Carpet Cloak Research Articles

Local discontinuous Galerkin methods for the carpet cloak model

Local discontinuous Galerkin methods for the carpet cloak model

Design of a broadband metamaterial-based acoustic lens using elaborated carpet cloak strategy

Design of a broadband metamaterial-based acoustic lens using elaborated carpet cloak strategy

Graphene metasurface for broadband, wide-angle and polarization-insensitive carpet cloak

Although great progress has been made in stealth research based on metamaterial/metasurface, it is still a great challenge to achieve broadband, wide-angle and full polarization stealth at the same time. In this paper, a metasurface carpet cloak based on graphene rings is proposed to address this issue. Simulation results demonstrate that the arched carpet cloak can achieve a polarization-insensitive invisibility with a large working bandwidth of 45% and a wide angular span of ±40° in terahertz range. The cloaking bandwidth can be further increased to 60% by adjusting graphene Fermi energy to control the phase dispersion of meta-atoms. By comparing the arched and triangular carpet cloaks and the arched carpet cloaks with different curvature, the sensitivity of cloaking performance to the shape of carpet cloak is demonstrated. The metasurface based on graphene ring can not only give a good solution for broadband, wide-angle and polarization-insensitive invisibility in terahertz to mid-infrared region, but also provide inspiration for the construction of other metasurface devices with excellent performance.

All dielectric terahertz carpet cloaking by phase compensation metasurfaces

Metasurfaces can control the phase, amplitude and polarization of electromagnetic waves, providing a new method for the design of cloaking devices. At present, most of the metasurface carpet cloaking devices are based on metal structure with larger ohmic loss, which severely limits the efficiency of the devices. Here, we propose to use all dielectric unit structure to construct metasurface layer. By using local phase compensation principle, all dielectric metasurface carpet cloaking can be designed in terahertz region. Based on the numerically simulation of near field and far field characteristics, the cloaking effect can be confirmed. The incident wavefront is reflected by the cloaking device and the plane wave characteristics can be recovered well. The cloaking effect can be effective in a broadband range from 0.3 to 0.58 THz. Moreover, the proposed all dielectric metasurface cloaking technology can be extended to the visible light and microwave range.

Extremely thin reflective metasurface for low-frequency underwater acoustic waves: Sharp focusing, self-bending, and carpet cloaking

An extremely thin metasurface is proposed for manipulating underwater reflected waves. Each metasurface unit is only 1/61.7 of the target wavelength in thickness and comprises an air cavity capped with a thin vibration plate held in place by rubber spacers on steel supports. The unit-cell design is thin, simple, and can be adjusted to obtain a full 2π phase shift in water waves that are reflected from the metasurface. It also provides this phase shift for a broad frequency range of 20–800 Hz for incident waves. The effectiveness of the design and the resolution of the expected effect is demonstrated for sharp focusing, self-bending, and directional carpet cloaking, which are applications with great potential in energy harvesting, underwater communication, and submarine stealth and antidetection.

Ge2Sb2Te5-based reconfigurable metasurface for polarization-insensitive, full-azimuth, and switchable cloaking.

Electromagnetic (EM) metasurface mantles afford an alternative avenue, allowing for the possibility of rendering arbitrary-shape objects unobservable. But the available mechanisms either depend on the states of polarization or the azimuth of wave incidence, or cannot dynamically manipulate cloaking responses without altering the structures. Herein, a three-dimensional closed-ring-based metasurface carpet cloak involving Ge2Sb2Te5 that circumvents current drawbacks of metasurface structures is proposed. By judiciously designing meta-atoms on the external surface of a spherical object, the scattered wavefront, including the distributions of EM fields and polarizations, can be reconstructed, resembling what is deflected from a flat plane. Enabled by the perfectly symmetric distribution of meta-atoms, the carpet cloak is demonstrated to work well under arbitrary states of polarization and arbitrary azimuthal angles of incident light. Meanwhile, by converting Ge2Sb2Te5 from the amorphous to crystalline state, the designed scheme is empowered with the ability to switch "ON" and "OFF" of stealth states. Furthermore, the unique design achieves invisibility over ±20∘ angular span in the mid-infrared range from 8800 to 9450 nm. The validated recipe empowers robust steps forward to achieve full-polarization, full-azimuth operation, and switchable cloaking in the real-world, showing great potential applications in stealth, camouflage, and illusion fields.

Underwater Acoustic Stealth by a Broadband 2-Bit Coding Metasurface

Underwater acoustic stealth is of significance for hiding targets. Since perfect cloaking achieved by carpet cloaks or metasurface carpet cloaks may only be suitable for static targets, here we propose a broadband 2-bit coding metasurface to reduce the scattering cross section (SCS) of an object by the destructive interferences among waves scattered by different coding elements. A simple and easy fabricated design is developed by drilling square holes into a steel plate. Utilizing the antiresonance induced by couplings among four square holes of the unit cell, we achieve an approximate $95.62\mathrm{%}$ fractional bandwidth of acoustic stealth with the SCS reduction below $\ensuremath{-}10\phantom{\rule{0.2em}{0ex}}\mathrm{dB}$, which has been confirmed by both the theoretical prediction and experimental measurement. Compared with the previously proposed underwater acoustic carpet cloaks by the transformation acoustics, the bandwidth of SCS reduction is greatly expanded by the destructive interferences. In addition, simulations have demonstrated that the proposed metasurface may also be suitable for an object with a nonplanar surface.

Optical cloaking and invisibility: From fiction toward a technological reality

Optical invisibility, which started in the pages of fiction before becoming an intriguing quest of humankind for over a century, has blossomed into a remarkable scientific journey toward reality over the last two decades. Perfect optical cloaking requires the total scattering of electromagnetic waves around an object at all angles, all polarizations, over a wide frequency range, irrespective of the medium. Such a device is still far-fetched, requiring the transformation of space around a cloaked region such that the phase velocity is faster than other areas to preserve the phase relationships. However, by simplifying the invisibility requirements, pioneering work on spherical transformation cloaks, carpet cloaks, plasmonic cloaks, and mantle cloaks has been realized in narrowband microwave, infrared, and even optical wavelengths. In this Tutorial, we review the theoretical basis for invisibility cloaking, from spherical transformational optics to non-Euclidian cases, and discuss their limitations. Subsequently, we highlight the recent trends in realizing reconfigurable intelligent cloaks to overcome the traditional limitations of wideband operation and parallel efforts in unidirectional cloaking. Because the human eye is insensitive to the phase and polarization of visible light, a class of ray optics cloaking devices has been recently developed by eliminating phase preservation requirements. Notably, we focus on the recent progress achieved on invisibility cloaks that function in natural incoherent light and can be realized using standard optical components. We conclude this Tutorial with a prospective of potential applications and the practicality of optical cloaks in everyday life.

Machine-learning based design of digital materials for elastic wave control

Materials for wave control need to be both anisotropic and spatially distributed. Traditional method is to first design a microstructure with anisotropic property, and then change geometric parameters of the microstructure according to analytical theory or numerical calculation. Unlike the traditional method, mechanical properties of digital materials can be easily tuned by changing the 0/1 ordering without changing the geometry of digital materials. However, determining suitable orderings of digital materials according to target properties remains a key challenge. In this paper, we establish a digital structural genome to solve this problem. By combining the developed machine learning method with finite element method, we can quickly calculate elastic wave properties of digital materials with all orderings and finally establish a digital structural genome quickly and accurately. The complete digital structural genome provides us with a fast approach to design anisotropy and spatial distribution of materials. Our research unequivocally shows that the establishment of a complete structural genome database of digital materials is of great significance for inverse design multifunctional structures, and can opens an avenue to achieve wave control on demand, such as corner cloak and acoustic carpet cloak.

Polarization-robust mid-infrared carpet cloak with minimized lateral shift

With the advent and rapid development of the transformation optics and metamaterials, invisibility cloaks have captivated much attention in recent years. While most cloaking schemes suffer from limited bandwidth, the carpet cloak, which can hide an object on a reflecting plane, can operate over a broadband frequency range. However, the carpet cloaks experimentally realized thus far still have several limitations. For example, the quasi-conformal mapping carpet cloak leads to a lateral shift of the reflected light ray, while the birefringent carpet cloak only works for a specific polarization. In this work, we propose a conformal transformation scheme to tackle these two problems simultaneously. As an example, we design a mid-infrared carpet cloak in a silicon platform and demonstrate its polarization-insensitive property as well as the minimized lateral shift over a broad frequency band from 24 to 28.3 THz.

Analysis of wave scattering from 2D curved metasurfaces using Floquet and Fourier series expansions

An efficient technique for calculating the scattering from curved metasurfaces using the extinction theorem in conjunction with the Floquet and Fourier series expansions is presented. Here, we treat the two-dimensional metasurfaces that have transversal polarizabilities with no variation along the y-axis. The boundary conditions at the metasurface are given by the generalized sheet transition conditions (GSTCs) whose susceptibilities are given in an arbitrary local coordinate system. First, we use the extinction theorem to provide integral equations of the scattering problem. The integral equations involve the Green's functions, tangential electric and magnetic fields and their normal derivatives in regions above and below the metasurface. Then, we employ the Floquet theorem that gives us the analytical periodic Green's functions of each region. Next, we employ the Fourier theorem to expand the tangential fields in terms of unknown Fourier coefficients. The GSTCs and the integral equations provide equations to be solved for the unknowns. The method can calculate scattering from both periodic and non-periodic metasurfaces. The technique is used to analyse different applied problems such as carpet cloaking, illusion, and radar echo width reduction. The method is fast and accurate and can efficiently treat metasurfaces with electrically large curved geometries with dimensions as large as 120 times the wavelength.

Polarization-insensitive 3D conformal-skin metasurface cloak

Electromagnetic metasurface cloaks provide an alternative paradigm toward rendering arbitrarily shaped scatterers invisible. Most transformation-optics (TO) cloaks intrinsically need wavelength-scale volume/thickness, such that the incoming waves could have enough long paths to interact with structured meta-atoms in the cloak region and consequently restore the wavefront. Other challenges of TO cloaks include the polarization-dependent operation to avoid singular parameters of composite cloaking materials and limitations of canonical geometries, e.g., circular, elliptical, trapezoidal, and triangular shapes. Here, we report for the first time a conformal-skin metasurface carpet cloak, enabling to work under arbitrary states of polarization (SOP) at Poincaré sphere for the incident light and arbitrary conformal platform of the object to be cloaked. By exploiting the foundry three-dimensional (3D) printing techniques to fabricate judiciously designed meta-atoms on the external surface of a conformal object, the spatial distributions of intensity and polarization of its scattered lights can be reconstructed exactly the same as if the scattering wavefront were deflected from a flat ground at any SOP, concealing targets under polarization-scanning detections. Two conformal-skin carpet cloaks working for partial- and full-azimuth plane operation are respectively fabricated on trapezoid and pyramid platforms via 3D printing. Experimental results are in good agreement with numerical simulations and both demonstrate the polarization-insensitive cloaking within a desirable bandwidth. Our approach paves a deterministic and robust step forward to the realization of interfacial, free-form, and full-polarization cloaking for a realistic arbitrary-shape target in real-world applications.

Phase-change reconfigurable metasurface for broadband, wide-angle, continuously tunable and switchable cloaking.

Being invisible at will has fascinated humanity for centuries and it has become more tangible with the development of metasurfaces, which have demonstrated the extraordinary ability of wavefront manipulation. However, state-of-the-art invisibility cloaks typically work in a deterministic system with a limited bandwidth and small incident angle ranges. Here, by integrating the phase-change material of Ge2Sb2Te5 and the wavefront tailoring functionality of a reflective metasurface, we have achieved a unique carpet cloak that is endowed with broadband invisibility from 6920 to 8220 nm, fully concealing objects over a wide angular span of ±25° and a prominent radar cross-section reduction. Furthermore, the central cloaking wavelength can be continuously tuned with Ge2Sb2Te5 film under different intermediate phases by precisely controlling external stimuli, which will provide a flexible and encouraging way to achieve active features once fabricated. Simulation results also show that the cloaking bandwidth can be significantly extended by triggering Ge2Sb2Te5 from the amorphous to crystalline states. Importantly, the hybrid metasurface can realize switching of "ON" and "OFF" states in terms of cloaking features by converting Ge2Sb2Te5 from the amorphous to the crystalline state. To the best of our knowledge, this is the first metasurface carpet cloak that utilizes the phase-change material of Ge2Sb2Te5 to achieve ultra-broadband, wide-angle, continuously tunable and switchable cloaking with low profiles, light weights, and easy access. This design of a reconfigurable cloak is expected to find potential applications in various areas such as vehicle cloaking, illusions and so on.

Non-closed acoustic cloaking devices enabled by sequential-step linear coordinate transformations

Hitherto acoustic cloaking devices, which conceal objects externally, depend on objects' characteristics. Despite previous works, we design cloaking devices placed adjacent to an arbitrary object and make it invisible without the need to make it enclosed. Applying sequential linear coordinate transformations leads to a non-closed acoustic cloak with homogeneous materials, creating an open invisible region. Firstly, we propose to design a non-closed carpet cloak to conceal objects on a reflecting plane. Numerical simulations verify the cloaking effect, which is completely independent of the geometry and material properties of the hidden object. Moreover, we extend this idea to achieve a directional acoustic cloak with homogeneous materials that can render arbitrary objects in free space invisible to incident radiation. To demonstrate the feasibility of the realization, a non-resonant meta-atom is utilized which dramatically facilitated the physical realization of our design. Due to the simple acoustic constitutive parameters of the presented structures, this work paves the way toward realization of non-closed acoustic devices, which could find applications in airborne sound manipulation and underwater demands.

Ultra-broadband passive acoustic metasurface for wide-angle carpet cloaking

Acoustic metasurfaces can reshape a reflected wavefront rather arbitrarily, despite being much thinner than the wavelength, thus allowing on-demand wavefront modulation in a variety of applications. Recent passive metasurfaces, however, have suffered from bandwidth limitations, thus restricting their range of operation. In this work, we propose the systematic design of ultra-broadband passive metasurfaces by combining the broadband local reflection rule with an optimization method. The validation of the technique is demonstrated by fabrication and measurement of an ultra-broadband carpet cloak. Numerical and experimental results show that the relative bandwidth of the optimized carpet cloak, thinner than one fifth of the maximum wavelength, can exceed 93.33%, a value that is much larger than that of previous passive metasurfaces. Furthermore, multi-frequency pulse incidence tests reveal the excellent time-domain broadband characteristics of the metasurface over a wide range of angles of incidence. The proposed strategy opens a new route for the design of advanced passive metasurfaces for ultra-broadband wave manipulation and thus promotes practical applications of broadband acoustical devices.

Design of two invisibility cloaks using transmissive and reflective metamaterial-based multilayer frame microstructures.

Ultrathin metamaterials provide new possibilities for the realization of cloaking devices because of their ability to control electromagnetic waves. However, applications of metamaterials in cloaking devices have been limited primarily to reflection-type carpet cloaks. Hence, a transmissive free-space cloak was developed using a multilayer frame structure, wherein highly transparent metamaterials were used to guide incident waves into propagating around an object. The cloaking effect was quantitatively verified using near-field and far-field distributions. Metamaterials allow for the cloaking shells of transmissive cloaks to be developed without spatially varying extreme parameters. Moreover, a transmissive invisible cloak with metamaterial-based mirrors was designed. The design principle of this cloak with a frame structure consists of four metamaterial-based mirrors and two metal mirrors. After covered with the designed metamaterials-based mirrors cloak, the outgoing electromagnetic wave is restored greatly as if the wave passes directly through the obstacle without distortion. This cloak used the metamaterials mirrors to adjust the reflected angle, so that the outgoing electromagnetic wave does not change direction, thereby achieving the cloaking effect.

An ultrathin metasurface carpet cloak based on the generalized sheet transition conditions

In this paper, we propose an arc-shaped metasurface carpet cloak based on generalized sheet transition conditions. The main idea behind this invisibility is phase compensation through local phase modulation, in which the front of the reflected wave is reshaped just as the incident wave hits the flat mirror. Secondly, we have implemented an arc-shaped metasurface carpet cloak based on a cavity structure unit with a thickness of one fifth wavelength. The cloak can still have a stealth effect in the range of incident angle from 0°to 30°and in the range of frequency from 8 GHz to 13 GHz. The numerical results of the arc-shaped metasurface carpet cloak agree well with the theoretical calculation of GSTCs and it can provide an important reference for the future carpet cloak field.

Wide‐band polarisation‐independent metasurface‐based carpet cloak

Although the ability of metasurfaces to control electromagnetic wavefronts has made it possible to design ultrathin carpet cloaks, the narrow operation bandwidth limits the performance of these cloaks. Here, the authors propose a new method to enhance operating bandwidth of metasurface-based carpet cloaks. The proposed cloak consists of super-cells providing appropriate phase shift to incident wave over a wide and continuous range of frequencies. Using the proposed method, a polarisation-independent wide-band single-layer carpet cloak is designed and fabricated. The performance of the proposed technique is verified using both numerical and experimental data. Experimental results show that the cloak operates well at all frequencies in the range of 9–10 GHz. The proposed method can be used for designing single-layer wide-band metasurface-based cloaks and illusion coatings at different frequency regimes.

An ultrathin acoustic carpet cloak based on resonators with extended necks

By employing a set of resonators with extended necks, an ultrathin acoustic carpet cloak with a thickness of about 1/30th of the operation wavelength is proposed to achieve effective cloaking around a bump at 1000 Hz. The cloak is composed of a periodic array of supercells. The supercell is constructed by a solid unit and seven inhomogeneous resonators with different extended necks. The elements in the supercell are capable of achieving distinct phase shifts ranging from 0 to 2π in a phase increment of π/4. To demonstrate the effectiveness of the cloak, both normal incidence and oblique incidence (incident angle equals to 30°) plane waves impinging on a cloaked triangular bump are investigated numerically and validated experimentally. The results show that it is possible to conceal the bump by wrapping over the surface of the bump through using the proposed ultrathin carpet cloak. As a resonant-element based acoustic device, the cloak exhibits invisibility in the frequency range from 980 to 1070 Hz. The proposed acoustic metasurface offers a simple and compact solution for acoustic cloaking and may hold promise in realistic applications.

Assembling hydrodynamic cloaks to conceal complex objects from drag

Transformation hydrodynamics and the corresponding metamaterials have been proposed as a means to exclude the drag force acting on an object. Here, we report a strategy to deploy the hydrodynamic cloaks in a more practical manner by assembling different-shaped cloaking parts. Our strategy is to first model a square-shaped cloak and a carpet cloak and then combine them to conceal a more complex-shaped space in the three-dimensional hydrodynamic flow. With the derivation of transformation hydrodynamics, the coordinate transformations for each hydrodynamic cloaking are demonstrated with the calculated viscosity tensors. The pressure and velocity fields of the square, triangular (carpet), and exemplary three-dimensional house-shaped cloaks are numerically simulated, thus showing a cloaking effect and reduced drag. This study suggests an efficient way of cloaking complex architectures from fluid-dynamic forces.