On the sensitivity of the 2D electromagnetic invisibility cloak

A computational study of the sensitivity of the two dimensional (2D) electromagnetic invisibility cloaks is performed with the finite element method. A circular metallic object is covered with the cloak and the effects of absorption, gain and disorder are examined. Also the effect of covering the cloak with a thin dielectric layer is studied.     

Ellipsoidal Thermal Concentrator and Cloak with Transformation Media

Thermal concentrators and cloaks with ellipsoidal shapes are designed by utilizing the transformation thermotics method and finite element simulations.The thermal conductivities for the concentrator and cloak are directly derive in Cartesian coordinates.The simulation results show that the ellipsoidal thermal concentrator can focus heat flux into a central region and that the ellipsoidal thermal cloak can guide heat flux around the cloaked region smoothly without disturbing the external temperature distribution and heat flux.The present method can be extended to design arbitrarily shaped thermal metadevices with novel properties.     

Cylindrical invisibility cloak with simplified material parameters is inherently visible

It was proposed that perfect invisibility cloaks can be constructed for hiding objects from electromagnetic illumination [J. B. Pendry, D. Schurig, and D. R. Smith, Science 312, 1780 (2006)10.1126/science.1125907]. The cylindrical cloaks experimentally demonstrated [D. Schurig, Science 314, 977 (2006)10.1126/science.1133628] and theoretically proposed [W. Cai, Nat. Photon. 1, 224 (2007)10.1038/nphoton.2007.28] have however simplified material parameters in order to facilitate easier realization as well as to avoid infinities in optical constants. Here we show that the cylindrical cloaks with simplified material parameters inherently allow the zeroth-order cylindrical wave to pass through the cloak as if the cloak is made of a homogeneous isotropic medium, and thus visible. To all high-order cylindrical waves, our numerical simulation suggests that the simplified cloak inherits some properties of the ideal cloak, but finite scatterings exist.     

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.     

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.     

Two-dimensional dissimilar electromagnetic cloak for irregular regions

A general method to design a 2D dissimilar cloak for irregular regions is presented by operating a nonlinear transformation in polar coordinates. The material parameters avoid discontinuities while the thickness of the cloak shell is effectively limited in elongated directions. Full-wave simulations of an elliptic cloak, a rectangular cloak, and an arbitrary-shape cloak are performed to verify the validity of the design. Both the material parameters and the scattering widths of different models are calculated and illustrated for comparison. This method provides a possible approach for designing complex shaped cloaks.     

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.     

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.     

Nonsingularity in two-dimensional cylindrical invisible cloaks

The method of designing electromagnetic invisible cloaks is usually based on the form-invariance of Maxwell's equations in coordinate transformation. The exterior boundary of a cylindrical invisible cloak is unchanged and the interior boundary is extended from that of a point to that of a cylindrical region in coordination transformation. This transformation process makes perfect cloaks, but it causes singularity in the constitutive material parameters of cloaks. This singularity makes the cloaks impossible to realize in practice. In order to remove this singularity, this paper sets a small cylindrical region replacing a point in the space transformation. The cylindrical region is so small that it does not affect the invisibility effects, but it can remove the singularity for material parameters. Full wave simulations based on the finite element method were used to verify the designed cloaks.     

Multi-window invisible cloaks

This paper reports that a general method of designing invisible cloaks is using variant constitutive material parameters to realize the space transformation. A hollow region can be hidden after this transformation. It was recently shown (Ma H, Qu S B, Xu Z and Wang J F 2009 \wx{Appl. Phys. Lett.}{94} 103501) that when the original point moves to the boundary of a cloak, the cloak can be designed to be open. Based on this theory, we propose multi-window invisible cloaks which can conceal a group of objects. Full wave simulations for invisible cloaks with regular and irregular shapes verified this method.     

Multifrequency super-thin cloaks

Bandwidth and thickness have become the most troublesome problems for EM cloaks. In this paper, we propose to solve the two problems using connected patches based on the microwave network model. By covering an obstacle with combined connected patches, cloaking effect can be achieved at multiple frequencies so as to expand the operating band. As an explicit example, a dual-band super-thin cloak using two different connected patch unit cells is demonstrated. Cloaking effect can be achieved at 3.50 GHz and 4.14 GHz simultaneously with an 8 dB transmission enhancement. The cloak design method provides a new route to broadening the bandwidth of thin EM cloaks.     

Examination of the Thermal Cloaking Effectiveness with Layered Engineering Materials

The concentrically layered thermal cloaks with isotropic materials could realize the equivalent thermal cloaking effect with Pendry's cloak,while the effectiveness is scarcely investigated quantitatively.Here we examine the cloaking effectiveness quantitatively by evaluating the standard deviation of the temperature difference between the simulated plane with the layered thermal cloak and Pendry's thermal cloak.The design rules for the isotropic materials in terms of thermal conductivity and layer thickness are presented.The present method could quantitatively evaluate the cloaking effectiveness,and could open avenues for analyzing the cloaking effect,detecting the(anti-) cloaks,etc.     

Two-Dimensional （2D） Polygonal Electromagnetic Cloaks

Transformation optics offers remarkable control over electromagnetic fields and opens an exciting gateway to design ＇invisible cloak devices＇ recently. We present an important class of two-dimensional （2D） cloaks with polygon geometries. Explicit expressions of transformed medium parameters are derived with their unique properties investigated. It is found that the elements of diagonalized permittivity tensors are always positive within an irregular polygon cloak besides one element diverges to plus infinity and the other two become zero at the inner boundary. At most positions, the principle axes of permittivity tensors do not align with position vectors. An irregular polygon cloak is designed and its invisibility to external electromagnetic waves is numerically verified. Since polygon cloaks can be tailored to resemble any objects, the transformation is finally generalized to the realization of 2D cloaks with arbitrary geometries.     

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.     

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).     

TE波辐射下超材料椭圆柱电磁斗篷特性分析

 利用有限元方法分析了TE波辐射下,超材料椭圆柱斗篷的电磁特性。研究发现：当超材料的介电常数、磁导率和电导率偏离理论值0.5%时,斗篷外的电场发生了明显变化;当材料的介电常数、磁导率和电导率为理论值时,不论在斗篷内放置什么特性的材料,也不论斗篷内壁上的边界条件如何,超材料椭圆柱斗篷都具有隐形作用。     

Analysis of elliptical thermal cloak based on entropy generation and entransy dissipation approach

In this work, we designed the elliptical thermal cloak based on the transformation thermotics. The local entropy generation rate distribution and entransy dissipation rate distribution were obtained, and the total entropy generation and entransy dissipation of different types of elliptical cloaks were evaluated. We used entropy generation approach and entransy dissipation approach to evaluate the performance of the thermal cloak, and heat dissipation analysis was carried out for models with different parameters. Finally, the optimized elliptical thermal cloak with minimum entropy generation and minimum entransy dissipation is found, and some suggestions on optimizing the structure of elliptical thermal cloak were given.     

Isotropic Thermal Cloaks with Thermal Manipulation Function

By extending the conventional scattering canceling theory,we propose a new design method for thermal cloaks based on isotropic materials.When the objects are covered by the designed cloaks,they will not disturb the temperature profile in the background zone.In addition,if different inhomogeneity coefficients are selected in the thermal cloak design process,these cloaks can manipulate the temperature gradient of the objects,i.e.,make the temperature gradients higher,lower,or equal to the thermal gradient in the background zone.Therefore,thermal transparency,heat concentration or heat shield effects can be realized under a unified framework.     

Numerical method of designing three-dimensional open cloaks with arbitrary boundary shapes

In this paper, three-dimensional (3-D) open cloaks were designed based on the coordinate transformation method. When the transformation center point is close to the boundary of the cloaks, the material parameter tensors in the near-boundary areas of the cloaks approximate to those in the background, so the near-boundary areas can be designed to be open. Full wave simulations based on finite element method verified the open cloaks we designed. Open cloaks can exchange information and materials with background media. Using the design method proposed in this paper, the boundary of open cloaks can be arbitrarily shaped, which greatly enhances the applicability of open cloaks.     

Tunable Visible Cloaking Using Liquid Diffusion

A method is introduced for the design of invisibility cloaks inspired by fluid dynamics that is different from traditional transformation optics. The inhomogeneous refractive index of the liquid cloak controlled by the natural liquid diffusion is analogous to its counterpart designed by transformation optics. Here, a tunable liquid visible cloak is experimentally presented by the natural diffusion of miscible flows. This method avoids the use of complex nanostructures in its solid counterpart, and provides a simple and low‐cost approach. This implies that optofluidics can be used as a technology to make real‐time reconfigurable transformation optic devices.