequency range, thus it can hide an object from the detection of acoustic waves. The acoustic wave can pass through the cloaking shell with an unchanged wavefront shape, which endues the cloaked object with duplex communication ability. More simulations on the acoustic far-field scattering patterns and the total scattering cross-section are performed to investigate the layer number and the frequency dependence of the cloaking effect, and the results show that the thinner layers exhibit a better cloaking effect. The proposal may significantly facilitate the experimental demonstration of the acoustic cloak.     

Bandwidth enhancement of transformation optics-based cloak with reduced parameters

In this paper, dispersive cloak design with broad bandwidth and minimal scattering cross section is proposed by appropriately selecting a radial permeability for each shell in a discretized reduced cloak. The dispersive medium is constructed by artificially varying the inner radius of the cloak with frequency, and this variation results into unique material properties at every frequency. The variation of inner radius of the cloak with frequency is artificial since the actual physical dimension of inner radius remains invariant. The relation between bandwidth and geometrical parameters of cloak is obtained by ensuring that transformation media must satisfy the condition that group velocity must remain less than the speed of light along every direction for a finite frequency range. The proposed cloak provides \(8.9\,\%\) bandwidth with respect to the center frequency for \(50\,\%\) reduction in total scattering cross section, and at the design frequency, the minimum scattering cross section obtained is \(0.266\). The proposed dispersive cloak design is verified by numerical full-wave simulations results which also confirm good cloaking performance.     

Simplified triangular ground plane cloak by oblique multilayer dielectrics

Based on the effective medium theory, the triangular ground plane cloak can be realized by thin layered systems. Two solutions of parameter setting of the layered cloak are suggested to demonstrate the invisibility performance of a hybrid incoming wave. The hybrid parameters are derived from the equivalent of both anisotropies of permittivity and permeability to the alternating layers. The performance of the designed layered cloak is validated by both TM and TE wave simulations with near-field distributions and average scattering power outflows on an observation semicircle. From the simulation results, the layered cloak with both hybrid parameters and improved hybrid parameters can reflect the incoming TM/TE waves in a specular direction, and the latter behaves with a better overall invisibility performance.     

Scattering characteristics of nonmagnetic invisibility cloak with minimized scattering

A kind of transformation functions is proposed to realize the nonmagnetic invisibility cloak with minimized scattering on the basis of generalized transformation. By matching the impedance at the outer surface of the cloak, the transformations with two parameters are determined. The good performance of the cloak is indicated by the full wave simulation based on the finite element method. Furthermore, based on the calculation of total scattering cross section, it is shown that the scattering cross section is very sensitive to the different parameters even though the impedance at the exterior boundary matches perfectly with the free space. In addition, from the effective media theory, an alternating layered system composed of two isotropic materials is proposed to realize experimentally the cloak.     

基于各向同性材料的层状椭圆柱形声隐身衣设计

基于等效介质理论, 提出了具有共焦层状结构的椭圆柱形声隐身衣设计方法. 理论分析与有限元数值模拟表明, 所设计隐身衣依然具有完美隐身衣典型特征, 可使刚性圆柱体散射场明显减小, 并且在隐身衣区域表现出波阵面弯曲的特性, 同时在隐身衣外部波阵面保持不变. 增加隐身衣离散层数可以拓宽其有效工作频带, 改善隐身效果. 由于是一种线变换隐身衣, 隐身效果受到了入射波方向的影响, 只有当入射波方向与椭圆长轴平行时效果最佳. 另外当椭圆柱焦距非常小的时候, 可近似认为是圆柱形隐身衣. 仿真实验结果证明了方法的正确性. 该研究为实现复杂形状声隐身衣提供了一种有效途径.     

Electromagnetic detection of a perfect cloak based on the material nonlinear response

Methods of detecting a perfect invisible cloak are analyzed. A classical frequency-domain method of detection is proposed. It is based on the detection of the difference between the nonlinear responses of the materials of the cloak and the background. The cloak treated here is perfect working within any frequency. The method for detecting the cloak in the frequency domain is demonstrated numerically for an example.     

Broadband acoustic cloak for ultrasound waves

Invisibility devices based on coordinate transformation have opened up a new field of considerable interest. We present here the first practical realization of a low-loss and broadband acoustic cloak for underwater ultrasound. This metamaterial cloak is constructed with a network of acoustic circuit elements, namely, serial inductors and shunt capacitors. Our experiment clearly shows that the acoustic cloak can effectively bend the ultrasound waves around the hidden object, with reduced scattering and shadow. Because of the nonresonant nature of the building elements, this low-loss (～6 dB/m) cylindrical cloak exhibits invisibility over a broad frequency range from 52 to 64?kHz. Furthermore, our experimental study indicates that this design approach should be scalable to different acoustic frequencies and offers the possibility for a variety of devices based on coordinate transformation.     

Two-dimensional metamaterial structure exhibiting reduced visibility at 500 nm

Metamaterials provide unprecedented freedom and flexibility in the creation of new structures, which control electromagnetic wave propagation in very unusual ways. Very recently various theoretical designs for an electromagnetic cloak were suggested and an experimental realization of a partial cloak operating in a two-dimensional cylindrical geometry were reported in the microwave frequency range. We report on an experimental two-dimensional reduced visibility structure that approximates the distribution of the radial component of the dielectric permittivity necessary to achieve nonmagnetic cloaking in the visible frequency range.     

Impedance-Matched Reduced Acoustic Cloaking with Realizable Mass and Its Layered Design

We present an impedance-matched reduced version of acoustic cloaking whose mass is in a reasonable range. A layered cloak design with isotropic material is also proposed for the reduced cloak. Numerical calculations from the transfer matrix methods show that the present layered cloak can reduce the scattering of an air cylinder substantially.     

圆柱形分层五模材料声学隐身衣的理论与数值分析

声学隐身衣物性参数分布的连续对制备造成了很大困难, 需要采用分层的方法进行近似. 研究分层对隐身衣性能的影响具有重要意义. 本文首先推导了圆柱形分层声学隐身衣散射声压场的理论解, 然后通过数值算例验证了理论推导的正确性, 最后针对层数、层厚分布对隐身衣性能的影响进行了计算研究. 结果表明, 选取恰当的层数和层厚分布可在不增加制备难度的同时改善隐身衣性能.     

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.     

Extraordinary Resonant Scattering in Imperfect Acoustic Cloak

We present a systematic study on the extraordinary resonant scattering in imperfect acoustic cloak by means of acoustic scattering theory. Analysis results demonstrate that the resonances are inevitable due to the perturbation to the ideal clo~k, and specific resonance modes are excited by specific order waves. The strength of resonance is determined by the magnitude of perturbation and each order wave＇s sensitivity to the perturbation. Further studies reveal the unique scattering characters of different resonance modes.     

Broadband cloaking in stratified seas

Here we show that floating objects in stratified fluids can be cloaked against broadband incident waves by properly architecting the bottom corrugations. The presented invisibility cloaking of gravity waves is achieved utilizing a nonlinear resonance concept that occurs between surface and internal waves mediated by the bottom topography. Our cloak bends wave rays from the surface into the body of the fluid. Wave rays then pass underneath the floating object and may be recovered back to the free surface at the downstream bearing no trace of diffraction or scattering. The cloak is the proper architecture of bottom corrugations only, and hence is surface noninvasive. The presented scheme is a nonlinear alternative to the transformation-based cloaking, but in the context of dispersive waves.     

Reciprocal invisibility cloak based on complementary media

The first invisibility cloak was proposed by Pendry et al. [Science 312, 1780 (2006)]. But the object enclosed in this original cloak is “blind", that is, it cannot see the outside world, since no electromagnetic waves can reach within the cloaked space. Based on the concept of complementary media, we propose a reciprocal invisibility cloak, in which the hidden object can see the outside world, but its presence cannot be detected by electromagnetic wave. The performance of the cloak has been verified by full-wave simulations.