Super-thin cloaks mediated by spoof surface plasmons

We demonstrated the possibility of designing super-thin electromagnetic cloaks based on spoof surface plasmon (SSP). Using a metamaterial layer, incident waves can be coupled into SSP efficiently at the air/metamaterial interface. Due to the strong surface confinement of SSP, EM waves are squeezed into and propagate in deep sub-wavelength scales. Implementation of an 8.2 GHz cloak less than 1/50 the cloaking diameter was presented using split ring resonator (SRR). Excellent cloaking effect was verified by simulations. Rather than isolating objects from the background, such cloaks can drastically enhance the field intensity around the cloaked object. This is of particular importance in applications such as weak wave detection and near-field sensing.     

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.     

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.     

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.     

Full-wave parallel dispersive finite-difference time-domain modeling of three-dimensional electromagnetic cloaking structures

A parallel dispersive finite-difference time-domain (FDTD) method for the modeling of three-dimensional (3-D) electromagnetic cloaking structures is presented in this paper. The permittivity and permeability of the cloak are mapped to the Drude dispersion model and taken into account in FDTD simulations using an auxiliary differential equation (ADE) method. It is shown that the correction of numerical material parameters and the slow switching-on of source are necessary to ensure stable and convergent single-frequency simulations. Numerical results from wideband simulations demonstrate that waves passing through a three-dimensional cloak experience considerable delay comparing with the free space propagations, as well as pulse broadening and blue-shift effects.     

Carpet Cloak Design for Rough Surfaces

Conventional carpet cloak structures have been utilized to conceal the objects located on a planar perfect electric conductor surface. We systematically investigate hiding arbitrarily shaped objects on a rough surface, as a more general and practical scenario. In addition, the required cloak is designed considering different boundary conditions for the surface beneath the object, despite the previous studies. To achieve an invisibility cloak, taking advantage of linear coordinate transformation, a simple homogeneous material is obtained to realize the cloak structure, facilitating the fabrication processes. Numerical simulations validate the performance of the proposed cloaking method. Therefore, the proposed structure is capable of cloaking in more general and complicated scenarios.     

A revisit to imperfect acoustic cloak of multi-layered shell structures considering sound speed and impedance matching

An imperfect multi-layered acoustic cloak is proposed for a two-dimensional cloaking zone based on feasible material properties. In this model, the matching of sound speed and acoustic impedance has been investigated, and the effects of material and geometric properties on the imperfect cloak have been studied for better design of the imperfect cloak. The imperfect cloak could be improved using appropriate changes in the design parameters. By increasing the thickness of the high density layer and with some changes in the sound speeds between the high density and the low density layers, the imperfect cloaking model showed better cloaking performance than Cummer–Schurig cloak. Also, present results show that the sound speed matching is more important for acoustic cloaking than the impedance matching. These results can be applied as a practical design guide for two-dimensional cloaks using multilayered structures composed of naturally existing materials.     

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.     

Construction and mechanism analysis on nanoscale thermal cloak by in-situ annealing silicon carbide film

In recent years,there is a strong interest in thermal cloaking at the nanoscale,which has been achieved by using graphene and crystalline silicon films to build the nanoscale thermal cloak according to the classical macroscopic thermal cloak model.Silicon carbide,as a representative of the third-generation semiconductor material,has splendid properties,such as the high thermal conductivity and the high wear resistance.Therefore,in the present study,we build a nanoscale thermal cloak based on silicon carbide.The cloaking performance and the perturbation of the functional area to the external temperature filed are analyzed by the ratio of thermal cloaking and the response temperature,respectively.It is demonstrated that silicon carbide can also be used to build the nanoscale thermal cloak.Besides,we explore the influence of inner and outer radius on cloaking performance.Finally,the potential mechanism of the designed nanoscale thermal cloak is investigated by calculating and analyzing the phonon density of states(PDOS)and mode participation rate(MPR)within the structure.We find that the main reason for the decrease in the thermal conductivity of the functional area is phonon localization.This study extends the preparation method of nanoscale thermal cloaks and can provide a reference for the development of other nanoscale devices.     

Dynamic study and applications of metamaterial systems

We investigate the dynamic characteristics of metamaterial systems, such as the temporal coherence gain of the superlens, the causality limitation on the ideal cloaking systems, the relaxation process and essential elements in the dispersive cloaking systems, and the extending of the working frequency range of cloaking systems. The key point of our study is the physical dispersive properties of metamaterials, which are well-known to be intrinsically strongly dispersive. With physical dispersion, new physical pictures can be obtained for the waves propagating inside metamaterial, such as the “group retarded time” for waves inside the superlens and cloak, the causality limitation on real metamaterial systems, and the essential elements for design optimization. Therefore, we believe the dynamic study of metamaterials will be an important direction for further research. All theoretical derivations and conclusions are demonstrated by powerful finite-difference time-domain simulations.     

隐声衣结构设计和实验研究新进展

隐声衣是一项使物体隐藏于声场的新技术,与传统吸声方式相比,隐声衣消除回波时不会在目标背后留下声影区。隐声衣的物理实现是重点研究方向之一,利用具有特殊性质的材料或结构消除散射是获得隐声效果的主要途径。文章综述了隐声衣研究在结构设计和实验方面的几项新发展。包括基于超常材料的隐声衣、基于温度递度的隐声衣、应用反演设计方法的隐声衣、有源隐声衣等,主要介绍隐声衣机理和结构设计方面的新思想,以及新型人工材料和人工结构在隐声衣研究中的应用。     

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.     

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.     

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.     

An integral equation approach to acoustic cloaking

The paper presents an original boundary integral equation (BIE) formulation for the analysis of the acoustic cloaking of a scatterer. The advantage of such an approach is the lower computational burden, especially when the analysis of a large portion of the hosting domain is required. The partial differential equation governing the propagation inside the cloak is recast in the form of non-homogeneous wave equation, with field sources depending on the mechanical properties of the cloak. The boundary integral formulation is derived using the standard procedure. The boundary element method (BEM) is used to derive the matrix transfer function of the cloak. The latter is applied to the incident field at the cloak's outer boundary to obtain the total field at arbitrary locations in the host. The formulation is applied to the simple case of a radially symmetric cloak embedding a circular obstacle. Numerical results are presented for sound-hard and sound-soft obstacles, including a study of the cloaking efficiency as a function of the frequency.     

一种适用于对称截面的主动式热斗篷的研究

直接求解导热方程对具有自适应热源的可控式热隐身技术进行数值研究，探索热流控制方法及隐身效果，推导出均匀背景下二维对称截面热斗篷热隐身区域自适应热源分布的通解.仿真结果表明：对于任意对称截面斗篷，自适应热源都能使得扰动后的温度场很好地恢复到背景温度场，达到热隐身的目的.     

Performance investigation of nanoscale thermal cloak by the perforated silicon film

In recent years, nanoscale thermal cloak, as a representative of nanoscale heat flux regulation devices, has attracted a lot of attention from researchers. However, the existing design methods are relatively complicated and all adopt constant temperature boundaries, the temperature changes constantly in the real environment, which greatly hinders its engineering applications. In this paper, inspired by phonon localization theory, we construct a nanoscale thermal cloak by a perforated silicon membrane and evaluate its cloaking performance and dynamic response. Results show that when the perforated area is fixed, the more the number of holes, the better the cloaking performance. In addition, the nanoscale thermal cloak still exhibits good cloaking performance in the dynamic environment. Finally, the cloaking mechanism is analyzed by calculating the phonon density of states (PDOS) and mode participation rate (MPR), and the reduction of thermal conductivity in the functional region is attributed to phonon localization.     

Experiments on elastic cloaking in thin plates

Following a theoretical proposal [M. Farhat et al., Phys. Rev. Lett. 103, 024301 (2009)], we design, fabricate, and characterize a cloaking structure for elastic waves in 1?mm thin structured polymer plates. The cloak consists of 20 concentric rings of 16 different metamaterials, each being a tailored composite of polyvinyl chloride and polydimethylsiloxane. By using stroboscopic imaging with a camera from the direction normal to the plate, we record movies of the elastic waves for monochromatic plane-wave excitation. We observe good cloaking behavior for carrier frequencies in the range from 200 to 400?Hz (one octave), in good agreement with a complete continuum-mechanics numerical treatment. This system is thus ideally suited for demonstration experiments conveying the ideas of transformation optics.     

Nanoscale thermal cloaking by in-situ annealing silicon membrane

With the advent of thermal metamaterials, many new thermal functionalities have been proposed, like thermal cloaking, concentrating, etc. However, these thermal functionalities are based on the transformation thermotics or scattering cancellation technique, which, derived from Fourier's law, cannot apply to the micro-/nanoscale counterparts. In this paper, we design a nanoscale thermal cloak based on a crystalline silicon (Si) membrane and investigate the in-plane phonon transport via non-equilibrium molecular dynamics (NEMD) simulation by in-situ tuning the thermal conductivity of the thermal cloak from crystalline Si to amorphous Si. The two-dimensional temperature profile is obtained, and the thermal cloaking effect is evaluated by the ratio of heat flux. By analyzing the phonon density of state (PDOS) and the mode participation ratio (MPR), the mechanism can be attributed to the phonon localization in the annealed cloaking region. The proposed nanoscale thermal cloak by in-situ tuned thermal conductivity, may trigger the development of nanoscale thermal functionalities and open avenues for and thermal management for nano-photonics and nano-electronics.     

Cancellation of acoustic scattering from an elastic sphere

Recent research has suggested the possibility of creating acoustic cloaks using metamaterial layers to eliminate the acoustic field scattered from an elastic object. This paper explores the possibility of applying the scattering cancellation cloaking technique to acoustic waves and the use of this method to investigate its effectiveness in cloaking elastic and fluid spheres using only a single isotropic elastic layer. Parametric studies showing the influence of cloak stiffness and geometry on the frequency dependent scattering cross-section of spheres have been developed to explore the design space of the cloaking layer. This analysis shows that an appropriately designed single isotropic elastic cloaking layer can provide up to 30 dB of scattering reduction for ka values up to 1.6. This work also illustrates the importance of accounting for the elasticity of the object and the relevant limitations of simplistic quasi-static analyses proposed in recent papers.