Cloaking matter waves around a Dirac monopole

In this Letter we study the quantum cloaking of a charged particle interacting with the field of a magnetic monopole. It is shown that the matter wave of the quantum particle can be perfectly cloaked and guided by the quantum cloak of free particles.     

Acoustic cloaking by the wave flow method

The methods for calculating acoustic cloaking that is implemented by the wave flow method are reviewed, and the efficiency of this technique for cloaking regions with symmetries of three types is analyzed. It is shown that the main problem in implementing acoustic cloaking is the formation of an anisotropic medium with inhomogeneous components of the density tensor and bulk modulus, which change in a wide range and have limiting values of ∞ or 0 at the cloaking-region boundaries. Some estimates are obtained, according to which a stratified medium composed of a sequence of layers with different densities and compressibilities appears to be more promising, because it (i) is more easy to implement and (ii) opens possibilities for broadband cloaking. Analysis of different versions of the cloaking-region symmetry revealed that, using a layered medium, one can implement efficient acoustic cloaking only in the case of a spherically symmetric region.     

Scattering theory derivation of a 3D acoustic cloaking shell

Through acoustic scattering theory we derive the mass density and bulk modulus of a spherical shell that can eliminate scattering from an arbitrary object in the interior of the shell--in other words, a 3D acoustic cloaking shell. Calculations confirm that the pressure and velocity fields are smoothly bent and excluded from the central region as for previously reported electromagnetic cloaking shells. The shell requires an anisotropic mass density with principal axes in the spherical coordinate directions and a radially dependent bulk modulus. The existence of this 3D cloaking shell indicates that such reflectionless solutions may also exist for other wave systems that are not isomorphic with electromagnetics.     

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.     

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.     

Illusion optics

The technique of “transformation optics” establishes a correspondence between coordinate transformation and material constitutive parameters. Most of the transformation optics mappings give metamaterials that have graded positive refractive indices that can steer light in curves defined by the coordinate transformation. We will focus on those “folded-geometry mappings” that give negative refractive index materials that have special wave scattering properties. One interesting example is a kind of remote illusion device that can transform the stereoscopic image of an object into the illusion of some other object of our choice. The conceptual device can create the illusion without touching or encircling the object. For any incident wave, the device transforms the scattered waves of the original object into that of the object chosen for illusion outside a virtual boundary. We will illustrate some possible applications of this type of metamaterial remote device, including “cloaking at a distance,” partial cloaking, cloaking from an embedded device, revealing a hidden object inside a container, turning the image of one object into that of another object, and seeing through a wall. The feasibility of building this remote illusion device by metamaterials will also be discussed.     

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.     

Illusion optics

The technique of “transformation optics” establishes a correspondence between coordinate transformation and material constitutive parameters. Most of the transformation optics mappings give metamaterials that have graded positive refractive indices that can steer light in curves defined by the coordinate transformation. We will focus on those “folded-geometry mappings” that give negative refractive index materials that have special wave scattering properties. One interesting example is a kind of remote illusion device that can transform the stereoscopic image of an object into the illusion of some other object of our choice. The conceptual device can create the illusion without touching or encircling the object. For any incident wave, the device transforms the scattered waves of the original object into that of the object chosen for illusion outside a virtual boundary. We will illustrate some possible applications of this type of metamaterial remote device, including “cloaking at a distance,” partial cloaking, cloaking from an embedded device, revealing a hidden object inside a container, turning the image of one object into that of another object, and seeing through a wall. The feasibility of building this remote illusion device by metamaterials will also be discussed.     

Sound-transparent anisotropic media for backscattering-immune wave manipulation

The scattering behavior of an anisotropic acoustic medium is analyzed to reveal the possibility of routing acoustic signals through the anisotropic layers with no backscattering loss. The sound-transparent effect of such a medium is achieved by independently modulating the anisotropic effective acoustic parameters in a specific order, and is experimentally observed in a bending waveguide by arranging the subwavelength structures in the bending part according to transformation acoustics. With the properly designed filling structures, the original distorted acoustic field in the bending waveguide is restored as if the wave travels along a straight path. The transmitted acoustic signal is maintained nearly the same as the incident modulated Gaussian pulse. The proposed schemes and the supporting results could be instructive for further acoustic manipulations such as wave steering, cloaking and beam splitting.     

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.     

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 carriers, such as the radar antenna system on an aircraft.1     

Total transmission and total reflection by zero index metamaterials with defects

We theoretically investigate microwave transmission through a zero-index metamaterial loaded with dielectric defects. The metamaterial is impedance matched to free space, with the permittivity and permeability tending towards zero over a given frequency range. By simply varying the radii and permittivities of the defects, total transmission or reflection of the impinging electromagnetic wave can be achieved. The proposed defect structure can offer advances in shielding or cloaking technologies without restricting the object's viewpoint. Active control of the observed exotic transmission and reflection signatures can occur by incorporating tunable refractive index materials such as liquid crystals and BaSrTiO3.     

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.     

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.     

Effective zero index in locally resonant acoustic material

Here in locally resonant acoustic material, it is shown that effective zero refractive index can be constructed by the resonant unit-cells with coherent degenerate monopole–dipole momenta. Due to strong local resonances, the material layers with effective zero refractive index can function as a resonant cavity of high Q factor, where a subtle deviation from the resonant frequency may result in distinct increase of reflection. Full-wave simulations are performed to demonstrate some unusual wave transport properties such as invisibility cloaking, super-reflection, local field enhancement, and wavefronts rotation.     

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.     

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.     

Cloaking by plasmonic resonance among systems of particles: cooperation or combat?

We study quasistatic cloaking by the mechanism of plasmonic resonance, for systems of coated cylinders. Our focus is on the nature of the resonant cloaking interaction: whether systems of particles can be made to cooperate in cloaking a polarizable particle from an applied uniform field. We show that in fact if the cloaking regions of the systems of particles overlap, then they tend to interact in a fashion detrimental to their cloaking of the polarizable particle. If the cloaking regions touch but do not overlap, then the system of particles can cloak a larger region than each would in isolation. To cite this article: R.C. McPhedran et al., C. R. Physique 10 (2009).     

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.     

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.