Cylindrical Cloak Research Articles

Dual-polarized printed cylindrical metasurface cloak at microwave frequencies

Dual-polarized printed cylindrical metasurface cloak at microwave frequencies

Численная оптимизация в задачах проектирования многослойных магнитных маскировочных оболочек

A multiparameter optimization problem of magnetic cloaking is solved for a two-dimensional model of magnetostatics. This problem arises when designing cylindrical multilayer cloaking devices filled with anisotropic or isotropic magnetic media with tensor, in the general case, magnetic permeabilities. Applying the optimization method for solving inverse problems the considered problem of magnetic cloaking is reduced to a finite-dimensional extremum problem. The results of applying the developed numerical algorithm based on the particle swarm optimization method to solve the extremum problem confirm its high efficiency and make it possible to establish the important property of the obtained optimal solutions.

Anapole state revealed by cloaking metallic cylinders with split ring resonators

This paper proposes the experimental demonstration of an anapole-based cylindrical electromagnetic cloaking scheme. An anapole state is excited by arranging around a cylindrical metallic target vertical split-ring resonators, forming an equivalent surface admittance boundary condition able to suppress the scattering. Using Mie formalism and multipole scattering theory, we identify the actual reason behind the cloaking operation, characterizing the anapole condition by the scattering contributions from toroidal and electric dipole moments. Numerical results are verified using full-wave simulation softwares and subsequently validated with back-scattering measurements inside an anechoic chamber.

Transformation design of in-plane elastic cylindrical cloaks, concentrators and lenses

We analyse the elastic properties of a class of cylindrical cloaks deduced from linear geometric transforms x→x′ in the framework of the Milton–Briane–Willis cloaking theory [New Journal of Physics 8, 248, 2006]. More precisely, we assume that the mapping between displacement fields u(x)→u′(x′) is such that u′(x′)=A−tu(x), where A is either the transformation gradient Fij=∂xi′/∂xj or the second order identity tensor I. The nature of the cloaks under review can be three-fold: some of them are neutral for a source located a couple of wavelengths away; others lead to either a mirage effect or a field confinement when the source is located inside the concealment region or within their coated region (some act as elastic concentrators squeezing the wavelength of a pressure or shear polarized incident plane wave in their core); the last category of cloaks is classified as an elastic counterpart of electromagnetic perfect cylindrical lenses. The former two categories require either rank-4 elastic tensor and rank-2 density tensor and additional rank-3 and 2 positive definite tensors (A=F) or a rank-4 elasticity tensor and a scalar density (A=I) with spatially varying positive values. However, the latter example further requires that all rank-4, 3 and 2 tensors be negative definite (A=F) or that the elasticity tensor be negative definite (and non fully symmetric) as well as a negative scalar density (A=I). We provide some illustrative numerical examples with the Finite Element package Comsol Multiphysics when A is the identity.

Water wave cloaking using a floating composite plate

The trajectory of surface gravity waves in the potential flow regime is affected by the gravitational acceleration, water density and sea bed depth. Although the gravitational acceleration and water density are approximately constant, the effect of water depth on surface gravity waves exponentially decreases as the water depth increases. In shallow water, cloaking an object from surface waves by varying the sea bed topography is possible, however, as the water depth increases, cloaking becomes a challenge because there is no physical parameter to be engineered and subsequently affects the wave propagation. In order to create an omnidirectional cylindrical cloaking device for finite-depth/deep-water waves, we propose an elastic composite plate that floats on the surface around a to-be-cloaked cylinder. The composite plate is made of axisymmetric, homogeneous and isotropic annular thin rings which provide adjustable degrees of freedom to engineer and affect the wave propagation. We first develop a pseudo-spectral method to efficiently determine the wave solution for a floating composite plate. Next, we optimise the physical parameters of the plate (i.e. flexural rigidity and mass of every ring) using an evolutionary algorithm to minimise the energy of scattered waves from the object and therefore cloak the inner cylinder from incident waves. We show that the optimised cloak reduces the energy of scattered waves as high as 99 % for the target wave number. We quantify the effectiveness of our cloak with different parameters of the plate and show that varying the flexural rigidity is essential to control wave propagation and the cloaking structure needs to be at least made of four rings with a radius of at least three times of the cloaked region. We quantify the wave drift force exerted on the structures and show that the optimised plate reduces the exerted force by 99.9 %. The proposed cloak, due to its structural simplicity and effectiveness in reducing the wave drift force, may have potential applications in cloaking offshore structures from water waves.

Multilayer acoustic invisibility cloak based on composite lattice

A concentric cylindrical cloak is showed here to achieve the acoustic cloaking phenomenon. The introduced structure consists of MNE layers and water in MNE substrate in the MHz frequency range. Due to avoiding the incoming acoustic waves by the shell, the object can be hidden inside the cylindrical area of any shape. In order to improve the quality of cloaking, we have optimized the desired shell by considering the manufacturing technology. We show that an optimized, acoustic cloak based on composite lattice structure can reduce the scattering of an object more than a 20-layer realization of acoustic cloak based on multilayer cylindrical structure. This design approach can substantially simplify the fabrication of cloaking shells. In this research, to study the acoustic distribution of the desired structure, finite element method (FEM) has been used to analyze the structure in two dimensions and a cloak of natural materials with isotropic properties has been designed using effective medium theory.

Polarized source model for the design of bulk active metamaterials

Active acoustic metamaterials have the potential for the broadband realization of complex property distributions, such as those prescribed by transformation acoustics. Especially promising are metamaterials composed of unit cells consisting of paired sensors and drivers that generate a programmed acoustic response to the local pressure and particle velocity. Previous experimental works have used this approach to implement active metamaterials of theoretically controllable effective bulk modulus and mass density, but were limited to only one or a few non-interacting cells and were not applicable to general bulk media. Here, we further develop some of the theory necessary to overcome these limitations and enable the design of bulk active metamaterials with desired properties, including steep gradients and mass density anisotropy. We present a highly simplified model of sensor-driver unit cells as interacting polarized sources that allow us to derive closed-form expressions directly relating the effective acoustic properties to the required gains, or polarizabilities. We validate our model for several example geometries and property distributions by comparing the scattered fields of the theoretical metamaterials with those of the equivalent continuous media. In particular, we show compatibility with transformation acoustics and demonstrate cylindrical cloaks using polarized sources.

Computer design of cylindrical cloaking shell for the magnetostatics model

Computer design of cylindrical cloaking shell for the magnetostatics model

Cylindrical Cloaking based on Arrangement Density Control of Multilayer Ceramic Capacitors

Cylindrical Cloaking based on Arrangement Density Control of Multilayer Ceramic Capacitors

Transformation acoustics with bulk media composed of polarized sources

Active acoustic metamaterials consisting of paired sensor-driver unit cells offer a promising path towards the practical realization of exciting transformation acoustics devices. The design of these cells is founded in a microscopic acoustic model that describes materials as collections of subwavelength polarized sources which respond to the local conditions of pressure and particle velocity. The current ability to express the polarizabilities that characterize these sources in terms of the effective macroscopic acoustic properties is limited to only a few simple cases and is not applicable to inhomogeneous bulk media of arbitrary geometries. Here, we address this challenge and derive general closed-form expressions relating the bulk modulus to the monopole polarizability and the mass density tensor to the dipole polarizability. Furthermore, we use these expressions to adapt transformation acoustics to the microscopic model. We demonstrate the accuracy of our approach by comparing the fields scattered by several devices, including cylindrical cloaks with steep property gradients and anisotropy, with the fields scattered by the devices' realizations with polarized sources.

Using plasmonic cloaking method on infinite cylindrical structures and its applications

In recent years, cloaking using materials with negative electric permittivity or magnetic permeability has been studied and researched. It has been demonstrated that covering an object with a cloak having an electric permittivity or magnetic permeability that is negative or less than unity can cause a reduction of the scattering cross-section (SCS) of the object. In this paper, we solve the scattering problem for an object with a single- or multilayer cylindrical cloak and thus obtain the fundamental equations necessary to design such cloaks under two conditions, viz. with and without consideration of the effects of coupling when solving the scattering problem. Using the obtained equations we demonstrate that this technique can indeed reduce the visibility of the object.

High-frequency cylindrical magnetic cloaks with thin layer structure

The article presents a design methodology of a thin layer, high-frequency cylindrical invisibility cloak intended for quasi-static magnetic fields. The proposed bilayer structure of a cloaking shell is synthesized using isotropic ferromagnetic and conductive materials. An analytical formula is derived to find the properties of a ferromagnetic layer in the wide frequency spectrum of an external time-harmonic magnetic field. The cloaking properties of the shell can be adjusted by the selection of conductive materials and estimated using an introduced formula. Analytical calculations are accompanied with numerical results of exemplary cloaking structures. The thin layer structures are able to maintain cloaking effect from a specific frequency, up to high frequencies of the magnetic field as long as displacement currents can be neglected. Additionally, the results of an experimental verification of the broadband magnetic cloak efficiency are reported. The cloaking shell composed of ferromagnetic polymer and conductive materials is subjected to the time-harmonic magnetic field. Three samples are presented and the magnetic field distortion, measured in the background area, is discussed. In order to obtain a desired effective electrical conductivity, a method of fabricating conductive layer, utilizing copper film on polytetrafluoroethylene base, is used. The experimental results are compared with both 3D and 2D numerical model of the cloak.

Perfect cylindrical cloak under gyration, non-inertial effects make perfect cloak visible

For the first time, electromagnetic wave interaction with a perfect non-inertial cloak is investigated. The electromagnetic detection of an ideal gyrating cloak, the effect of rotational speed on the scattering pattern, and the determination of rotational direction and rotational velocity magnitude are investigated in the present paper. It is shown that a rotating cloak is electromagnetically detectable due to the magneto-electric and Sagnac effects. These phenomena create bending in the passing wave pattern and generate cylindrical wavelets that are observable in the electromagnetic scattering pattern. It is shown, the gyrating cloak’s speed and direction can be determined using the amplitude of wavelets and bending location in the pulses passed because they increase with growing angular frequency. At low frequency gyration, the distortion of waveform pattern is humble and negligible; however, when the rotational frequency increases, the pattern distortion becomes huge, wavelets are formed, and their intensity grows. The direction of rotation can influence the scattering pattern; thus, the rotation direction could be identified with the scattering pattern analysis. The simulation is computed using the FDTD method by transforming Maxwell’s equations from the non-inertial framework to the lab framework.

The cloaking method in non-uniform static fields of cylindrical and spherical invisibility cloaks

The cloaking methods of infinite length coaxial cylindrical and concentric spherical invisibility cloaks in the non-uniform static field are investigated. First, the arbitrary potential function at the infinite boundary is expanded into a series of orthogonal potential functions. When there is only the nth order orthogonal potential function distribution at the infinite boundary, the analytical cloaking conditions of both the infinite length coaxial cylindrical and concentric spherical cloaks are derived. Furthermore, based on the analytical cloaking conditions, the approaching cloaking method is proposed in a complex non-uniform static field which composed of multiple orders potential functions. This method can acquire the optimal cloaking condition and avoid the iteration process by using numerical methods such as the finite element method etc. Finally, three examples are given to verify the correctness of the analytical cloaking conditions and the effectiveness of the approaching cloaking method proposed in this paper.

Design and finite element simulation of information-open cloaking devices

In this paper, based on the transformation optics theory as well as the complementary media ideas, we design a novel type of information-open (IO) cloaking devices by using nonlinear coordinate transformations. Finite element methods are used to design layered IO cylindrical cloaking devices. The equidistant and hierarchical layered designs of the IO cloaking devices are considered, and their cloaking performances are verified. Finally, by comparing the finite element numerical results, we find that satisfactory IO cloaking performances can be obtained with fewer layers by the hierarchical layered design. As far as we know, this is the first work on the hierarchical design of IO cloaking devices via nonlinear transformations.

Demonstration of Fano resonance-based miniaturized cylindrical cloaking scheme

An experimental realization of a magnetic Fano-like resonance-based cylindrical cloaking scheme in the microwave frequency regime is presented in this paper. Fano-like resonance is excited by the coupling between the background electric dipole mode and the dark magnetic resonant mode in the composite. The excitation of Fano interference patterns significantly reduces scattering from the composite at resonance in the microwave regime and hence the target becomes undetectable from backscattering measurements. Multipolar scattering theory has been used to clarify the excitation of this Fano resonance and the designed cloak is characterized using monostatic and bistatic scattering measurements inside an anechoic chamber.

Cloaking In-Plane Elastic Waves with Swiss Rolls.

We propose a design of cylindrical cloak for coupled in-plane shear waves consisting of concentric layers of sub-wavelength resonant stress-free inclusions shaped as Swiss rolls. The scaling factor between inclusions’ sizes is according to Pendry’s transform. Unlike the hitherto known situations, the present geometric transform starts from a Willis medium and further assumes that displacement fields in original medium and in transformed medium remain unaffected (). This breaks the minor symmetries of the rank-4 and rank-3 tensors in the Willis equation that describe the transformed effective medium. We achieve some cloaking for a shear polarized source at specific, resonant sub-wavelength, frequencies, when it is located in close proximity to a clamped obstacle surrounded by the structured cloak. The structured medium approximating the effective medium allows for strong Willis coupling, notwithstanding potential chiral elastic effects, and thus mitigates roles of Willis and Cosserat media in the achieved elastodynamic cloaking.

Cylindrical thermal invisibility cloak based on transformation thermodynamics

The paper provides a theoretical proof that the methods of transformation optics can be used for thermodynamics. Thermal conductivity tensor is calculated for the development of the thermal invisibility cloak, which directs the heat flux around a certain area. The finite element method was used to simulate the cloak with parameters close to real. The simulation was performed by the finite element method for two cases: with gradient k and with copper/mica layered structure.

Design and analysis of nonlinear-transformation-based broadband cloaking for acoustic wave propagation

A coordinate-transformation method can be used to design invisibility cloaks for many types of waves, including acoustic waves. The traditional method for designing a cloak depends on a transformation from a virtual space to a physical space. Previous acoustic cloaks that are mainly designed with linear-transformation-based acoustics have drawbacks that acoustic wave trajectories in the cloaks cannot be controlled and tuned. This work uses a nonlinear mapping from a ray trajectory perspective to construct acoustic cloaks with tunable non-singular material properties. Use of a ray trajectory equation is a straightforward and alternate way to study propagation characteristics of different types of waves, which allows more flexibility in controlling the waves. A broadband cylindrical cloak for acoustic waves in an inviscid fluid is realized with layered non-singular, homogeneous, and isotropic materials based on a nonlinear transformation. Some advantages and improvements of the invisibility nonlinear-transformation cloak over a traditional linear-transformation cloak are analyzed. The invisibility capability of the nonlinear-transformation cloak can be tuned by adjusting a design parameter that is shown to have influence on the acoustic wave energy flowing into the region inside the cloak. Numerical examples show that the nonlinear-transformation cloak is more effective for making a domain undetectable by acoustic waves in an inviscid fluid and shielding acoustic waves from outside the cloak than the linear-transformation cloak in a broad frequency range. The methodology developed here can be used to design nonlinear-transformation cloaks for other types of waves.

Optimization method in material bodies cloaking with respect to static physical fields

Abstract Inverse problems associated with designing cylindrical DC electrical cloaking shells are studied. Using the optimization method, these inverse problems are reduced to corresponding control problems in which electrical conductivities play the role of passive controls. Admissibility of the optimization method for solving inverse design problems is justified. A numerical algorithm based on the particle swarm optimization is proposed, and the results of numerical experiments are discussed. Optimization analysis shows that high cloaking efficiency of the shell can be achieved either using a highly anisotropic single-layer shell or using a multilayer shell with isotropic layers. In the latter case, the resulting cloaking shell admits simple technological realization using natural materials.