Experimental investigation of omnidirectional multiphysics bilayer invisibility cloak with anisotropic geometry

Thermal-electric bilayer invisibility cloak can prevent the heat flux and electric current from touching the object without distorting the external temperature and electric potential fields simultaneously.In this paper,we design an omnidirectional thermal-electric invisibility cloak with anisotropic geometry.Based on the theory of neutral inclusion,the anisotropic effective thermal and electric conductivities of confocal elliptical bilayer core-shell structure are derived,thus obtaining the anisotropic matrix material to eliminate the external disturbances omnidirectionally.The inner shell of the cloak is selected as an insulating material to shield the heat flux and electric current.Then,the omnidirectional thermal-electric cloaking effect is verified numerically and experimentally based on the theoretical anisotropic matrix and manufactured composite structure,respectively.Furthermore,we achieve the thermal-electric cloaking effect under a specific direction of heat flux and electric current using the isotropic natural materials to broaden the selection range of materials.The method proposed to eliminate anisotropy and achieve the omnidirectional effect could also be expanded to other different physical fields for the metadevices with different functions.

Electrodynamics of transformation-based invisibility cloaking

The existing knowledge on the electrodynamics of invisibility cloaking based on transformation optics is reviewed from an integrated science and engineering perspective.Several significant electromagnetic problems that have resulted in intense discussions in the past few years are summarized in terms of propagation,scattering,radiation and fabrication.Finally,the road ahead toward invisibility cloaking and transformation optics is discussed from the viewpoint of the author.

Revealing the Transformation Invariance of Full-Parameter Omnidirectional Invisibility Cloaks

Searching for an optimal solution among many nonunique answers provided by transformation optics is critical for many branches of research,such as the burgeoning research on invisibility cloaks.The past decades have witnessed rapid development of transformation optics,and different kinds of invisibility cloaks have been designed and implemented.However,the available cloaks realized thus far have been mostly demonstrated with reduced parameters,which greatly impact the predefined cloaking performance.Here,we report a general design strategy to realize full-parameter omnidirectional cloaks that can hide arbitrarily shaped objects in free space.Our approach combines a singular transformation with transformation-invariant metamaterials.The cloaking device with extreme parameters is implemented using a metallic array structure.In the experiment,two cloak samples are designed and fabricated,one with nondiscrete cloaking regions and the other with separated hidden regions.Near-unit transmission of electromagnetic waves with arbitrary incident angles is experimentally demonstrated along with significantly suppressed scattering.Our work challenges the prevailing paradigms of invisibility cloaks and provides deep insight into how transformation optics could be harnessed to obtain easily-accessible metadevices.

Cloaking an object in a half space without reflection

A novel cloaking scheme to hide an object in a half space from electromagnetic （EM） detection without reflection is firstly presented. The proposed cloaking scheme contains a couple of matching strips, which consist of an isotropic material layer and an anisotropic UPML layer, located right under the bottom surface of a semi-cylindrical cloaking shell. Simple expressions for the material parameters of the cloaking scheme are derived. Numerical simulations are also performed, and a good cloaking effect is achieved. The cloaking scheme is effective to hide the local object with strong scattering characters placed on mobile carders, such as the radar antenna system on an aircraft.

Achieving acoustic cloak by using compressible background flow

We propose a scheme of acoustic spherical cloaking by means of background irrotational flow in compressible fluid.The background flow forms a virtual curved spacetime and directs the sound waves to bypass the cloaked objects. To satisfy the laws of real fluid, we show that spatially distributed mass source and momentum source are necessary to supply. The propagation of sound waves in this system is studied via both geometric acoustics approximation and full wave approach.The analytic solution of sound fields is obtained for plane wave incidence. The results reveal the effect of phase retardation（or lead） in comparison with the ordinary transformation-acoustic cloak. In addition, the ability of cloaking is also evaluated for unideal background flows by analyzing the scattering cross section.

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.

Experimental demonstration of an invisible cloak with irregular shape by using tensor transmission line metamaterials

We present the design and the experimental demonstration of an invisible cloak with irregular shape by using tensor transmission line（TL） metamaterials. The fabricated cloak consists of tensor TL unit cells exhibiting anisotropic effective material parameters, while the background medium consists of isotropic TL unit cells. The simulated and the measured field patterns around the cloak show a fairly good agreement, both demonstrate that the fabricated cloak can shield the cloaked interior area from electromagnetic fields without perturbing the external fields. The scattering of the cloaked perfect electric conductor（PEC） is minimized. Furthermore, the nonresonant property of the TL structure results in a relatively broad bandwidth of the realized cloak, which is clearly observed in our experiment.

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 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.

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 trans-formation 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 re- gion 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.

Optimization of loss and gain multilayers for reducing the scattering of a perfect conducting cylinder

Reduction of electromagnetic scattering from a conducting cylinder could be achieved by covering it with optimized multilayers of normal dielectric and plasmonic material. The plasmonic material with intrinsic losses could degrade the cloaking effect. Using a genetic algorithm, we present the optimized design of loss and gain multilayers for reduction of the scattering from a perfect conducting cylinder. This multilayered structure is theoretically and numerically analyzed when the plasmonic material with low loss and high loss respectively is considered. We demonstrate by full-wave simulation that the optimized nonmagnetic gain-loss design can greatly compensate the decreased cloaking effect caused by loss material,which facilitates the realization of practical electromagnetic cloaking, especially in the optical range.

Total Reflection and Cloaking by Triangular Defects Embedded in Zero Index Metamaterials

In this work,we investigate wave propagation through a zero index meta-material(ZIM)waveguide embedded with triangular dielectric defects.We provide a theoretical guidance on how to achieve total reflection and total transmission(i.e.,cloaking)by adjusting the defect sizes and/or permittivities of the defects.Our work provides a systematical way in manipulating wave propagation through ZIM in addi-tion to the widely studied dielectric defects with cylindrical and rectangular geome-tries.

Approximate acoustic cloaking in inhomogeneous isotropic space

We consider the approximate acoustic cloaking in an inhomogeneous isotropic background space.By employing transformation media,together with the use of a sound-soft layer lining right outside the cloaked region,we show that one can achieve the near-invisibility by the"blow-up-a-small-region"construction.This is based on novel scattering estimates corresponding to multiple multi-scale obstacles located in an isotropic space.We develop a novel system of integral equations to decouple the nonlinear scattering interaction among the small obstacle components,the regular obstacle components and the inhomogeneous background medium.

Theory, experiment and applications of metamaterials

In this review article, a brief introduction on the theory, experiments and applications of metamaterials is presented. The main focuses are concentrated on the composing meta-atoms, the method of transformation optics, the experimental demonstration of negative refraction, and the realizations of invisibility cloaks and electromagnetic black hole. At the end of this review, some typical applications of metamaterials, including high-performance antennas made of zero-refractive-index materials, inhomogeneous metamaterial lenses, and planar metasurfaces, are introduced in details.