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.     

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.     

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.     

任意横截面柱形热斗篷研究与设计

基于变换热力学,导出了具有非共形任意横截面的柱形热斗篷热导率表达式,并在此基础上设计了具有非共形横截面的柱形热斗篷.全波仿真结果表明,热斗篷迫使外部热流绕过斗篷,导致隐身区域热通量为零,从而具有热保护功能;同时,热流绕过斗篷后将恢复原来的温度场分布,使其具有完美热隐身功能.此外,基于所导出的变换媒质热导率表达式,设计并求解了具有规则共形和非规则共形横截面的柱形热斗篷,发现它们同样具有热保护和完美热隐身功能.这表明通过选择适当的边界函数,所得变换媒质热导率表达式可用于设计任意横截面柱形热斗篷,具有普遍的适用性,这种技术在计算机芯片、卫星和航天器等的热保护中有潜在应用.     

While rotating while cloaking

Invisible optical and thermal cloaking have been explored as the typical demonstrations of the transformation optics and thermotics theory. However, the existing cloaks are realized by only one-coordinate transformation, and the cloaking layout, i.e. the form of electromagnetic wave/heat passing around the invisible region, is single for a long time. Here, we propose a new rotated thermal cloak which can unify the conventional cloaking and rotating together, and realize the while-rotating–while-cloaking effect. The required anisotropic thermal conductivity tensor is deduced from the new geometric mapping. Though rotated, the heat flux can be tuned around the central invisible region perfectly by the proposed rotated thermal cloak. The underlying physics is explored by comprehensive analysis of the distribution of the thermal conductivity tensor, which is further compared with those of the conventional cloak and rotator. The experimental feasibility is also discussed by validating the practical while-rotating–while-cloaking effect through a proof-of-concept design. The proposed rotated thermal cloak is expected to extend the possibility of cloaking scheme, and open avenues for the multiple coordinate transformation in counterpart physical fields, like optics, electrics, acoustics, magnetics, mechanics, etc.     

Invisible anti-cloak with elliptic cross section using phase complement

Based on the theory of phase complement,an anti-cloak with circular cross section can be made invisible to an object outside its domain.As the cloak with elliptic cross section is more effective to make objects invisible than that with circular cross section,a scaled coordinate system is proposed to design equivalent materials of invisible anticloak with elliptic cross section using phase complement.The cloaks with conventional dielectric and double negative parameters are both simulated with the geometrical transformations.The results show that the cloak with elliptic cross section through phase complement can effectively hide the outside objects.     

基于变换热力学的三维任意形状热斗篷设计

在变换热力学的基础上,通过坐标变换的方法,推导出三维任意形状热斗篷导热系数的通解表达式,并进行了全波仿真验证.结果表明:热流均能绕过保护区域流出,保护区域的温度保持不变,而且热斗篷外的温度场并没有破坏,具有很好的热保护和热隐身的效果.这一方法把变换热力学从二维拓展到三维,具有普遍的适用性.同时,这种技术为热流流动路径和目标温度场的控制奠定了理论基础,在微芯片、电动机的保护以及目标热隐身上有潜在应用.     

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.     

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.     

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).     

Cloaks,editors, and bubbles: applications of spacetime transformation theory

Spacetime or ‘event’ cloaking was recently introduced as a concept, and the theoretical design for such a cloak was presented for illumination by electromagnetic waves [McCall et al., J. Opt. 2011]. Here it is described how event cloaks can be designed for simple wave systems, using either an approximate ‘speed cloak’ method, or an exact full‐wave one. Further, details of many of the implications of spacetime transformation devices are discussed, including their (usually) directional nature, spacetime distortions (as opposed to cloaks), and how leaky cloaks manifest themselves. More exotic concepts are also addressed, in particular concepts that follow naturally on from considerations of simple spacetime transformation devices, such as spacetime modeling and causality editors. A proposal for implementing an interrupt‐without‐interrupt concept is described. Finally, the design for a time‐dependent ‘bubbleverse’ is presented, based on temporally modulated Maxwell's Fisheye transformation device (T‐device) in a flat background spacetime.     

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.     

The design of metamaterial cloaks embedded in anisotropic medium

By using coordinate transformation method, this paper obtains an useful equation of designing meta-material cloaks embedded in anisotropic medium. This equation is the generalization of what was introduced early by Pendry et al (2006 \textit{Science} {312 1780) and can be more widely used. As an example of its applications, this paper deduces the material parameter equation for cylinder cloaks embedded in anisotropic medium, and then offers the numerical simulation. The results show that such a cylinder cloak has perfect cloaking performance and therefore verifies the method proposed in this paper.     

Bandwidth evaluation of dispersive transformation electromagnetics based devices

In this paper, the transient responses of some devices which are based on transformation electromagnetics are studied, such as invisible cloaks and concentrators, by using the Finite-Difference Time-Domain (FDTD) numerical technique. In particular, effects of the inherent losses as well as the coating size of the ideal cylindrical cloak on its bandwidth and cloaking performance are examined. In addition, it is demonstrated that the performance of transformation electromagnetics based devices is affected by the material parameters in the design, although they may behave nicely under monochromatic plane wave illuminations. The obtained results are of interest for the future practical implementation of these structures.     

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.     

新型椭圆形互补隐身斗篷设计

基于变换光学理论和互补媒质理论,提出了新型椭圆形互补隐身斗篷的设计方法,并得到了本构参数张量表达式,利用基于有限元算法的电磁仿真软件对该模型进行了全波仿真验证,结果证实了所得到的表达式的正确性,这种隐身斗篷不仅能够实现对位于其内部的物体隐身,而且电磁波能透进隐身斗篷内部进而可以与外界进行通信。     

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.     

Quasi-invisible thermal cloak based on homogenous materials

Invisible thermal cloak, which cancels distortions of temperature distribution caused by objects, has many potential applications in thermal engineering. In this letter, we theoretically proposed and simulatively verified a new design method for quasi-invisible thermal cloak. Different from conventional transformation thermodynamics that focus on complete invisibility, our method only decreases the effective scale of objects to small enough and realizes a quasi-invisible cloaking effect in thermal conduction regime. However, this quasi-invisible cloak has the same effect as that of invisible thermal cloak in practical engineering. More important, our cloak is easy to construct by natural materials due to its homogenous thermal properties and can cloak objects with different shapes and properties. These characters make the clock more comfortable for engineering applications.     

二维椭圆形散射转移斗篷的设计研究

基于变换光学理论设计了椭圆形散射转移斗篷,并推导了相应的本构参数张量表达式,根据得到的本构参数,利用基于有限元算法的软件分别对椭圆形介质柱、导体柱进行了全波仿真验证.仿真结果证实了所得本构参数的正确性,验证了散射转移斗篷可以模拟任意物体,包括介质、导体的幻象,这些结果为设计隐身斗篷提供了一种新的理论途径.     

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.