Carpet Cloak Research Articles

Potential application of a homogeneous and anisotropic slab as an angle insensitive absorbing material

In this paper, a flat and incident angle independence absorbing material is proposed and numerically verified in the optical spectrum. A homogeneous and anisotropic dielectric slab as a non-reflecting layer is first reviewed, and a feasible realization strategy of the slab is then given by using layered isotropic materials. When the loss components of the constitutive materials are not zero, the slab will work as an angle insensitive absorbing layer, and the absorption rate augments with increase of the losses. As the numerical verifications, the field distributions of a metallic cylinder and a triangular metallic object individually covered by the designed absorbing layer are demonstrated. The simulation results show that the designed absorbing layer can efficiently absorb the incident waves with the property of incident angle independence at the operation frequency. This homogeneous slab can be used in one and two dimensional situations for the realization of an invisibility cloak, a carpet cloak and even a skin cloak, if it is used to conformally cover target objects.

Acoustic metasurfaces with rapid change profiles

Acoustic metasurfaces with subwavelength unit cells modulating the phase of sound wave were developed to realize beam forming, steering, focusing, and recently carpet cloaking. However, these metasurfaces were used for small phase gradient applications and the ability to cloak or mimic reflection surfaces with rapid change profiles remained unexplored. Here, we analytically and experimentally investigate the effect of rapid change in-plane phase gradient on the design of acoustic metasurfaces for carpet cloaking and diffusing applications. Helmholtz resonators with different neck radii are used to form the ultrathin metasurface in our study. The variation of neck radius of the Helmholtz resonator results in different reflection phase of acoustic wave. In both analytical and experimental results, the large in-plane phase gradient does not affect the carpet cloak design, which is solely depending on the phase correction of the local profile height. On the contrary, the impedance mismatch resulted from the rapid change profile yields additional scattering to the reflection from such rapid change profiles. The recreation of the reflection pattern of rapid change profiles from a flat surface using metasurface needs to consider this multiple scattering, which requires additional amplitude modulation from the metasurface.

A thin and conformal metasurface for illusion acoustics of rapidly changing profiles

Recently developed metasurfaces have been used for surface engineering applications. However, the ability to cloak or mimic reflective surfaces with a large in-plane phase gradient remains unexplored. One major challenge is that even with a small incidence angle, the strong acoustic impedance variation induced by the random height profile creates additional scattering and increases the complexity of the analysis and design of the metasurface. Here, we introduce an acoustic metasurface with 1/12 wavelength thickness to realize an acoustic carpet cloak for a randomly rapid-change surface and a virtual acoustic diffuser from a flat surface using a set of Helmholtz resonators. The limitation of the metasurface for large phase gradient application is explored based on a nonlocal model that considers the contributions from neighboring surface profiles. This study extends the integration of smart acoustic surface and may find applications of surface engineering such as in architectural acoustics.

Design and demonstration of an underwater acoustic carpet cloak

The carpet cloak, which is designed to hide the objects placed on a reflecting surface, has become a topic of considerable interest. Inspired by those theoretical works, the experimental realization of acoustic carpet cloak in air host has been reported. However, due to the difficulty in obtaining the unit cell in reality, the underwater carpet cloak still remains in simulation thus far. Here, we design and fabricate a realizable underwater acoustic carpet cloak. By introducing a scaling factor, the structure of the carpet cloak, which is comprised of layered brass plates, is greatly simplified at the cost of some impedance match. The experimental results demonstrate a good performance of the proposed carpet cloak in a wide frequency range. Our work paves the way for future applications in the practical underwater devices.

Three-dimensional ultra-wideband carpet cloak using multi-layer dielectrics

In this letter, a feasible three-dimensional ultra-broadband carpet cloak is presented. To realize the structure, use of multilayer dielectrics are proposed. The results prove that the three-dimensional conducting bump can be concealed from all the viewing angles in an ultra-broadband frequency range of 8–30 GHz. An experimental results confirm the performance of the proposed pyramidal cloak in practice. © 2017 Wiley Periodicals, Inc. Microwave Opt Technol Lett 59:1284–1288, 2017

Electrically tunable metasurface based on Mie-type dielectric resonators

In this paper, we have designed a metasurface based on electrically tunable Mie-type resonators and theoretically demonstrated its tunable response to electromagnetic waves with varying frequency. The metasurface consists of disk-like ferroelectric resonators arrayed on a metal film and the upper surface of resonators is covered by ion gel film which is transparent for incident electromagnetic wave. Using the metal film and ion gel film as electrodes, the permittivity of the resonators can be adjusted by an external electric field and, as a result, the reflection phase of the resonators can be dynamically adjusted in a relatively wide range. By programmable controlling the electric field strength applied on resonators of metasurface, a 2π phase ramp can be realized and, thereby, the arbitrary reflection behavior of incident waves with varied frequency is obtained. Because of the tunability, this metasurface can also be used to design adaptive metasurface lens and carpet cloak.

All-dielectric ultrathin conformal metasurfaces: lensing and cloaking applications at 532 nm wavelength.

Metasurfaces are ideal candidates for conformal wave manipulation on curved objects due to their low profiles and rich functionalities. Here we design and analyze conformal metasurfaces for practical optical applications at 532 nm visible band for the first time. The inclusions are silicon disk nanoantennas embedded in a flexible supporting layer of polydimethylsiloxane (PDMS). They behave as local phase controllers in subwavelength dimensions for successful modification of electromagnetic responses point by point, with merits of high efficiency, at visible regime, ultrathin films, good tolerance to the incidence angle and the grid stretching due to the curvy substrate. An efficient modeling technique based on field equivalence principle is systematically proposed for characterizing metasurfaces with huge arrays of nanoantennas oriented in a conformal manner. Utilizing the robust nanoantenna inclusions and benefiting from the powerful analyzing tool, we successfully demonstrate the superior performances of the conformal metasurfaces in two specific areas, with one for lensing and compensation of spherical aberration, and the other carpet cloak, both at 532 nm visible spectrum.

Experimental Demonstration of Metasurface‐Based Ultrathin Carpet Cloaks for Millimeter Waves

A metasurface carpet cloak for millimeter-wave range with polarization-independent performance is experimentally demonstrated. It is shown that the cloak is able to mimic the ground plane by fully restoring the amplitude and phase distributions for both transverse electric and transverse magnetic polarizations, with a relatively wide frequency and angular widths response. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Topology-optimized carpet cloaks based on a level-set boundary expression.

The concept of topology-optimized carpet cloaks is presented using level-set boundary expressions. Specifically, these carpet cloaks are designed with the idea of minimizing the value of an objective functional, which is here defined as the integrated intensity of the difference between the electric field reflected by a flat plane and that controlled by the carpet cloak. Made of dielectric material, our cloaks are designed to imitate reflections from a flat plane, and with some cloaks, the value of the objective functional falls below 0.12% of that for a bare bump on a flat plane. These optimal carpet cloaks spontaneously satisfy the time-reversal symmetry of the scattered field during the optimization process. The profiles associated with optimal configurations are controlled by adjusting a regularization parameter. With this approach, a variety of configurations with different structural characteristics can be obtained.

New procedure to design low radar cross section near perfect isotropic and homogeneous triangular carpet cloaks.

A new design procedure for near perfect triangular carpet cloaks, fabricated based on only isotropic homogeneous materials, is proposed. This procedure enables us to fabricate a cloak with simple metamaterials or even without employing metamaterials. The proposed procedure together with an invasive weed optimization algorithm is used to design carpet cloaks based on quasi-isotropic metamaterial structures, Teflon and AN-73. According to the simulation results, the proposed cloaks have good invisibility properties against radar, especially monostatic radar. The procedure is a new method to derive isotropic and homogeneous parameters from transformation optics formulas so we do not need to use complicated structures to fabricate the carpet cloaks.

Acoustic carpet cloak based on an ultrathin metasurface

An acoustic metasurface carpet cloak based on membrane-capped cavities is proposed and investigated numerically. This design has been chosen for allowing ultrathin geometries, although adapted to airborne sound frequencies in the range of 1 kHz ($\ensuremath{\lambda}\ensuremath{\approx}30$ cm), surpassing the designs reported in the literature in terms of thinness. A formulation of generalized Snell's laws is first proposed, mapping the directions of the incident and reflected waves to the metasurface phase function. This relation is then applied to achieve a prescribed wavefront reflection direction, for a given incident direction, by controlling the acoustic impedance grading along the metasurface. The carpet cloak performance of the proposed acoustic metasurface is then assessed on a triangular bump obstacle, generally considered as a baseline configuration in the literature.

Broadband reflected wavefronts manipulation using structured phase gradient metasurfaces

Acoustic metasurface (AMS) is a good candidate to manipulate acoustic waves due to special acoustic performs that cannot be realized by traditional materials. In this paper, we design the AMS by using circular-holed cubic arrays. The advantages of our AMS are easy assemble, subwavelength thickness, and low energy loss for manipulating acoustic waves. According to the generalized Snell’s law, acoustic waves can be manipulated arbitrarily by using AMS with different phase gradients. By selecting suitable hole diameter of circular-holed cube (CHC), some interesting phenomena are demonstrated by our simulations based on finite element method, such as the conversion of incoming waves into surface waves, anomalous reflections (including negative reflection), acoustic focusing lens, and acoustic carpet cloak. Our results can provide a simple approach to design AMSes and use them in wavefront manipulation and manufacturing of acoustic devices.

Ultra-broadband carpet cloak for transverse-electric polarization

Magnetism is a necessity in constructing macroscopic metamaterial invisibility cloaks that are theoretically designed by transformation optics, but will generally limit the cloaking bandwidth to an impractically narrow range. To meet the broad bandwidth demand, magnetism has been fully abandoned in previous demonstrations of macroscopic carpet cloaking, whose approach, however, cannot apply to a transverse-electric (TE) polarization. To fill this gap, here we experimentally demonstrate an ultra-broadband magnetic carpet cloak for the TE polarization. The cloak is made of non-resonant closed-ring metamaterials with little dispersion and the cloaking performance is confirmed with both time-domain simulation and frequency scanning measurement over a broad bandwidth corresponding to a pulse signal illumination.

Experiments on metasurface carpet cloaking for audible acoustics

We present experiments on acoustic carpet cloaking by using a metasurface made of graded Helmholtz resonators. The thin metasurface, placed over the object to hide, is designed such that the reflection phase shifts of the resonators at the resonance frequency are tuned to compensate the shape of the object to cloak. Experimental as well as numerical results show the efficiency of the cloak at the resonance frequency. The reflection of a short pulse is also reported to inspect the broadband character of the cloak.

A metasurface carpet cloak for electromagnetic, acoustic and water waves.

We propose a single low-profile skin metasurface carpet cloak to hide objects with arbitrary shape and size under three different waves, i.e., electromagnetic (EM) waves, acoustic waves and water waves. We first present a metasurface which can control the local reflection phase of these three waves. By taking advantage of this metasurface, we then design a metasurface carpet cloak which provides an additional phase to compensate the phase distortion introduced by a bump, thus restoring the reflection waves as if the incident waves impinge onto a flat mirror. The finite element simulation results demonstrate that an object can be hidden under these three kinds of waves with a single metasurface cloak.

進化的戦略を用いたカーペットクロークのトポロジー最適化

This paper presents a topology optimization for carpet cloaks by means of an evolutionary strategy. Carpet cloaks are the cloaking devices which can render an bump on a flat plane invisible by imitating the reflection from the flat plane. The objective function is defined as the integrated intensity of the difference between the reflected wave from flat plane and that from carpet cloaks covering the bumps. The structures of carpet cloaks made of dielectrics are expressed by level set functions and the structural boundaries between air and dielectrics are expressed by the iso-surfaces of the level set functions. Level set functions are optimized by CMA-ES which is the most powerful evolutionary algorithm for multimodal problem. Obtained optimal configurations realize good performance under the explicit perimeter constraint.

Manipulating scattering features by metamaterials

We present a review on manipulations of electromagnetic scattering features by using metamaterials or metasurfaces. Several approaches in controlling the scattered fields of objects are presented, including invisibility cloaks and radar illusions based on transformation optics, carpet cloak using gradient metamaterials, dc cloaks, mantle cloaks based on scattering cancellation, “skin” cloaks using phase compensation, scattering controls with coding/programmable metasurfaces, and scattering reductions by multilayered structures. Finally, the future development of metamaterials on scattering manipulation is predicted.

Broadband Carpet Cloak Designed using Nanocomposite Metamaterials for 3- 5��������Wavelength Range

In this paper, a broadband carpet cloak using nanocomposite layers to operate in whole 3–5-μm wavelength band is designed successfully. For this purpose, spherical nanometals or doped semiconductors with desired plasma frequency are proposed. In this way, the refractive index of the cloak as an analytic function is considered by optical conformal mapping. In a two-dimensional structure, we approximated the obtained refractive index inside each pixel to be constant. Each pixel is considered as an effective medium that possesses nanoparticles with desired plasma frequency. Therefore, plasma frequency of nanoparticles should be adjusted proportional to the desired value of refractive index in each two-dimensional pixels, which is approximated to be constant (two-dimensional layers approximation). The cloak region is compacted for broadband performance. To compact the cloak region, refractive index values should be greater than 1 and realization of these required values is possible by designing nonresonant structures.

Planar Distributed Full-Tensor Anisotropic Metamaterials for Transformation Electromagnetics

Planar distributed full-tensor anisotropic metamaterials for cloaks of invisibility based on transformation electromagnetics are proposed. The proposed metamaterials are composed of nonresonant transmission lines and are advantageous in full control of the off-diagonal components of the permeability tensor as well as broadband and low-loss characteristics. The explicit design formulas for the metamaterials are given based on the equivalent circuit derived directly from Maxwell's equations. A carpet cloak of invisibility is designed and the validity of the design theory is confirmed by circuit simulations. In addition, the carpet cloak is implemented in microstrip line technology and its performance is demonstrated experimentally at microwave frequencies.

Reflectionless quasiconformal carpet cloak via parameterization strategy

In this work the possibility of using both the parameterization function and the least squares method to achieve a quasiconformal carpet cloak via a transformation optical design is demonstrated. The parameterization strategy allows us to obtain a continuous interface around the media, and, therefore, the resulting structure is reflectionless. Polynomial series are added to the coordinate transformation functions, providing a number of degrees of freedom (DoF) without modifying the device boundary conditions. The anisotropy reduction effects over the coordinate transformation are analyzed for different numbers of DoF in the parameterization function. The method’s connection with the Riemann mapping theorem is also studied. Our results indicate that anisotropy can be reduced to very close to zero with increasing DoF without incurring edge reflections around the carpet cloak.