Combining the absorptive and radiative loss in metasurfaces for multi-spectral shaping of the electromagnetic scattering

Compatible camouflage for dual-band guided-laser radar and infrared via a metamaterial perfect absorber

Laser-guided detector and infrared detection have attracted increasing attention in a wide range of research fields, including multispectral detection, radiative cooling, and thermal management. Previously reported absorbers presented shortcomings of lacking either tunability or compatibility. In this study, a metamaterial perfect absorber based on a Helmholtz resonator and fractal structure is proposed, which realizes tunable perfect absorptivity (α 1.06μ m >0.99,α 10.6μ m >0.99) of guided-laser radar dual operating bands (1.06 µm and 10.6 µm) and a low infrared average emissivity (ε¯3-5μ m =0.03,ε¯8-14μ m =0.31) in two atmospheric windows for compatible camouflage. The proposed perfect absorber provides a dynamically tunable absorptivity without structural changes and can be applied to optical communication, military stealth or protection, and electromagnetic detection.

High-temperature resistant electromagnetic protection bilayer structure based on the low-reflection metasurface and wave-absorbing material

In this paper, we propose a high-temperature resistant bilayer structure for electromagnetic protection with low reflection, consisting of a metasurface and an absorbing layer. The bottom metasurface decreases the reflected energy by using a phase cancellation mechanism to make electromagnetic wave scattering in the 8-12 GHz range. While the upper absorbing layer assimilates the incident electromagnetic energy through electrical losses and simultaneously regulates the reflection amplitude and phase of the metasurface to enhance scattering and expand its operating bandwidth. Research shows that the bilayer structure achieves a low reflection of -10 dB in the range of 6.7-11.4 GHz due to the combined effect of the above two physical mechanisms. In addition, long-term high-temperature and thermal cycling tests verified the stability of the structure in the temperature range of 25-300°C. This strategy provides the feasibility of electromagnetic protection in high-temperature conditions.

Tunable Infrared Detection, Radiative Cooling and Infrared-Laser Compatible Camouflage Based on a Multifunctional Nanostructure with Phase-Change Material

The nanostructure composed of nanomaterials and subwavelength units offers flexible design freedom and outstanding advantages over conventional devices. In this paper, a multifunctional nanostructure with phase-change material (PCM) is proposed to achieve tunable infrared detection, radiation cooling and infrared (IR)-laser compatible camouflage. The structure is very simple and is modified from the classic metal-dielectric-metal (MIM) multilayer film structure. We innovatively composed the top layer of metals with slits, and introduced a non-volatile PCM Ge₂Sb₂Te₅ (GST) for selective absorption/radiation regulation. According to the simulation results, wide-angle and polarization-insensitive dual-band infrared detection is realized in the four-layer structure. The transformation from infrared detection to infrared stealth is realized in the five-layer structure, and laser stealth is realized in the atmospheric window by electromagnetic absorption. Moreover, better radiation cooling is realized in the non-atmospheric window. The proposed device can achieve more than a 50% laser absorption rate at 10.6 μm while ensuring an average infrared emissivity below 20%. Compared with previous works, our proposed multifunctional nanostructures can realize multiple applications with a compact structure only by changing the temperature. Such ultra-thin, integratable and multifunctional nanostructures have great application prospects extending to various fields such as electromagnetic shielding, optical communication and sensing.

Broadband radar cross section reduction by an absorptive metasurface based on a magnetic absorbing material

A highly feasible approach to achieve a broadband radar cross section (RCS) reduction using a simple magnetic metasurface is presented. A magnetic absorbing material (MAM) with high permittivity and magnetic loss is introduced into the metasurface design instead of the more common dielectric material to considerably reduce its thickness. The metasurface is composed of an optimized two-dimensional array of MAM meta-atoms and a metal plate in back. The meta-atoms share a simple square ring shape but with variable geometrical parameters, forming strong absorption in different frequency bands with large reflection phase differences. By hybridizing the absorption and phase-cancelation technique, a 10-dB RCS reduction from 3.4 to 18 GHz is achieved at a thickness of only 4 mm. Further experimental measurements are provided to evaluate the performance. Our work provides a promising way to broaden the bandwidth of RCS reduction with low density, reduced thickness, and stable performance, which can be utilized in harsh physical and chemical environments.

Broadband low-scattering metasurface using a combination of phase cancellation and absorption mechanisms

In this paper, a broadband low-scattering metasurface is proposed by using a combination of phase cancellation and absorption mechanisms. The metasurface is composed of two structural layers. One layer adopts the geometric phase cell that can obtain a different reflection phase by changing its orientation. Through the random phase distribution design, electromagnetic diffusion can be realized to reduce the backward scattering energy. The other layer is made of a resistive frequency selective surface (RFSS) that can absorb the incident wave by converting it into Ohmic loss. The above two physical mechanisms respectively play the great roles at two distinct frequency bands, and finally make our metasurface achieve the RCS reduction over a wide frequency band ranging from 13 to 31.5 GHz. Both simulation and experimental results are in good agreement, which fully demonstrates our design method. The analysis of the scattering patterns, electric-field distribution and power loss density are given to explain the hybrid RCS-reduction mechanism of our metasurface.

Broadband and Tunable RCS Reduction using High-order Reflections and Salisbury-type Absorption Mechanisms

In this paper, a broadband and tunable radar cross section (RCS) reduction structure is proposed by using the hybrid physical mechanism that is based on high-order reflections and Salisbury-type absorption. Our design combines the high-index grating structure with a traditional Salisbury screen in which the lossy sheet is made of a graphene structure. When it is illuminated by a plane wave with normal incidence, the Salisbury screen can absorb the incoming wave, and the introducing high-index grating structure could further reduce the backward scattering wave by generating high-order reflection beams, which broadens the RCS reduction bandwidth. In addition, the RCS reduction level can be dynamically controlled by tuning the surface resistance of the graphene layer. Simulated results show that the proposed structure can realize tunable RCS reduction between 4.1 and 18 GHz under normal incidence with different graphene resistances. Experimental results are in accordance with those of the simulation results. In addition, the scattering field distributions and the plots of surface power loss density have been illustrated to analyze the RCS-reduction mechanism of our structure.

Ultra-Wideband and Wide-Angle Microwave Metamaterial Absorber

In order to extend the performance of radar absorbing materials, it is necessary to design new structures with wideband properties and large angles of incidence which are also as thin as possible. The objective of this work, realized within the framework of the SAFAS project (self-complementary surface with low signature) is, then, the development of an ultra-wideband microwave absorber of low thickness. The design of such material requires a multilayered structure composed with dielectric layers, metasurfaces, and wide-angle impedance matching layers. This solution has been realized with on-the-shelf materials, and measured to validate the concept. At normal incidence, the bandwidth ratio, defined for a magnitude of the reflection coefficient below -10 dB, is 4.7:1 for an absorber with a total thickness of 11.5 mm, which corresponds to λ/7 at the lowest operating frequency. For an incidence of 60°, this bandwidth ratio is reduced to 3.8:1, but the device remains ultra-wideband.

Uneven-Layered Coding Metamaterial Tile for Ultra-wideband RCS Reduction and Diffuse Scattering

In this paper, a novel uneven-layered coding metamaterial tile is proposed for ultra-wideband radar cross section (RCS) reduction and diffuse scattering. The metamaterial tile is composed of two kinds of square ring unit cells with different layer thickness. The reflection phase difference of 180° (±37°) between two unit cells covers an ultra-wide frequency range. Due to the phase cancellation between two unit cells, the metamaterial tile has the scattering pattern of four strong lobes deviating from normal direction. The metamaterial tile and its 90-degree rotation can be encoded as the ‘0’ and ‘1’ elements to cover an object, and diffuse scattering pattern can be realized by optimizing phase distribution, leading to reductions of the monostatic and bi-static RCSs simultaneously. The metamaterial tile can achieve −10 dB RCS reduction from 6.2 GHz to 25.7 GHz with the ratio bandwidth of 4.15:1 at normal incidence. The measured and simulated results are in good agreement and validate the proposed uneven-layered coding metamaterial tile can greatly expanding the bandwidth for RCS reduction and diffuse scattering.

Broadband Polarization Conversion Metasurface Based on Metal Cut-Wire Structure for Radar Cross Section Reduction

A class of linear polarization conversion coding metasurfaces (MSs) based on a metal cut-wire structure is proposed, which can be applied to the reduction properties of radar cross section (RCS). We firstly present a hypothesis based on the principle of planar array theory, and then verify the RCS reduction characteristics using linear polarization conversion coding MSs by simulations and experiments. The simulated results show that in the frequency range of 6⁻14 GHz, the linear polarization conversion ratio reaches a maximum value of 90%, which is in good agreement with the theoretical predictions. For normal incident x- and y-polarized waves, RCS reduction of designed coding MSs 01/01 and 01/10 is essentially more than 10 dB in the above-mentioned frequency range. We prepare and measure the 01/10 coding MS sample, and find that the experimental results in terms of reflectance and RCS reduction are in good agreement with the simulated ones under normal incidence. In addition, under oblique incidence, RCS reduction is suppressed as the angle of incidence increases, but still exhibits RCS reduction effects in a certain frequency range. The designed MS is expected to have valuable potential in applications for stealth field technology.

Pixelated Checkerboard Metasurface for Ultra-Wideband Radar Cross Section Reduction

In this paper we designed and fabricated a metasurface working as a radar cross section (RCS) reducer over an ultra wide band of frequency from 3.8 to 10.7 GHz. The designed metasurface is a chessboard-like surface made of alternating tiles, with each tile composed of identical unit cells. We develop a novel, simple, highly robust and fully automated approach for designing the unit cells. First, a topology optimization algorithm is used to engineer the shape of the two unit cells. The area of each unit cell is pixelated. A particle swarm optimization algorithm is applied wherein each pixel corresponds to a bit having a binary value of 1 or 0 indicating metallization or no metallization. With the objective of reducing the RCS over a specified frequency range, the optimization algorithm is then linked to a full wave three-dimensional electromagnetic simulator. To validate the design procedure, a surface was designed, fabricated and experimentally tested showing significantly enhanced performance than previous works. Additionally, angular analysis is also presented showing good stability and wide-angle behavior of the designed RCS reducer. The automated design procedure has a wide range of applications and can be easily extended to design surfaces for antennas, energy harvesters, noise mitigation in electronic circuit boards amongst others.

Dual-band wide-angle metamaterial perfect absorber based on the combination of localized surface plasmon resonance and Helmholtz resonance

In this article, a dual-band wide-angle metamaterial perfect absorber is proposed to achieve absorption at the wavelength where laser radar operates. It is composed of gold ring array and a Helmholtz resonance cavity spaced by a Si dielectric layer. Numerical simulation results reveal that the designed absorber displays two absorption peaks at the target wavelength of 10.6 μm and 1.064 μm with the large frequency ratio and near-unity absorptivity under the normal incidence. The wide-angle absorbing property and the polarization-insensitive feature are also demonstrated. Localized surface plasmons resonance and Helmholtz resonance are introduced to analyze and interpret the absorbing mechanism. The designed perfect absorber can be developed for potential applications in infrared stealth field.

Dynamical beam manipulation based on 2-bit digitally-controlled coding metasurface

Recently, a concept of digital metamaterials has been proposed to manipulate field distribution through proper spatial mixtures of digital metamaterial bits. Here, we present a design of 2-bit digitally-controlled coding metasurface that can effectively modulate the scattered electromagnetic wave and realize different far-field beams. Each meta-atom of this metasurface integrates two pin diodes, and by tuning their operating states, the metasurface has four phase responses of 0, π/2, π, and 3π/2, corresponding to four basic digital elements “00”, “01”, “10”, and “11”, respectively. By designing the coding sequence of the above digital element array, the reflected beam can be arbitrarily controlled. The proposed 2-bit digital metasurface has been demonstrated to possess capability of achieving beam deflection, multi-beam and beam diffusion, and the dynamical switching of these different scattering patterns is completed by a programmable electric source.

Multi-spectral Metasurface for Different Functional Control of Reflection Waves

Metasurface have recently generated much interest due to its strong manipulation of electromagnetic wave and its easy fabrication compared to bulky metamaterial. Here, we propose the design of a multi-spectral metasurface that can achieve beam deflection and broadband diffusion simultaneously at two different frequency bands. The metasurface is composed of two-layered metallic patterns backed by a metallic ground plane. The top-layer metasurface utilizes the cross-line structures with two different dimensions for producing 0 and π reflection phase response, while the bottom-layer metasurface is realized by a topological morphing of the I-shaped patterns for creating the gradient phase distribution. The whole metasurface is demonstrated to independently control the reflected waves to realize different functions at two bands when illuminated by a normal linear-polarized wave. Both simulation and experimental results show that the beam deflection is achieved at K-band with broadband diffusion at X-Ku band.