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.

Mesoporous multi-valence manganese oxides composite nanotubes boosting long-life lithium-ion batteries.

Multi-component nano-oxide composite materials may present special synergistic effects as anode materials for lithium-ion batteries. Mesoporous ß-MnO2/Mn3O4 composite nanotubes are built here via controlling the deoxidation process of carbon-coating to induce a partial phase transition of high valence manganese dioxides. Compared to single ß-MnO2 nanotubes or Mn3O4@C nanotubes, the mesoporous ß-MnO2/Mn3O4@C composite nanotubes exhibit superior electrochemical properties. 679 mA h g-1 of reversible specific capacity and 86% of capacity retention after 1000 cycles at 1 A g-1 current density are obtained. The excellent performance is attributed to the unique multiple phase transitions regulation phenomena of manganese oxide occurring in the ß-MnO2/Mn3O4 composite material during the electrochemical processes, which significantly extends the cycle life of the ß-MnO2/Mn3O4 composite material.

Phase-to-pattern inverse design paradigm for fast realization of functional metasurfaces via transfer learning.

Metasurfaces have provided unprecedented freedom for manipulating electromagnetic waves. In metasurface design, massive meta-atoms have to be optimized to produce the desired phase profiles, which is time-consuming and sometimes prohibitive. In this paper, we propose a fast accurate inverse method of designing functional metasurfaces based on transfer learning, which can generate metasurface patterns monolithically from input phase profiles for specific functions. A transfer learning network based on GoogLeNet-Inception-V3 can predict the phases of 28×8 meta-atoms with an accuracy of around 90%. This method is validated via functional metasurface design using the trained network. Metasurface patterns are generated monolithically for achieving two typical functionals, 2D focusing and abnormal reflection. Both simulation and experiment verify the high design accuracy. This method provides an inverse design paradigm for fast functional metasurface design, and can be readily used to establish a meta-atom library with full phase span.

Absorptive frequency selective surface with two alternately switchable transmission/reflection bands.

The traditional frequency selective surface (FSS) needs further improvement with the development of stealth technology, and the design of multifunctional FSSs is essential. In this letter, an active absorptive FSS (AFSS) has been designed based on the absorption structure of the spoof surface plasmon polariton (SSPP) and the switching activity of the active FSS. The active FSS embedded with PIN diodes realizes the shift of two transmission/reflection frequency bands by controlling the bias voltage of the feed network, which switches from one band-pass response (at around 3.06 GHz) to the other (at around 4.34 GHz). And when one of the transmission windows switches to the other, the original transmission window closes. The upper plasmonic structure achieves a continuous and efficient absorption band from 6.31 to 8.34 GHz. A sample was also fabricated and carried out to verify the numerical simulation, and the experimental and simulation results are consistent. This work provides new ideas for the design of active AFSS and promotes its application in common aperture radome, antenna isolation, and electromagnetic shielding.

Single-layer metasurface for ultra-wideband polarization conversion: bandwidth extension via Fano resonance.

In this paper, we propose a method of designing ultra-wideband single-layer metasurfaces for cross-polarization conversion, via the introduction of Fano resonances. By adding sub-branches onto the unit cell structure, the induced surface currents are disturbed, leading to coexistence of both bright and dark modes at higher frequencies. Due to the strong interaction between the two modes, Fano resonance can be produced. In this way, five resonances in all are produced by the single-layer metasurface. The first four are conventional and are generated by electric and magnetic resonances, whereas the fifth one is caused by Fano resonance, which further extends the bandwidth. A prototype was designed, fabricated and measured to verify this method. Both the simulated and measured results show that a 1:4.4 bandwidth can be achieved for both x- and y-polarized waves, with almost all polarization conversion ratio (PCR) above 90%. This method provides an effective alternative to metasurface bandwidth extension and can also be extended to higher bands such as THz and infrared frequencies.

Electromagnetic wave absorption and compressive behavior of a three-dimensional metamaterial absorber based on 3D printed honeycomb.

Lightweight structures with multi-functions such as electromagnetic wave absorption and excellent mechanical properties are required in spacecraft. A three-dimensional metamaterial absorber consisting of honeycomb and resistive films was proposed and fabricated through 3D printing and silk-screen printing technology. According to simulation and experiment results, the present three-dimensional metamaterial absorber can realize an absorptivity of more than 90% in a wide band of 3.53-24.00 GHz, and improve absorbing efficiency for transverse magnetic (TM) waves of oblique incidence angle from 0° to 70°. The compression test results reveal that compressive strength of the 3D printed honeycomb can reach 10.7 MPa with density of only 254.91 kg/m3, and the energy absorption per volume W v and per unit mass W m are 4.37 × 103 KJ/m3 and 17.14 KJ/Kg, respectively. The peak compressive strength and energy absorption per mass are at least 2.2 and 3 times comparing to metallic lattice cores with the same density. Outstanding electromagnetic wave absorption and mechanical performance make the present three-dimensional metamaterial absorber more competitive in engineering applications.

Spatial k-dispersion engineering of spoof surface plasmon polaritons for customized absorption.

Absorption of electromagnetic waves in a medium is generally manipulated by controlling the frequency dispersion of constitutive parameters. However, it is still challenging to gain the desired constitutive parameters for customized absorption over a broad frequency range. Here, by virtue of spoof surface plasmonic polaritons (SPPs), we demonstrate capabilities of the spatial k-dispersion engineering for producing the customized broadband absorption. Incident waves can be efficiently converted to the spoof SPPs by plasmonic arrays, and their propagation and/or absorption can be controlled by engineering the spatial dispersion of k-vector. Based on this feature, we show how such concept is employed to achieve broadband as well as frequency-selective broadband absorptions as examples. It is expected that the proposed concept can be extended to other manipulations of propagating electromagnetic waves over a broad frequency range.

k-dispersion engineering of spoof surface plasmon polaritons for beam steering.

In this paper, we propose to achieve beam steering by k-dispersion engineering of spoof surface plasmon polaritons (spoof SPP) at microwave frequencies. The planar plasmonic metamaterials (PPMs) are employed to couple and guide spoof SPP. High-efficiency transmission based on spoof SPP coupling is realized via matching the wave-vectors of the spoof SPP and the space wave. The transmission phase can be modulated by k-dispersion engineering of the spoof SPP with great freedom. Due to the independent phase shift produced by the spoof SPP on the PPMs, the phase gradient achieved by using the PPMs as the sub-unit cells can be altered by changing the repetition period of the sub-unit cells. Two phase gradient materials (PGMs) are achieved by using nine different PPMs as the sub-unit cells with the repetition period q = 4mm and 4.5mm. Both the simulated and measured results demonstrated the excellent performances of the PGMs on high efficiency, wideband, tunable beam steering.