The power of controlling objects with mind has captivated a popular fascination to human beings. One possible path is to employ brain signal collecting technologies together with emerging programmable metasurfaces (PM), whose functions or operating modes can be switched or customized via on-site programming or pre-defined software. Nevertheless, most of existing PMs are wire-connected to users, manually-controlled and not real-time. Here, we propose the concept of remotely mind-controlled metasurface (RMCM) via brainwaves. Rather than DC voltage from power supply or AC voltages from signal generators, the metasurface is controlled by brainwaves collected in real time and transmitted wirelessly from the user. As an example, we demonstrated a RMCM whose scattering pattern can be altered dynamically according to the user’s brain waves via Bluetooth. The attention intensity information is extracted as the control signal and a mapping between attention intensity and scattering pattern of the metasurface is established. With such a framework, we experimentally demonstrated and verified a prototype of such metasurface system which can be remotely controlled by the user to modify its scattering pattern. This work paves a new way to intelligent metasurfaces and may find applications in health monitoring, 5G/6G communications, smart sensors, etc.

We design a 3D acoustic metamaterial having a coiling resonant structure with high symmetry. Eigenstate analysis reveals that such a 3D metamaterial has two significant Mie-type eigenmodes, monopole and dipolar resonances. Large blocking of sound waves in the low-frequency range between monopole and dipolar resonances is observed numerically and experimentally. The effective properties extracted from the reflection and transmission coefficients show negative bulk modulus around the monopole resonant frequency and negative mass density around the dipolar resonant frequency. By employing the proposed two-scale model, the metamaterial system demonstrates the functionalities of sound cloaking and super-tunneling within a finite space.

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.

In this letter, we propose to extend the bandwidth of polarization conversion (PC) by merging the bands of different polarization converters. Intuitively, it is quite natural that the bands of two polarization converters be merged if they are close enough. Unfortunately, there is always a narrow band with low efficiency between the two bands, which results from Fano resonance. We establish a theoretical model to analyze the underlying mechanism of the Fano resonance. We find that the Fano resonance can be suppressed by reducing the resonant frequency of the converter operating at a higher frequency band. In this way, the anti-symmetric surface current can be eliminated and the two PC bands can be merged as a wide band. We designed, fabricated, and measured a prototype. Both the simulation and experimental results verify this method. This work provides an effective alternative to the design of wide-band or even ultra-wideband polarization converters.

As the traditional frequency-selective surface (FSS) cannot meet the current needs of stealth, in the eletromagnetic environment today the design of multifunctional FSSs is particularly important. Considering this, in this paper we design a kind of transmission and absorption integrated structure based on the dispersion engineering of the spoof surface plasmon polariton (SSPP) and high-order FSS. It can achieve high-efficiency transmission in the band of 9.8–11.4 GHz and absorption in the band of 14.0–18.0 GHz for both x- and y -polarized waves. The effective angle range is 0° to 60°. Compared with classical planar design, it possesses excellent mechanical properties. The transmission and absorption integrated structure is also fabricated and measured to prove the feasibility of the idea. Due to its thin thickness and character, it can be used in radar cross-section reduction aspects. More importantly, through the shape design of the radome, it can achieve scattering in a lower frequency band.

In this paper, recent developments of metamaterials and metasurfaces for RCS reduction are reviewed, including basic theory, working principle, design formula, and experimental verification. Super-thin cloaks mediated by metasurfaces can cloak objects with minor impacts on the original electromagnetic field distribution. RCS reduction can be achieved by reconfiguring scattering patterns using coding metasurfaces. Novel radar absorbing materials can be devised based on field enhancements of metamaterials. When combined with conventional radar absorbing materials, metamaterials can expand the bandwidth, enlarge the angular range, or reduce the weight. Future tendency and major challenges are also summarized.

In this letter, we propose a method of extending the bandwidth of radar absorbing materials (RAMs) by integrating the metasurface and an impedance-matching lattice. The metasurface operates at lower frequencies and is sandwiched as an interlayer between a RAM sheet and an impedance-matching lattice. Absorption at lower frequencies will be enhanced due to the strong resonance of the metasurface, which extends the bandwidth to lower frequencies.To enhance absorption at higher frequencies, a square lattice of RAM patches, rather than a complete sheet, is placed on top of the metasurface to improve the impedance matching. As well as the dissipation of the RAM patches themselves, a high absorption at higher frequencies can also be achieved, which extends the bandwidth towards higher frequencies. Therefore, through such an integrated design, a high absorption of radar waves can be achieved in an ultra-wide band. To verify this method, we designed, fabricated and measured a prototype based on a commercial magnetic rubber RAM. Both the simulated and measured results show that the prototype can achieve an absorption rate of over 90% on average in 2.5-18.0 GHz. Moreover, such a design not only maintains the softness of rubber RAMs, but also reduces the total aerial density, which will facilitate practical applications. This work provides an alternative method for designing microwave absorbers for ultra-wideband applications.

Amplitude–phase control for circular polarized (CP) waves is experiencing a research upsurge in electromagnetics owing to the kaleidoscopic electromagnetic responses and promising application prospects of circular polarizations, and chiral metasurfaces are more facile to achieve a series of intriguing chiral phenomena than natural materials. However, it is difficult for most existing chiral metasurfaces to independently tailor the amplitude and phase of left-handed circular polarized and right-handed circular polarized waves at the same frequency as they suffer the drawbacks of large thickness, multiple layers, and complex structure. Herein, an innovative strategy of single-layer achiral metasurfaces of thickness 0.13λ0 is proposed to independently and simultaneously manipulate the amplitude and phase of orthogonal CP waves. As a proof of concept, an amplitude and phase controlled dual-channel meta-hologram is designed to reconstruct diverse images with high fidelity under orthogonal CP illumination, and the simulated and experimental results collectively validate the availability of our methodology. Significantly, the meta-hologram is also applicable to full polarization states according to the decomposition of electromagnetic waves. The inspiring design of single-layer achiral metasurfaces provides a simple and effective approach to explore chiral effects, and they possess enormous application potential in multitudinous microwave devices.