Mantle Cloaking Research Articles

Experimentally approaching to reciprocal cloaks via annulus-dielectric-metamaterials

Transformation optics has brought a wave of fanaticism among researchers to realize the first-ever invisible cloak over the past decade. Several relevant devices, including internal cloaks, external cloaks, and mantle cloaks, have already been proposed, with a few being experimentally demonstrated. Still, the proposed cloaks possess their insufficiencies to realize a reciprocal cloak. To better illustrate the cloak, here we report a new approach to a transformation-optics-based reciprocal cloak with designed dielectric-annulus-based metamaterials. Our results indicate we can easily control the anisotropic constitutive parameters at will. In addition, the as-fabricated cloak enables concurrent vision and movements of a hidden object, evidenced by both simulated and measured field distributions. More importantly, the cloak demands no bespoke design and is applicable to arbitrary electric-large objects. We affirm that we provide a method to developing the first-ever reciprocal cloak in experiments, and believe that the cloak could be further operated at visible ranges through the methodology we introduced, paving a route toward future applications in the fields of optics, physics, materials science, etc.

Scattering suppression and cloak for electrically large objects using cylindrical metasurface based on monolayer and multilayer mantle cloak approach

The authors proposed analytical models applicable to monolayer and multilayer mantle cloaks for suppressing the scattering fields of electrically large conducting objects using printed ultrathin metasurfaces. To analyse the proposed structure, the two-sided impedance surface method is used and the scattering cross section (SCS) is obtained using Mie theory. It is shown that using surface impedance expression obtained from the analytical model for monolayer mantle cloak they can design a proper unit cell of a metasurface to reduce significantly total scattering from the cylinder. The proposed mantle cloaks in this work are mainly designed to minimise simultaneously several scattering multi-poles of a relatively large object around considerably broad bandwidth. Numerical results and full-wave simulation show good cloaking performance in designed frequency. The simulated results using their analytical models as well as the full-wave results show that the multilayer mantle cloak is an optimised candidate for cloaking of electrically large objects. The proposed multilayer mantle cloak confirms 3.6 dB SCS gain to cancel the scattering fields of a perfect electric conductor cylinder with a radius of 1.425λ0 at the designed frequency. Finally, they consider a cluster of closely spaced and overlapping three cloaked cylinders for suppression of electromagnetic fields.

Mantle Cloaks Based on the Frequency Selective Metasurfaces Designed by Bayesian Optimization

We propose a full optimization procedure for designing mantle cloaks enclosing arbitrary objects, using sub-wavelength conformal frequency selective surface (FSS). Rely on the scattering cancellation principle of mantle cloak characterized by an average surface reactance, a personal computer can achieve this design procedure. By combing a Bayesian optimization (BO) with an electromagnetic solver, we can automatically find the optimal parameters of a conformal mantle cloak which can nearly cancel the scattering from the enclosed objects. It is shown that the results obtained by our method coincide with those from a rigorous analytical model and the numerical results by full parametric scanning. The proposed methodology opens up a new route for realizing ultra-wideband illusion scattering of electromagnetic wave, which is important for stealth and microwave applications.

A Microwave Invisibility Cloak: The Design, Simulation, and Measurement of a Simple and Effective Frequency-Selective Surface-Based Mantle Cloak

In recent years, there has been a growing interest in developing invisibility cloaks that can conceal an object. These techniques are often based either on coating a dielectric or conducting object with a homogeneous plasmonic layer of negative permittivity [1] or on creating multilayer structure [2] that cancels the scattering of the cloaked object (i.e., scattering cancelation technique). The technique may also be based on an inhomogeneous layer that bends electromagnetic waves around the region occupied by the cloaked object without interacting with it (i.e., transformation optics technique) [3].

Wideband Elliptical Metasurface Cloaks in Printed Antenna Technology

In this paper, we apply the mantle cloaking method to reduce the mutual coupling between two closely spaced wideband antennas operating at microwave frequencies. We show that by introducing two dielectric elliptical shells around the antennas and covering them with two strongly coupled elliptically shaped metasurfaces comprised of I-shaped printed unit cells, it is possible to significantly increase the original bandwidth of the antennas and, at the same time, reduce the mutual electromagnetic interaction. This effect is observed both in a reduction of mutual S-parameters between the antennas and in the restoration of the original radiation patterns, as if the antennas were almost completely isolated from each other. Broadband metasurface/cloak designs are proposed for two geometries, with the free-standing strip monopole antennas placed over a ground plane and with the metasurface cloaks integrated with microstrip antennas.

Analytical Approach for Making Multilayer and Inhomogeneous Structures Invisible Based on Mantle Cloak

Abstract We investigate an analytical approach based on transmission line model to design mantle cloak for multilayer and/or inhomogeneous dielectric objects. Our proposed model is verified with different numerical results. The results matched well with each other. Also, the required impedance for making an inhomogeneous material invisible is calculated analytically. Finally, some complex structures are implemented using CST simulator and suitable performance is reached. So, it is proved that mantle cloak is applicable for making complex inhomogeneous materials invisible.

Closed-form harmonic contrast control with surface impedance coatings for conductive objects

The problem of suppressing the scattering from conductive objects is addressed in terms of harmonic contrast reduction. A unique compact closed-form solution for a surface impedance $Z_s(m,kr)$ is found in a straightforward manner and without any approximation as a function of the harmonic index $m$ (scattering mode to suppress) and of the frequency regime $kr$ (product of wavenumber $k$ and radius $r$ of the cloaked system) at any frequency regime. In the quasi-static limit, mantle cloaking is obtained as a particular case for $kr \ll 1$ and $m=0$. In addition, beyond quasi-static regime, impedance coatings for a selected dominant harmonic wave can be designed with proper dispersive behaviour, resulting in improved reduction levels and harmonic filtering capability.

Wideband and multi-frequency infrared cloaking of spherical objects by using the graphene-based metasurface.

The ultrathin graphene metasurface is proposed as a mantle cloak to achieve wideband tunable scattering reduction around the spherical (three-dimensional) objects. The cloaking shell over the metallic or dielectric sphere is structured by a periodic array of graphene nanodisks that operate at infrared frequencies. By using the polarizability of the graphene nanodisks and equivalent conductivity method, the metasurface reactance is obtained. To achieve the cloaking shell for both dielectric and conducting spheres, the metasurface reactance as a function of nanodisks dimensions, graphene's Fermi energy, and permittivity of the surrounding areas can be tuned from the inductive to capacitive situation. Inhomogeneous metasurfaces including graphene nanodisks with different radii provide wideband invisibility due to extra resonances. We could significantly increase the 3-dB bandwidth more than the homogenous case by simpler realistic designs compared to the multi-layer structures. The analytical results are confirmed with full-wave numerical simulations.

Use of Mantle Cloaks to Increase Reliability of Satellite-to-Ground Communication Link

In this paper, we show how mantle cloaking can be effectively used in satellite scenarios to reduce the deteriorating effects of the deployable equipment on the link budget between a miniaturized satellite and its ground station. The proposed idea is applied to a nanosatellite scenario and is validated through a simple analytical analysis of its communication link with the ground. The effectiveness of the designed cloaking metasurfaces is evaluated through a proper combination of numerical strategies taking into account the electrically large domain of the problem and the electrically small features of the metasurfaces. This innovative solution can be used to enhance the performances of existing satellite communication systems or to design new extremely compact nanosatellite platforms equipped with multifunctional antennas and sensors.

Nonlinear Mantle Cloaking Devices for Power-Dependent Antenna Arrays

In this letter, we investigate the concept and the applications of power-dependent mantle cloaking devices. The idea is to exploit the intrinsic nonlinearity of a diode pair for the realization of metasurfaces whose cloaking behavior can be turned on and off by acting on the power level of the impinging electromagnetic field. A simple analytical model, taking into account the parasitic elements of the diodes, is proposed, and its effectiveness evaluated through full-wave simulations. Then, the nonlinear mantle cloaks are used for the design of power-dependent antenna arrays whose radiation pattern can be tuned by changing the input power of the antenna. These innovative radiating systems may find applications in different scientific and technical fields.

Kirchhoff's current law as local cloaking condition: theory and applications

Starting from the electric field integral equation (EFIE), it is shown how, for quasi-static cloaking (i.e. zero scattered fields), the EFIE can be reduced to classical Kirchhoff's current law (KCL). The KCL, if considered according to all-dielectric or metal-dielectric structures, is demonstrated to realise devices based on plasmonic or mantle cloaking, respectively. The KCL generalises scattering cancellation theory for arbitrary shape devices in homogeneous backgrounds and it can be extended as a local cloaking condition beyond quasi-static regime. A cloaking device can be seen as a node (i.e. no current source or sink) that ensures all current densities to be compensated even for more complicated low scattering devices.

Invisibility and cloaking structures as weak or strong solutions of Devaney-Wolf theorem

Inspired by a general theorem on non-radiating sources demonstrated by Devaney and Wolf, a unified theory for invisible and cloaking structures is here proposed. By solving Devaney-Wolf theorem in the quasi-static limit, a weak solution is obtained, demonstrating the existence of Anapole modes, Mantle Cloaking and Plasmonic Cloaking. Beyond the quasi-static regime, a strong solution of Devaney-Wolf theorem can be formulated, predicting general non-scattering devices based on directional invisibility, Transformation Optics, neutral inclusions and refractive index continuity. Both weak and strong solutions are analytically demonstrated to depend on the concept of contrast, mathematically defined as a normalized difference between constitutive parameters (or wave-impedance property) of a material and its surrounding background.

A New Implementation of Frequency Selective Surface Cloak for Cylindrical Structures

The main purpose of this paper is to design, implement and measure a new sample of mantle cloak. A new method called mantle cloak is introduced by cloaking an object by a single, conformal meta-surface which can drastically suppress the scattering of the desired object. In this paper, a grid lattice is placed around a dielectric object as the cloaking structure. Previously, this FSS has been utilized for the cloaking of PEC object; but, when patch structure is placed around the dielectric it has inductive equivalent circuit with appropriate designing, it can be used for cloaking of dielectric cylinder. Numerical simulations of near field and far field of the designed structure are derived to prove the effectiveness of our meta-surface cloak. Also, measurement results of S11 and S21 related to cloaked structure and their comparison with single dielectric proves suitable performance of the designed cloak. The results show more than 20% bandwidth for this structure. This means that this structure is suitable for broadband operations.

Mantle cloaking for co-site radio-frequency antennas

We show that properly designed mantle cloaks, consisting of patterned metallic sheets placed around cylindrical monopoles, allow tightly packing the same antennas together in a highly dense telecommunication platform. Our experimental demonstration is applied to the relevant example of two cylindrical monopole radiators operating for 3G and 4G mobile communications. The two antennas are placed in close proximity, separated by 1/10 of the shorter operational wavelength, and, after cloaking, are shown to remarkably operate as if isolated in free-space. This result paves the way to unprecedented co-siting strategies for multiple antennas handling different services and installed in overcrowded platforms, such as communication towers, satellite payloads, aircrafts, or ship trees. More broadly, this work presents a significant application of cloaking technology to improve the efficiency of modern communication systems.

Waveguide Characterization of S-Band Microwave Mantle Cloaks for Dielectric and Conducting Objects.

We present the experimental characterization of mantle cloaks designed so as to minimize the electromagnetic scattering of moderately-sized dielectric and conducting cylinders at S-band microwave frequencies. Our experimental setup is based on a parallel-plate waveguide system, which emulates a two-dimensional plane-wave scattering scenario, and allows the collection of near-field maps as well as more quantitative assessments in terms of global scattering observables (e.g., total scattering width). Our results, in fairly good agreement with full-wave numerical simulations, provide a further illustration of the mantle- cloak mechanism, including its frequency-sensitivity, and confirm its effectiveness both in restoring the near-field impinging wavefront around the scatterer, and in significantly reducing the overall scattering.

Correction: Corrigendum: Thermal invisibility based on scattering cancellation and mantle cloaking

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.

Decoupling antennas in printed technology using elliptical metasurface cloaks

In this paper, we extend the idea of reducing the electromagnetic interactions between transmitting radiators to the case of widely used planar antennas in printed technology based on the concept of mantle cloaking. Here, we show that how lightweight elliptical metasurface cloaks can be engineered to restore the intrinsic properties of printed antennas with strip inclusions. In order to present the novel approach, we consider two microstrip-fed monopole antennas resonating at slightly different frequencies cloaked by confocal elliptical metasurfaces formed by arrays of sub-wavelength periodic elements, partially embedded in the substrate. The presence of the metasurfaces leads to the drastic suppression of mutual near-field and far-field couplings between the antennas, and thus, their radiation patterns are restored as if they were isolated. Moreover, it is worth noting that this approach is not limited to printed radiators and can be applied to other planar structures as well.

Reduction of Mutual Coupling Between Strip Dipole Antennas at Terahertz Frequencies With an Elliptically Shaped Graphene Monolayer

In this letter, we propose to use the concept of the mantle cloaking method in order to reduce the mutual coupling between strip dipole antennas at low-terahertz (THz) frequencies. Here, we show that by covering each antenna with an elliptically shaped graphene monolayer, the electromagnetic interaction between the antennas can be suppressed drastically. The material properties of the graphene monolayer are properly tuned to provide the required surface reactance that leads to the restoration of the radiation patterns of the antennas as if they were isolated and uncoupled. The tunability of graphene with respect to its chemical potential allows us to reduce the mutual coupling between the antennas over a range of frequencies.

Design of cloaked Yagi-Uda antennas

Exploiting the appealing features of the mantle cloaking approach, we propose a general procedure that allows designing Yagi-Uda antennas with low scattering profile. The proposed radiators are able to operate efficiently within a given frequency band, while being electromagnetically undetectable in another desired frequency range. The general design procedure of such cloaked antennas is first presented, considering that both antenna and cloak requirements must be simultaneously fulfilled. Then, we prove that a cloaked Yagi-Uda antenna can be installed in close proximity to other radiators without affecting their performance. Due to these unprecedented characteristics, the proposed radiators may have a strong impact in the design of compact overcrowded platforms for both terrestrial and satellite applications.

Electromagnetic cloak to restore the antenna radiation patterns affected by nearby scatter

We have theoretically verified the feasibility of the concept of mantle cloak for very high frequency (VHF) antenna communications. While the applicability of the concept has been demonstrated for an infinitely long cylindrical obstacle and infinitely long electric source [Y.R. Padooru, A.B. Yakovlev, and P.-Y. Chen and Andrea Alù, J. Appl. Phys., 112, 104902, (2012)], the use of this cloak in realistic conditions is not straightforward. In this paper as an electric source we consider a typical VHF monopole antenna mounted on ground plane together with a metallic cylindrical obstacle. The both ground plane and obstacle affect the antenna radiation scattering. Nevertheless, we could show that the mantle cloak can bee successfully applied to restore the radiation patterns of antenna even when the source, the cylindrical metallic obstacle, and the ground plane have finite length. We have studied the antenna adaptation in the presence of the cloaked obstacle and found that the complete radiation system is still functional in the bandwidth that is reduced only by 11%.