Metasurface Carpet Cloak Research Articles

Broadband cross circularly polarized carpet invisibility cloak based on Pancharatnam–Berry phase reflective metasurface

We present a broadband circularly polarized reflective metasurface carpet cloak. The metasurface unit cell is composed of two crossed H-shaped metal patterns. When the incident wave is left-handed circularly polarization (LCP), the metasurface can achieve high-efficiency cross-polarization conversion in the wide frequency range. The cross-polarization reflection efficiency is higher than 94%. By using the proposed cross-polarized reflection unit cell structure, the phase gradient metasurfaces based on the Pancharatnam–Berry phase principle is proposed, and a broadband reflective metasurface invisibility cloak is constructed. The simulation results show that when the LCP light is incident vertically, the proposed carpet cloak can significantly suppress the scattering and reconstruct the wavefront in the broadband frequency range.

3D Printing of A Thermally Programmable Conformal Metasurface (Adv. Mater. Technol. 7/2022)

Curved Metasurfaces In article number 2101479, Qingxuan Liang and co-workers demonstrate a thermally programmable conformal metasurface carpet cloak printed by the composite fused deposition molding. The thermal programming method effectively reduces the complexity of conformal manufacturing and ensures the electromagnetic performances of the metasurface. This work provides a viable implementation strategy for complex curved metasurface and will substantially promote the applications of the conformal invisibility cloaks.

Cross-Polarization Carpet Cloaking Based on Polarization Conversion Metasurfaces

The principle of phase compensation on the metasurfaces is applied to realize the cloaking function, which provides a new idea for the further development of invisibility technology. However, the uppermost layer of the metasurface carpet cloak previously studied is mostly a metal structure with ohmic loss and low coupling efficiency. In practical applications, these ordinary invisibility cloaks are easy to be corroded, and does not endure high temperature, and only has the function of invisibility under the action of the same polarization. Here, we propose to use the unit structure of high-refractive-index dielectric silicon resonators to construct the invisibility cloak, which can better reduce loss, improve reflection efficiency, and can be better applied to practical applications. A unique cross polarization stealth device with the broadband cloaking effect is designed to effectively resist cross polarization detectors. Objects of arbitrary shape can be hidden under the triangular carpet stealth device designed to achieve the purpose that the detector cannot detect, so as to achieve stealth function. The working wavelength of the cloak proposed is at 1550nm, achieving the effect of cross polarization invisibility in the communication band.

Broadband Generation of Polarization-Immune Cloaking via a Hybrid Phase-Change Metasurface

Metasurface-enabled cloaking offers an alternative platform to render scatterers of arbitrary shapes indiscernible. However, specific propagation phases generated by the constituent elements for cloaking are usually valid for a single or few states of polarization (SOP), imposing serious restrictions on their applications in broadband and spin-states manipulation. Moreover, the functionality of a conventional metasurface cloak is locked once fabricated due to the absence of active elements. Here, we propose a hybrid phase-change metasurface carpet cloak consisting of coupled phase-shift elements setting on novel phase-change material of Ge2Sb2Se4Te1 (GSST). By elaborately arranging meta-atoms at either 0 or 90 degrees on the external surface of the hidden targets, the wavefront of its scattered lights can be thoroughly rebuilt for arbitrary SOP exactly as if the incidence is reflected by a flat ground, ensuring the targets’ escape from polarization-scanning detections. Furthermore, the robustness of phase dispersion of meta-atoms endows the metasurface cloak wideband indiscernibility ranging from 7.55 to 8.35 µm and tolerated incident angles at least within ±25°. By reversibly switching of the phase states of Ge2Sb2Se4Te1, the stealth function of our design can be turned on and off. The generality of our approach will provide a straightforward platform for polarization-immune cloaking, and may find potential applications in various fields such as electromagnetic camouflage and illusion and so forth.

3D Printing of A Thermally Programmable Conformal Metasurface

AbstractThe curved surfaces conformality has become the coming barrier to the implementation of the metasurface carpet cloaks. A curved conformal strategy that can balance the electromagnetic performances and the manufacturing challenges of metasurface simultaneously is highly in demand, but yet underexplored. Herein, a novel thermal programming 3D‐printing method for curved conformal metasurface is proposed. Owing to the glass‐rubber transition characteristics of thermoplastic polymers, the advantages of rigid substrate and flexible substrate are fully utilized to achieve the conformality of metasurface, which ingeniously reduces the manufacturing complexity and ensures the electromagnetic performances. As a proof, an arched metasurface carpet cloak composed of the easy‐bending unit cells is designed and fabricated by the composite fused deposition molding of polylactic acid and Sn‐Bi materials with the thermal programming process. Experiments prove that the maximum reduced total radar cross section of the conformal cloak reaches about −9.4 dB while the 3 dB reduction bandwidth is ≈26.6%. The excellent cloaking performance of the metasurface sample demonstrates the effectiveness of this curved conformal method. This work provides a viable implementation strategy for complex curved metasurface and will substantially promote the applications of the conformal invisibility cloaks.

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.

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.

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.

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.

Tunable arc-shaped acoustic metasurface carpet cloak

In this paper, a tunable arc-shaped acoustic metasurface (AAM) carpet cloak has been proposed and systematically investigated. The AAM carpet cloak consists of covers, rotors, bolts and nuts. The rotors can be rotated continuously within the covers to form a reconfigurable hook channel mechanism. By setting various rotational angles, we construct two-dimensional (2D) tunable AAM carpet cloaks to achieve excellent stealth performances under different operating conditions. Numerical simulations and experimental results for the 2D carpet cloaks show good agreements. Furthermore, simulated results for three-dimensional (3D) carpet cloaks have demonstrated that tapered objects are successfully escaping from being detected. The tunable hook channel mechanism can be flexibly applied to arbitrarily curved metasurface carpet cloaks, making them closer to practical invisibility acoustic devices.

Broadband and wide-angle terahertz carpet cloaks based on pattered graphene metasurfaces

Conventional metasurface carpet cloak generally has a narrow working bandwidth and a small incident angle range, which is far from meeting the requirements of modern military for broadband and wide-angle cloaking. Based on graphene apertures and graphene patches, we propose the design of metasurface carpet cloaks for terahertz wave and explore their application potential in realizing broadband and wide-angle cloaking. Simulation results demonstrate that the carpet cloaks based on graphene apertures and patches can achieve a large working bandwidth of 35.7% and 52.6%, and a wide angular span of ±30° and ±40°, respectively. The cloaking performance and the working bandwidths of the two graphene metasurface carpet cloaks can be tuned by changing the graphene Fermi energy uniformly. The carpet cloak constructed by graphene patches has a larger bandwidth and a wider angle domain than that by graphene apertures, which can be understood by comparing the phase dispersion of the two graphene metasurfaces.

3D acoustic metasurface carpet cloak based on groove structure units

A 3D acoustic metasurface carpet cloak (AMCC) based on groove structure units is proposed and investigated. The key idea behind this acoustic invisibility is the phase compensation via local phase modulation, and thus a cloak with a thickness of only of a half wavelength could be designed for objects with arbitrary geometric shapes and sizes. We first demonstrate the cloaking effect of hiding a conical object numerically, and find that the working bandwidth of the designed AMCC is from 6200–7500 Hz. Further experimental results are in good agreement with the numerical simulations, verifying that the AMCC works well for the normal incident wave and the small-angled incident waves. As another example, we also confirm that the proposed AMCC can hide a semispherical object. The present AMCC is thin, has a simple geometrical structure, and is easy to implement, which benefits future realistic applications.

Experimental Demonstration of a 3D‐Printed Arched Metasurface Carpet Cloak

AbstractMetasurfaces have an extraordinary wavefront manipulation capability to generate various functionalities with ultrathin planar structures and provide an unprecedented means of easily achieving invisibility carpet cloaks in practice. However, due to the limitation of the fabrication method, great challenges exist in the implementation of curved or spherical metasurface‐based cloaks with good performance. Here, a novel fabrication method that combines 3D printing via stereolithography and injection molding of liquid metal is proposed, and an arched metasurface carpet cloak operating in the microwave domain is realized successfully as a demonstration. Experiments show that the proposed ultrathin and polarization‐independent cloak has good stealth performance in terms of bandwidth and angular span. Notably, the fabrication method is also suitable for obtaining other metasurfaces and metamaterials with arbitrary shapes, which may greatly promote the practical application of artificial electromagnetic structures.

Broadband Metasurface Carpet Cloak in the Near Infrared Region

The advent of metasurface gives a great push to the realization of extremely thin cloak. However, narrow bandwidth remains an unsolved problem. To solve this problem, we propose a broadband metasurface carpet cloak which can work well in the near-infrared band with circularly-polarized incidence. A novel phase compensation method is presented in this letter to show that a single metasurface can meet the phase requirements for cloaking in a broad spectrum. Through finite-difference time-domain simulations, we demonstrate that the cloak conceals a bump well in the broad spectrum from 1200 to 1680 nm.

Multi-frequency metasurface carpet cloaks.

Metasurfaces provide an alternative way to design three-dimensional arbitrary-shaped carpet cloaks with ultrathin thicknesses. Nevertheless, the previous metasurface carpet cloaks work only at a single frequency. To overcome this challenge, we here propose a macroscopic metasurface carpet cloak. The cloak is designed with a metasurface of a few layers that exhibit a special spatial distribution of the conductance and inductance in the unit cell; therefore, it can fully control the reflection phases at several independent frequencies simultaneously. Because of this, the present metasurface cloak can work at dual frequencies based on multi-resonance principle. The proposed design methodology will be very useful in future broadband macroscopic cloaks design with low profiles, light weights, and easy access.

Metasurface Approach to External Cloak and Designer Cavities

A metasurface, with only a single layer of artificial atoms for ease of fabrication, can become a practical surface-equivalent route to transformation optical (TO) applications. The previous design paradigm for a metasurface carpet cloak is based on straightforward phase compensation, hampering more general wave manipulations. Here, we propose a theoretical approach in designing a metasurface using the concept of complementary media as an intermediate step. The metasurface, effectively storing all the original information in TO media, enables specific TO applications that normally require complementary media. A passive external metasurface cloak is numerically demonstrated here, which can hide an object on top of a reflective metasurface, mimicking a flat mirror. Furthermore, our scheme enables metasurfaces to be used to construct arbitrary standing waves on-demand, which will be useful for constructing tailor-made cavity modes, optical trapping, and illusion-type TO applications, which can project hologr...

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.

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.

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.