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To hide a metallic object from outside observers, we designed cloaking structures to manipulate the propagation path of the microwave at a specific frequency. Dielectric materials are used to realize the cloaking effect and their electromagnetic properties are evaluated to reduce the size of the cloaking structure as well as to take the loss effect into account. We used the structural design method based on the phase field method to control the electric field flow around a target object for cloaking. It is aimed to minimize the scattered electric field measured at the region located behind the target object for a normal incident wave in the X-band frequency range. Numerical examples are given to verify the suggested design process and its results. [ABSTRACT FROM AUTHOR]

The presence of extreme material properties and undesired scattering make practical implementation of transformation optics based cloaks an extremely challenging issue. To overcome these problems, a simplified quadratic cloak has been proposed which provides finite material properties at the inner radius of the cloak and impedance match with the free space at the outer radius of the cloak simultaneously. The simultaneous occurrence of finite material properties and impedance match with the free space at the respective boundaries reduces the scattering cross section significantly. The material properties of the proposed simplified quadratic cloak can be realized with two dimensional (2D) metamaterials. The performance of the proposed cloak is examined by plotting the normalized total scattering cross section (SCSt,norm), and the scattering patterns in azimuthal plane. The proposed simplified quadratic cloak shows 10 dB reduction of the scattered field in both the forward and backward directions with respect to the perfect electric conductor (PEC) cylinder. [ABSTRACT FROM AUTHOR]

New connections between static elastic cloaking, low-frequency elastic wave scattering and neutral inclusions (NIs) are established in the context of two-dimensional elasticity. A cylindrical core surrounded by a cylindrical shell is embedded in a uniform elastic matrix. Given the core and matrix properties, we answer the questions of how to select the shell material such that (i) it acts as a static elastic cloak, and (ii) it eliminates low-frequency scattering of incident elastic waves. It is shown that static cloaking (i) requires an anisotropic shell, whereas scattering reduction (ii) can be satisfied more simply with isotropic materials. Implicit solutions for the shell material are obtained by considering the core–shell composite cylinder as a neutral elastic inclusion. Two types of NI are distinguished, weak and strong with the former equivalent to low-frequency transparency and the classical Christensen and Lo generalized self-consistent result for in-plane shear from 1979. Our introduction of the strong NI is an important extension of this result in that we show that standard anisotropic shells can act as perfect static cloaks, contrasting previous work that has employed 'unphysical' materials. The relationships between low-frequency transparency, static cloaking and NIs provide the material designer with options for achieving elastic cloaking in the quasi-static limit. [ABSTRACT FROM AUTHOR]

In many applications, a cloaked resonator is highly desired, which can harvest and maximize the energy within the resonator without being detected. This paper presents the resonator cloaking achieved by topology optimization-based inverse design methodology. The resonator cloaking is inversely designed by solving the topology optimization problem with minimizing the ratio of the scattering field energy outside the cloak and the cloaked resonating field energy. By inversely designing the resonator cloaking with relative permittivity 2 for both the resonator and cloak, the topology optimization-based inverse design methodology is demonstrated, where the incident angle sensitivity is considered to derive incident angle insensitive design. Then, the proposed methodology is applied for the cases with resonator and cloak materials chosen from dielectrics with low, moderate and high permittivity, respectively. The derived results demonstrate that the resonator cloaking can be categorized into three types, which are the Fabry-Pérot resonance cloaking, Mie resonance cloaking and hybrid resonance cloaking. [ABSTRACT FROM AUTHOR]

This paper demonstrates the design of ultra-transparent anisotropic media for broadband acoustic invisibility. The required parameter distributions of the proposed invisibility cloak are derived by coordinate transformation, and its practical implementation simply consists of two-dimensional solid arrays in fluids. By studying the acoustic properties of the structures in the quasi-static limit, it is found that similar anisotropic features and transparent effects can be achieved with two kinds of structures, i.e., anisotropic solid cells arranged in the square lattice and cylindrical cells arranged in the highly anisotropic lattice. The acoustic impedance of the anisotropic cloaking media can be perfectly matched to the background fluid. The performances of the designs are quantitatively evaluated and compared in broadband by the averaged invisibility parameter. The full threedimensional structure is also analyzed for demonstrating the practical feasibility of the scheme. The results show that the anisotropic structures are effective for manipulating the acoustic field and suppressing the wave scattering. [ABSTRACT FROM AUTHOR]

An efficient iterative analytical-numerical method is proposed for three-dimensional (3D) electromagnetic scattering from an inhomogeneous object buried beneath a two-dimensional (2D) randomly dielectric rough surface. In the hybrid method, the electric and magnetic currents on the dielectric rough surface are obtained by current-based Kirchhoff approximation (KA), while the scattering from the inhomogeneous object is rigorously studied by finite element method (FEM) combined with the boundary integral method (BIM). The multiple interactions between the buried object and rough surface are taken into account by updating the electric and magnetic current densities on them. Several numerical simulations are considered to demonstrate the algorithm’s ability to deal with the scattering from the inhomogeneous target buried beneath a dielectric rough surface, and the effectiveness of our proposed method is also illustrated. [ABSTRACT FROM AUTHOR]

Non-linear shape external cloak using a parabolic transformation function is proposed with finite material properties. A simple one directional folding operation is utilised that provides homogeneous material properties in the complementary region, and also eases the creation of anti-object in the complementary region from an object present in the cloaked region. In the core region, the axial material parameter is spatially invariant and other parameters are spatially varying but possess finite value. Due to finite small material range and its one directional spatial variation, the proposed cloak even with discrete material properties in core region shows nearly zero scattering in contrast to earlier reported cloak. Further, off-diagonal elements of the core constitutive tensors in the Cartesian coordinate system are removed by converting them to diagonal elements in the principal coordinate system. Thus, only three diagonal elements are needed in the principal coordinate system to create the core medium, which are finite and realisable using two-dimensional metamaterials. [ABSTRACT FROM AUTHOR]

We predict scattering cancellation in diffusive transport of acoustics waves propagating through multiple scattering media in the stationary limit. This would enable sensing of diffusive sound without disrupting the exterior acoustic field. We present design schemes for making spherical or cylindrical core-shell structures with multiple layers, characterized by homogenous and isotropic diffusion coefficients, neutral to an arbitrary applied multipole field. The double-layered sphere is found to support transparency to two concurrent multipole fields and unique cloaking solutions of arbitrary multipole order. One extra degree of freedom is provided by every layer added to the core-shell structure which may be exploited with our iterative formula for effective diffusivity for cloaking of additional field terms. From this we pass over to the long wavelength limit of ballistic sound and provide formulas for effective mass densities of multi-layered structures in spherical and cylindrical geometries with respect to multipole pressure fields. [ABSTRACT FROM AUTHOR]

ABSTRACT Metamaterial-based single layer rectangular cloaking operation in the epsilon-negative (ENG) region is experimentally demonstrated. In this study, a new metamaterial unit cell is presented that exhibits epsilon-negative property in the C-band of microwave regime. The metamaterial was then utilized in designing a single layer rectangular cloak that hides a metal cylinder in the ENG region of the metamaterial. The cloak provides less than one normalized total scattering width for the cloaked object. The proposed cloaking operation was experimentally demonstrated over a 500 MHz area in the C-band region of microwave spectra. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:2522-2525, 2016 [ABSTRACT FROM AUTHOR]

Transformational acoustics offers the theoretical possibility of cloaking obstacles within fluids, provided metamaterials having continuously varying bulk moduli and densities can be found or constructed. Realistically, materials with the proper, continuously varying anisotropies do not presently exist. However, discretely layered cloaks having constant material parameters within each layer may be a viable alternative in practice. The present work considers a range of cloaks, from those comprised of fluid layers that are isotropic in bulk moduli with anisotropic density (inertial cloaks) to those having anisotropic bulk moduli and isotropic density (pentamode cloaks). In this paper an analytical solution is obtained for the case of plane wave scattering from a submerged rigid cylinder covered with a multilayered cylindrical cloak composed of discrete anisotropic fluid layers. An investigation of the parameter space defining such cloaks is undertaken with the goal of minimizing the far-field scattered pressure, using layer constituent anisotropic properties (density and bulk modulus) constrained to lie within reasonable ranges relative to those of water. [ABSTRACT FROM AUTHOR]

In this paper, an efficient integral equation method is developed for the response and scattering characteristics of cloaked 2-D planar microstrip receiving antennas. The featured technique combines Green’s function for the strip-free structure with the decomposition of the strip into cylindrical thin wires and a radiation integral evaluation for the scattered field. In essence, the desired low-profile operation of the antenna is achieved by overlaying a superstrate slab atop the antenna. Moreover, an optimization process determines the constitutive parameters and thickness of the slab, so that a significantly reduced scattering behavior is obtained. Finally, the performance of the receiving antenna is investigated for a specific range of frequencies around the operating one, while the accuracy of the proposed algorithm is successfully verified through several numerical simulations. [ABSTRACT FROM AUTHOR]

In this review, a brief introduction is given to the development of acoustic superlens cloaks that allow the cloaked object to receive signals while its presence is not sensed by the surrounding, which can be regarded as “cloaking an acoustic sensor”. Remarkably, the designed cloak consists of single-negative materials with parameters independent of the background medium or the sensor system, which is proven to be a magnifying superlens. This has facilitated significantly the design and fabrication of acoustic cloaks that generally require double-negative materials with customized parameters. Such innovative design has then been simplified further as a multi-layered structure comprising of two alternately arranged complementary media with homogeneous isotropic single-negative materials. Based on this, a scattering analyses method is developed for the numerical simulation of such multi-layered cloak structures, which may serve as an efficient approach for the investigation on such devices. [ABSTRACT FROM AUTHOR]

• Antiplane elastic wave cloaking of circular cylindrical elastic inclusion where layered cloak has an imperfect interface. • Multilayered cloak consisting of many bi-material cells constructed using homogenization theory. • Recursive procedure is used for the multilayered scatterer. • Effectiveness of the wave cloak depends on the location of the imperfect interface and the level of imperfect contact. • Wave cloak is more effective when the imperfect interface is closer to the circular cylinder. The cloaking of an elastic circular cylinder embedded in a homogeneous, linear, isotropic, elastic medium from antiplane elastic waves, where the layered cloak has an imperfect interface, is investigated in this paper. The material properties of the cloak are determined using the transformation or change-of-variables method and the homogenization theory of composites is used to construct a multilayered cloak consisting of many bi-material cells. The location of the imperfect interface can be at the interface between the elastic cylinder and the cloak, or at any interface between the bi-material cells, or at the interface between the cloak and the surrounding medium. This problem is solved by using the concept of multiple scattering with wave expansion coefficient matrices and the modified recursive procedure for a multilayered scatterer with an imperfect interface which is developed in this paper. Numerical results show that the effectiveness of the wave cloak depends on the location of the imperfect interface and the level of imperfect contact. The wave cloak is more effective when the imperfect interface is closer to the circular cylinder. In particular, when the imperfect interface is at the boundary of the circular cylinder, effective cloaking can be achieved independent of the level of imperfect contact. When the imperfect interface is located in the middle of the cloak, effective cloaking can only be achieved when the level of imperfect contact is very small, i.e., closer to a perfectly bonded interface. [ABSTRACT FROM AUTHOR]

This communication presents a new numerical method for the efficient solution of marching on in time (MOT)-based time domain integral equations (TDIE) for electromagnetic scattering from perfect electrically conducting (PEC) object with large smooth surfaces. A set of space-delayed temporal basis functions is proposed for the temporal discretization, and the curvilinear Rao-Wilson-Glisson (CRWG) basis functions for the spatial discretization. The space-delayed temporal basis functions are firstly extended to the time domain electric field integral equations (TDEFIE), then easily to the time domain combined field integral equations (TDCFIE). Numerical results demonstrate that the new method is stable and effective for the transient scattering problems from PEC objects with large smooth surfaces and a single plane wave incidence. A significant reduction of the number of spatial unknowns is achieved by using the space-delayed temporal basis functions. [ABSTRACT FROM PUBLISHER]

An analytical full wave model of parallel, randomly centered carbon nanotubes (CNTs) embedded in a dielectric slab was developed to calculate the electromagnetic scattering from CNT composites. The model simulates empty and metal-filled single walled CNTs as infinitesimally thin wires, parallel and infinitely long in the z direction, but randomly located in the x–y plane. The wires have the equivalent impedance of CNTs and are embedded in a dielectric slab with finite thickness in the x direction but infinite in the y and z directions. The electromagnetic radiation propagates in the x direction, and the entire model is periodic in the y direction. The main goal of this work is to investigate how the variations in the random locations and impedances of the CNTs can lead to variations in the electromagnetic scattering from the composite. Composites with metal-filled CNTs exhibit more variations in the electromagnetic scattering than unfilled CNTs. The model also showed that increasing the density of CNTs, decreasing the losses of the dielectric matrix, and the random incorporation of nonidentical CNTs increases the variations in the electromagnetic scattering. The model is finally used to demonstrate how simple fabrication tolerances can affect the variations in electromagnetic scattering. [ABSTRACT FROM PUBLISHER]

We propose a theoretical study on the electromagnetic wave scattering from three-dimensional layered structures with an arbitrary number of rough interfaces by using the small perturbation method and the small slope approximation. The interfaces are characterized by Gaussian height distributions with zero mean values and Gaussian correlation functions. They can be correlated or not, isotropic or not. The electromagnetic field in each medium is represented by a continuum of plane waves and a perturbation theory is used for solving the boundary value problem and determining the first-order scattering amplitudes by recurrence relations. The scattering amplitudes under the first-order small slope approximation are deduced from results derived from the first-order small perturbation method. We analyze with the small slope approximation model the combined influence of the anisotropy and cross-correlation upon the electromagnetic signature of a natural stratified structure. [ABSTRACT FROM PUBLISHER]

This paper presents a new hybridization between MoM-GEC and some asymptotic methods. In fact, a new hybrid current function based on Physical Optic (PO) and a modal method is developed. The approach consists in approximating the total current on an invariant metallic pattern on two parts; the inside of metal is governed by PO method; however, the edges are modeled by infinite cylinders and described by Hankel functions (modal method). The considered single test function is required then by MoM method to replace a lot of sinusoidal or triangular test functions, in order to get a rapid convergence and less computational time. For validation purposes, the new developed hybrid approach is applied to compute scattering in different structures. The obtained input impedances, currents and fields distributions are in agreement with those obtained by MoM method. Considerable gain in computational time and memory resources is achieved. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Enhancement of wave-like characteristics of heavily damped diffuse photon density waves in a random medium by amplification can induce strongly localised resonances. These resonances can be used to either suppress or enhance scattering from an inhomogeneity in the random medium by cloaking the inhomogeneous region by a shell of random medium with the correct levels of absorption or amplification. A spherical core-shell structure consisting of a shell of a random amplifying medium is shown to enhance or suppress specific resonant modes. A shell with an absorbing random medium is also shown to suppress scattering which can also be used for cloaking the core region. [ABSTRACT FROM AUTHOR]

We first review classical results on cloaking and mirage effects for electromagnetic waves. We then show that transformation optics allows the masking of objects or produces mirages in diffusive regimes. In order to achieve this, we consider the equation for diffusive photon density in transformed coordinates, which is valid for diffusive light in scattering media. More precisely, generalizing transformations for star domains introduced in (Diatta and Guenneau J. Opt.13 024012) for matter waves, we numerically demonstrate that infinite conducting objects of different shapes scatter diffusive light in exactly the same way. We also propose a design of an external light-diffusion cloak with spatially varying sign-shifting parameters that hides a finite size scatterer outside the cloak. We next analyze non-physical parameters in the transformed Fick’s equation derived in (Guenneau and Puvirajesinghe R. Soc. Interface10 20130106), and propose the use of a non-linear transform that overcomes this problem. We finally investigate other forms of invariant transformed diffusion-like equations in the time domain, and touch upon conformal mappings and non-Euclidean cloaking applied to diffusion processes. [ABSTRACT FROM AUTHOR]

We theoretically and numerically analyze thermal invisibility based on the concept of scattering cancellation and mantle cloaking. We show that a small object can be made completely invisible to heat diffusion waves, by tailoring the heat conductivity of the spherical shell enclosing the object. This means that the thermal scattering from the object is suppressed, and the heat flow outside the object and the cloak made of these spherical shells behaves as if the object is not present. Thermal invisibility may open new vistas in hiding hot spots in infrared thermography, military furtivity, and electronics heating reduction. [ABSTRACT FROM AUTHOR]

Concealing objects by making them invisible to an external electromagnetic probe is coined by the term "cloaking." Cloaking devices, having numerous potential applications, are still facing challenges in realization, especially in the visible spectral range. In particular, inherent losses and extreme parameters of metamaterials required for the cloak implementation are the limiting factors. Here, we numerically demonstrate nearly perfect suppression of scattering from arbitrary-shaped objects in spatially dispersive metamaterial acting as an alignment-free concealing cover. We consider a realization of a metamaterial as a metal-dielectric multilayer and demonstrate suppression of scattering from an arbitrary object in forward and backward directions with perfectly preserved wave fronts and less than 10% absolute intensity change, despite spatial dispersion effects present in the composite metamaterial. Beyond the usual scattering suppression applications, the proposed configuration may be used for a simple realization of scattering-free detectors and sensors. [ABSTRACT FROM AUTHOR]

A simple, thin, flexible mantle cloak for conducting rods based on scattering cancellation is analyzed, designed and experimentally realized. We show strong scattering suppression at all angles of incidence, for both far-field plane-wave and near-field Gaussian excitations. The required effective shunt surface impedance is realized by a subwavelength patch array, targeting the suppression of the dominant omnidirectional scattering contribution of a conductive rod. Full-wave simulations predict a total radar cross-section reduction better than 14 dB in the lossless case and nearly 8 dB when considering a lossy substrate in the cover. Measurements of the realized cloak are consistent and validate these numerical predictions. The proposed geometry is also shown to be an ideal platform for monolithic integration of varactor diodes, allowing real-time tuning of the effective surface capacitance of the cloak. We show with numerical simulations the possibility of tunable scattering suppression over 1 GHz of bandwidth by seamlessly integrating varactor diodes in our mantle cloak design. [ABSTRACT FROM AUTHOR]