Multi-spectral compatible metasurface with low infrared emissivity, independent microwave complex-amplitude control, and high visible transparency.

With the rapid development of communication technology and detection technology, it is difficult for devices operating in a single spectrum to meet the application requirements of device integration and miniaturization, resulting in the exploration of multi-spectrum compatible devices. However, the functional design of different spectra is often contradictory and difficult to be compatible. In this work, a transparent slit circular metasurface with a high filling ratio is proposed to achieve the compatibility of microwave, infrared and visible light. In the microwave, based on the Pancharatnam-Berry phase theory, the continuous amplitude and binary phase can be customized only by rotating the slit angle to achieve an Airy beam function at 8-12 GHz. In the infrared, the mean infrared emissivity is reduced to 0.3 at 3-14 µm by maintaining high conductive filling ratio, and in visible light, based on the transparency of materials, the mean transmittance can achieve 50% at 400-800 nm. All the results can verify the multi-spectral compatibility performance, which can also verify the validity of our design method. Importantly, the multi-spectral compatible metasurface contributes an option for multifunctional integration, which can be further applied in communication, camouflage, and other fields.

Broadband 2D phase-gradient metasurface for linearly-polarized waves by suppressing Lorentz resonance of meta-atoms.

Metasurfaces have exhibited versatile capacities of controlling electromagnetic (EM) waves due to the high degree of freedom of designing artificially engineered meta-atoms. For circular polarization (CP), broadband phase gradient metasurfaces (PGMs) can be realized based on P-B geometric phase by rotating meta-atoms; while for linear polarization (LP), realization of broadband phase gradients has to resort to P-B geometric phase during polarization conversion and polarization purity has to be sacrificed for broadband properties. It is still challenging to obtain broadband PGMs for LP waves without polarization conversion. In this paper, we propose the design of 2D PGMs by combining the inherently wideband geometric phases and non-resonant phases of meta-atom, under the philosophy of suppressing Lorentz resonances that usually bring about abrupt phase changes. To this end, an anisotropic meta-atom is devised which can suppress abrupt Lorentz resonances in 2D for both x- and y-polarized waves. For y-polarized waves, the central straight wire is in perpendicular to electric vector Ein of incident waves, Lorentz resonance cannot be excited although the electrical length approaches or even exceeds half a wavelength. For x-polarized waves, the central straight wire is in parallel with Ein, a split gap is opened on the center of the straight wire so as to avoid Lorentz resonance. In this way, the abrupt Lorentz resonances are suppressed in 2D and the wideband geometric phase and the gradual non-resonant phase are left for broadband PGM design. As a proof of concept, a 2D PGM prototype for LP waves was designed, fabricated and measured in microwave regime. Both simulated and measured results show that the PGM can achieve broadband beam deflection for reflected waves for both x- and y-polarized waves in broadband, without changing the LP state. This work provides a broadband route to 2D PGMs for LP waves and can be readily extended to higher frequencies such as terahertz and infrared regimes.

Dispersion-boosting wideband electromagnetic transparency under extreme angles for TE-polarized waves.

Half-wave wall is the most common method of achieving electromagnetic (EM) transparency. Transmission windows can be formed when reflected waves are out of phase. Due to the interference mechanism, these windows are dependent on the frequency and incident angle of EM waves, leading to limited bandwidth, especially under extreme angles. In this letter, we propose to extend the bandwidth of the transmission window under extreme angles by utilizing dispersion. To this end, long metallic wires are embedded into the half-wave wall matrix, without increasing the physical thickness. Due to the plasma-like behavior of metallic wires under TE-polarization, the effective permittivity of the half-wave wall, rather than keeping constant, increases with frequency nonlinearly. Such a dispersion will boost wideband transparency in two aspects. On one hand, an additional transmission window will be generated where the effective permittivity equals that of the air; on the other hand, the 1st- and 2nd-order half-wave windows will be made quite closer. By tailoring the dispersion, the three windows can be merged to enable wideband transparency under extreme incident angles. A proof-of-principle prototype was designed, fabricated, and measured to verify this strategy. Both simulated and measured results show that the prototype can operate in the whole Ku-band under incident angle [70°, 85°] for TE-polarized waves. This work provides an effective method of achieving wideband EM transparency under extreme angles and may find applications in radar, communications, and others.

Ultra-wideband RCS reduction based on coupling effects between beam diffuse and absorptive structures.

A hybrid design method for broadband radar cross section (RCS) reduction is proposed and successfully demonstrated based on the coupling effects between diffuse and absorptive structures. The reflection energy is distributed into more directions away from the source direction by the one-bit diffuse coding metasurface (CM). The two-layer resistive frequency selective surface (RFSS) is employed in the one-bit CM structure, reducing the amplitude of the co- and cross-polarized reflected waves under circularly polarized wave incidence by converting it into ohmic loss. In addition, the bandwidth of RCS reduction is further broadened through the coupling effects between the metallic patterns and the two-layer RFSS. The coupling effect shows that the absorption rate of the composite structure is significantly improved compared to the only RFSS structure. A lightweight CM loaded with RFSS (the area density is 597 g/m2) was fabricated, analyzed, simulated, and measured. The results show that the proposed mechanism can effectively break the bandwidth constraints of traditional diffusion and absorption methods. Furthermore, the proposed mechanism significantly expands the bandwidth of RCS reduction. The proposed metasurface can achieve a 10 dB RCS reduction in an ultra-wideband from 7.3 to 44.2 GHz with about 143.3% fractional bandwidth. Moreover, the metasurface also has good performances under wide-angle oblique incidences. Under the condition of maintaining lightweight, the design provides an idea for broadening the frequency band.

Feasible strategy for simultaneously achieving excellent frequency selective characteristic and ultralight mechanical properties.

Materials with both excellent frequency selective characteristic and ultralight mechanical properties are highly urgent demanded for its potential applications such as absorbing materials, artificial magnetic conductors, antenna and so on. However, although the research about materials with only excellent frequency selective characteristic or ultralight mechanical properties is advanced, in most cases, it is still a challenge that making a material possesses excellent frequency selective characteristic and ultralight mechanical properties simultaneously. So how to make the two properties achieving a high level simultaneously is a hot topic which remains to be solved. Herein, we proposed a novel and feasible strategy for achieving simultaneously excellent frequency selective characteristic and ultralight mechanical properties material. According to our strategy, the composite we designed behaviors as a FSS which can realize highly efficiency stop bands in 16.09-16.4GHz and 17.11-17.36GHz. At the same time, the composite can be regarded as an ultralight mechanical metamaterial. The relativity density of the composite can reduce to 431.99 Kg/m3, which have a distinct advantage compared with the dielectric layers that conversional FSS used. Moreover, Its elasticity modulus can reach 112.25 MPa and its bending stiffness can reach 90.54 N/mm. These performances show that although the density of the composite is reduced, the composite can still keep well mechanical properties. The strategy we proposed gives a good solution to the problem existing in the materials which desire both excellent frequency selective characteristic and ultralight mechanical properties. The composite is a designing example which can be applied in engineering. So the strategy is a guideline for researchers to achieve composite which owns both excellent frequency selective characteristic and ultralight mechanical properties.

High temperature infrared-radar compatible stealthy metamaterial based on an ultrathin high-entropy alloy.

In this work, a high temperature infrared (IR) and radar compatible stealthy metamaterial based on ultrathin high-entropy alloy are proposed. From room temperature to 600°C, the fabricated radar absorption layer (RAL) can have wideband absorption in X-band (8.2-12.4 GHz) with average absorption 78% owing to magnetic resonance and ohmic loss. The ultrathin high-entropy alloy film is further design as infrared shielding layer (ISL) due to low-emissivity property. The ISL and RAL consist of the IR-microwave compatible stealth metamaterial. It can give rise to the strong reduction of both radar wave reflection and infrared thermal emission. Its bandwidth (absorption over 90%) is 2.15 GHz. In the infrared atmosphere window, it can suppress a half of thermal radiation. This is realized by the subtle combination between the RAL and specifically designed ISL that control the infrared emission and microwave absorption. These results show that they are practically very promising for the application of a radar-infrared bi-stealth technology in high temperature environment.

Polarization-multiplexed full-space metasurface simultaneously merging with an ultrawide-angle antireflection and a large-angle retroreflection.

Multifunctional electromagnetic (EM) metasurfaces are capable of manipulating electromagnetic waves with kaleidoscopic functions flexibly, which will significantly enhance integration and applications of electronic systems. However, most known design schemes only realize the reflection or transmission functions under a specific angle range, which wastes the other half EM space and restricts wider applications of multifunctional metadevices. Herein, an encouraging strategy of broadband and wide-angle EM wavefronts generator is proposed to produce two independent functions, i.e., antireflections for transverse electric (TE) waves and retroreflection for transverse magnetic (TM) waves, which utilizes band-stop and bandpass responses of the metasurface, respectively. As a feasibility verification of this methodology, a three-layer cascaded metasurface, composed of anisotropic crossbar structures patterned on the two surfaces of a dielectric substrate with sandwiched orthogonal metal-gratings, is designed, fabricated, and measured. Both the simulated and experimental results are in good accordance with theoretical analyses. This full-space metasurface opens up a new route to multifunctional metasurfaces and will further promote engineering applications of metasurfaces.

Large-angle broadband transmission of electromagnetic waves through dielectric plates by embedding meta-atoms.

In many practical applications, dielectric electromagnetic (EM) windows are usually under large-angle incidence of EM waves rather than normal incidence. To guarantee normal operation of devices inside, high transmission must be maintained under large incident angles, especially for TE-polarized waves. In this work, we propose a method of achieving broadband transmission of TE-polarized waves under large incident angles by embedding meta-atoms within dielectric plates. To this end, long metallic wires and S-shaped structures are embedded in the original dielectric plate, the former of which will dilute the effective permittivity due to plasma oscillation and the latter will increase the effective permeability due to induced strong current loops under large incident angles. In this way, two consecutive transmission peaks can be generated, forming a broad transmission band under large incident angles. A proof-of-principle Ku-band prototype was designed, fabricated, and measured to verify this strategy. Both simulated and measured results show that the prototype can operate in the whole Ku-band under incident angle [60°, 85°] for TE-polarized waves, with significantly enhanced transmission. This work provides an effective method of enhancing large-angle transmission of EM waves and may find applications in radar, communications and others.

Tailoring transmission window in a dynamic way with a multi-degree-of-freedom.

With the rapid development of wireless technology, the revolution of tailoring transmission window in dynamic way for the next generation communication systems is urgently required. However, the degree-of-freedom for switching transmission spectra of an effective medium still needs further investigation. Here, we propose a paradigm of solving this difficult academic issue via the method of bias-voltage-driven. Leveraging PIN diodes and varactor diodes into the predesigned positions of plasmonic meta-structures, the macro-control of transmission windows switch and the detailed dispersion manipulation can be separately achieved by synergy modulation of feed networks. Both the numerical simulations and experimental verifications are conducted to support the effectiveness of the proposed method. Significantly, the proposed paradigm presents great potential for applications in intelligent radome, adaptive communication systems, and other EM scenarios with multi-degree-of-freedom.

Tailoring permittivity using metasurface: a facile way of enhancing extreme-angle transmissions for both TE- and TM-polarizations.

The transmission of electromagnetic (EM) waves through a dielectric plate will be decreased significantly when the incident angle becomes extremely large, regardless of transverse electric (TE)- or transverse magnetic (TM)- polarization. In this regard, we propose a facile way of tailoring the permittivity of the dielectric material using metasurface to enhance the transmissions of both TE- and TM-polarized waves under extremely large incidence angles. Due to parallel or antiparallel electric fields induced by the metasurface, the net electric susceptibility is altered, and hence the effective permittivity can be tailored to improve the impedance matching on the two air-dielectric interfaces, which enhances the wave transmissions significantly under extreme incident angles. As an example, we apply this method to a typical ceramic-matrix composite (CMC) plate. By incorporating orthogonal meta-gratings into the CMC plate, its effective permittivity is reduced for the TE-polarized waves but increased for the TM-polarized waves under the extreme incidence angle, which can reduce the impedance for the TE-polarization and increase the Brewster angle for the TM-polarization. Therefore, the impedance matchings for both TE- and TM-polarizations are improved simultaneously and dual-polarized transmission enhancements are achieved under the extreme angles. Here, the transmission responses have been numerically and investigated using the finite-difference-time-domain (FDTD) method. A proof-of-principle prototype is designed, fabricated, and measured to verify this method. Both numerical simulations and measurement results show that the prototype can operate under extremely large incidence angles θi∈[75°,85°] with significant transmission enhancement for both TE- and TM-polarizations compared to the pure dielectric plate. This work provides a facile way to enhance the transmissions under extreme angles and can be readily extended to terahertz and optical frequencies.

Transmission enhancement of a half-wave wall under extreme angles by synergy of double lorentz resonances.

The a half-wave wall is usually adopted as the transparent window for electromagnetic (EM) waves ranging from microwave to optical regimes. Due to the interference nature, the bandwidth of the half-wave wall is usually quite narrow, especially under extreme angles for TE-polarized waves. It is usually contradictory to expand the bandwidth and to keep high transmission. To overcome this contradiction, we propose to extend the transmission bandwidth of half-wave walls under extreme angles by introducing Lorentz-type resonances using metasurfaces. The impedance of the half-wave wall is firstly analyzed. To improve the impedance matching, the impedance below and above the half-wave frequency should be increased. To this end, metallic wires and I-shaped structures are incorporated into the half-wave wall as the mid-layer. Due to the Lorentz-type resonance of the metallic wire, effective permittivity below the half-wave frequency can be reduced while that above the half-wave frequency can be increased due to Lorentz-type resonance of the I-shaped structures, both under large incident angles. In this way, the impedance matching, and thus the transmission, can be improved within an extended band. A proof-of-principle prototype was designed, fabricated, and measured to verify this strategy. Both simulated and measured results show that the prototype can operate in 14.0-19.0GHZ under incident angle [70°, 85°] with significant transmission enhancement for TE-polarized waves. This work provides an effective method of enhancing the transmission of EM waves and may find applications in radomes, IR windows, and others.

Quasi-omnidirectional retroreflective metagrating for TE-polarized waves based on wave-vector reversions.

Structuring elements of gratings brings more freedom in manipulating diffraction waves, e.g., retroreflection using diffraction orders other than the 0th order. Most retroreflective metagratings (RMs) can achieve retroreflection only under one particular direction, limiting their applications. In this paper, we propose a quasi-omnidirectional RM based on wave-vector reversion for TE-polarized waves. The metagrating element is composed of four rotationally-symmetric sub-elements, which is composed of one probe and two directors on its two sides. The substrate-air-metal layer can reverse kz while directors can reverse kx. Therefore, the wave-vector k of reflected waves can be completely reversed by the sub-element, providing necessary momentum for retroreflection. The -2nd diffraction order of the metagrating is tailored to channel out waves with reversed k, leading to retroreflection. Due to the element's four-fold rotational symmetry, retroreflection can be achieved along four directions, covering all of the four quarters of azimuth angle. We demonstrate prototypes in Ku band, and the average backscattering enhancement compared with a metal plane with the same area (SAMP) along the four directions reaches up to 31.3 dB with incident angle 50.0° at 15.0 GHz. Both simulated and measured results verify our design. This work provides another perspective on retroreflection and may find applications in retroreflective functional devices.

Generating diverse functionalities simultaneously and independently for arbitrary linear polarized illumination enabled by a chiral transmission-reflection-selective bifunctional metasurface.

A multifunctional metasurface is capable of manipulating electromagnetic waves and achieving kaleidoscopic functions flexibly, which significantly improves the integration and utilization of a single metasurface and has become one of the hotspots in electromagnetics. However, the majority of designs to date can only operate for limited polarization states in half-space and are difficult to show diverse functions at the same time, which restrict the widespread applications of multifunctional metadevices. Herein, an inspiring strategy of a chiral transmission-reflection-selective bifunctional metasurface is proposed to generate two independent functions in co-polarized reflection channel for left-handed circular polarized (LCP) incidence utilizing rotation-induced geometric phase modulation and in co-polarized transmission channel for right-handed circular polarized (RCP) incidence utilizing scaling-induced propagation phase modulation, and both functions appear concurrently under arbitrary linear polarized (LP) incident waves. To verify the feasibility of this methodology, three proof-of-concept metadevices composed of a dual-mode orbital angular momentum (OAM) generator, a bifocal metalens and an integrated metadevice of OAM generator and metalens are constructed and their performances in simulations and experiments are in good accordance with the theoretical ones. This exotic design of bifunctional metasurface will open up a promising way for multifunctional metadevices in engineering applications.

Extremely angle-stable transparent window for TE-polarized waves empowered by anisotropic metasurfaces.

Impedance mismatch generally exists upon interfaces between different media. This is especially true for TE-polarized waves with large incident angles since there is no Brewster effect. As a result, high-efficiency transmission can only be guaranteed within limited incident angle range. It is desirable that transparent windows possess robust angle-stability. In this work, we propose a strategy of realizing transparent windows with extreme angle-stability using anisotropic metasurfaces. Different from traditional isotropic materials, anisotropic metasurfaces require specific three-dimensional permittivity and permeability parameters. Theoretical formulas are derived to realize a highly efficient transmission response without angular dispersion. To validate our design concept, a two-layer cascaded electromagnetic anti-reflector is designed, and it exhibits a characteristic impedance matching for nearly all incidence angles under TE-polarization illumination. As a proof-of-concept, a prototype of extremely angle-stable transparent window is fabricated and measured. Compared with the pure dielectric plate, the reflection coefficients are on average reduced by 40% at 13.5 GHz for TE-polarized waves from 0° to 80°. Therefore, we think, anisotropic cascaded electromagnetic transparent windows are capable of tailoring the electromagnetic parameter tensors as desired, and provide more adjustable degrees of freedom for manipulating electromagnetic wavefronts, which might open up a promising way for electromagnetic antireflection and find applications in radomes, IR windows and others.

Polarization meta-converter for dynamic polarization states shifting with broadband characteristic.

Polarization, as an important property of light, has been widely discussed in modern detecting and radar systems. A polarization converter that can be used to achieve dynamic control is regarded as an excellent alternative for implementing the integrated functionalities of communication and stealth. In this work, we propose a paradigm of meta-converter for dynamic polarization states shifting from linear-to-linear (LTL) to linear-to-circular (LTC) polarization. The strategy is achieved by loading voltage-controlled PIN diodes on the double-arrows metallic meta-resonators. The operation modes can be switched by changing the bias voltage. When the PIN diodes are turned on, the polarization meta-converter (PMC) will reflect and convert a linearly polarized electromagnetic (EM) wave into a circularly polarized one in 5.6-15.5 GHz with an axial ratio (AR) below 3dB. When the PIN diodes are turned off, the PMC will reflect and convert a linearly polarized EM wave into the orthogonal counterpart in 7.6-15.5 GHz with a polarization conversion ratio (PCR) over 88%. Simulations and experimental results show a good agreement, which manifests the feasibility of our proposed meta-converter. Moreover, the proposed PMC has great potential for polarization-dependent communication and stealth systems.

Dual-polarization multi-angle retroreflective metasurface with bilateral transmission windows.

Metasurfaces have provided unprecedented degrees of freedom in manipulating electromagnetic (EM) waves and also granted high possibility of integrating multiple functions into one single meta-device. In this paper, we propose to incorporate the retroreflection function with transmission function by means of metasurface design and then demonstrate a dual-polarization multi-angle retroreflective metasurface (DMRM) with bilateral transmission bands. To achieve high-efficiency retroreflections, the compact bend structures (CBSs), which exhibit high reflections around 10.0 GHz in X band, are added onto the substrate of the DMRM. Two selected metasurface elements are periodically arranged so as to form 0-π-0 phase profile. By delicately adjusting the periodicity, high-efficiency retroreflections can be produced for both TE and TM-polarized waves under both vertical incidence and oblique incident angles ±50.0°, with an average efficiency of 90.2% at the designed frequency. Meanwhile, the two metasurface elements exhibit high transmission properties and minor phase disparities in S, C and Ku bands, resulting in bilateral transmission windows. Prototypes were designed and fabricated. Both simulated and measured results verified our design. This work provides an effective means of integrating retroreflection functions with other functions and may find applications in target tracking, radomes and other sensor integrated devices in higher frequency or even optical frequency bands.

Orbital angular momentum generator with multiple retroreflection channels enabled by an angle-selective metasurface.

Vortex beams carrying orbital angular momentum (OAM) have aroused great attention on account of the remarkable potential in the field of communication. It has the characteristics of higher spectrum efficiency, greater channel capacity and stronger anti-interference, which will revolutionize the wireless communications in the future. However, target tracking on a vortex generator in practical applications is becoming a challenge because the backscattering of electromagnetic (EM) waves under oblique incidence is too small for detection. Currently, the main way to solve this problem is to load an extra retroreflector such as Luneburg lens, which in turn leads to increased weights and bulky volumes. In this paper, we propose a vortex generator simultaneously with retroreflective characteristics utilizing an angle-selective metasurface. The meta-atom can achieve broadband polarization conversion under normal incidence and efficient retroreflection under oblique incidence. Without the need for an additional retroreflection phase arrangement, an OAM generator composed of such meta-atoms can be achieved in 15.0-21.0GHz under both x- and y-polarized normal incidence. Meanwhile, four retroreflection channels are opened under oblique illumination of both transverse electric (TE) and transverse magnetic (TM) waves at 20.0GHz. Both the simulated and measured results show excellent performances. The integration of an OAM generator and retroreflector will greatly reduce the weight and volume and save in the cost of production, which will promote the development of miniaturized, multi-role, and even intelligent functional devices.

Ultra-wideband flexible transparent metamaterial with wide-angle microwave absorption and low infrared emissivity.

Optically transparent metamaterials with the performance of infrared radar compatible stealth have been designed and manufactured on the basis of the continuous in-depth research on single-band stealth technology. In this paper, metamaterials are designed through theoretical calculations and modeling simulations. The designed structure can achieve higher than 90% broadband (8.7-32 GHz) absorption at wide-angle (45 degrees), emissivity of 0.3 in infrared atmospheric window, and optical transparency. In addition, the material can be bent, which greatly expands its application scenarios. The experimental results are consistent with the theoretical calculation and simulation results.

Compatible stealth design of infrared and radar based on plasmonic absorption structure.

In this paper, a metamaterial structure with radar and infrared (IR) compatible stealth characteristics is designed based on the principle of plasmonic absorbing structure (PAS). Due to the lack of reports on PAS-based IR radar compatible stealth, this article combines PAS and IR frequency selective surfaces to achieve the desired purpose. Through mathematical modeling and dispersion engineering of the unit cell proposed, a PAS with ultra-wideband wave absorption is realized. The low emissivity of the IR atmospheric window band is realized by means of the simulation and analysis of the IR frequency selective surface with different indium tin oxide (ITO) occupation ratios. The absorptivity of designed structure is higher than 90% from 4GHz to 28.6GHz, and the emissivity of the IR atmospheric window is only 0.3. The experience of the fabricated sample is consistent with the theoretical analysis and the simulation. Our method enriches the implementation strategies of radar-IR compatibility and has reference significance for multi-spectrum compatible stealth.

Active meta-device for angular dispersion elimination of dual-polarized transmission windows.

Metasurface-based strategy of tailoring electromagnetic waves has aroused huge attention in both academic and engineering communities owing to great potential in a large portfolio of applications. Commonly, however, the artificially designed metasurfaces are sensitive to the oblique incident waves which results in the angular dispersion and inevitably deteriorates the performances. Here, we propose a paradigm of an active meta-device to effectively eliminate the angular dispersion in two orthogonal polarization states of transmission waves. By loading varactor diodes into a transmissive meta-atom, the transmission responses for traverse electric (TE) and traverse magnetic (TM) waves are actively tunable by a voltage-driven manner. Accordingly, the blue shifts of transmission windows can be ingeniously compensated via tailoring the corresponding dispersion characteristics of varactor diodes. A triple-layer meta-atom loaded with varactor diodes is designed as a dual-polarization proof-of-principle, in which the varactor diodes can be applied to independently control two polarization states. The numerical simulations and experimental verification are in good agreement, indicating the proposed paradigm possesses the potential in versatile applications, including radome, wireless communications, and other dispersionless systems.