Dual-Channel Surface Waves Directional Radiation with Customizable Intensity and Switchable Pattern.

Achieving the conversion from surface waves (SWs) to propagating waves has captivated long-standing interest, and various ingenious metasurfaces benefiting from the powerful control capability for electromagnetic waves are able to realize efficient SWs directional radiation. Nevertheless, most existing schemes still suffer from the bottlenecks of single radiation channel, uncontrollable radiation intensity, and immutable radiation pattern, which immensely hinder their practical application in high-integration intelligent devices. Herein, a series of appealing strategies are proposed to achieve the dual-channel SWs directional radiation with customizable radiation intensity and switchable radiation pattern. The dual-channel SWs radiation metadevice based on the phase modulation metasurface is designed to directionally radiate SWs in left-handed circular polarized channel and right-handed circular polarized channel and possesses the broadband frequency scanning characteristic. More strikingly, the intensity-customizable dual-channel SWs radiation metadevice loaded with lumped resistors can control the realized gain of two circular polarized radiation beams, and the pattern-switchable dual-channel SWs radiation metadevice loaded with PIN diodes can dynamically adjust the radiation direction of the radiation beams. Numerous simulations and experiments of the proof-of-concept prototypes with modular design corroborate the theoretical predictions. Our methodology shows unprecedented flexibility in regulating SWs directional radiation and has enormous potential in engineering applications.

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.

Single-Layer Achiral Metasurface with Independent Amplitude-Phase Control for Both Left-Handed and Right-Handed Circular Polarizations.

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.

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.

Broadband surface wave coupler with low infrared emission and microwave reflection.

Metasurfaces possess excellent capabilities to flexibly manipulate electromagnetic waves in multiple frequency domains, which show great potential application in multispectral stealth. Herein, a broadband surface waves coupler based on the design of thin Pancharatnam-Berry (PB) phase gradient metasurfaces (PGMs) of thickness 0.12λ0 is proposed to reduce infrared emission and microwave reflection simultaneously. Low infrared emission results from the high filling ratio of the indium-tin-oxide (ITO) on the surface, and low microwave reflection results from the conversion from propagating waves to surface waves. Intriguingly, this design is also capable of acting as a simple circular polarized (CP) discriminator because orthogonal CP waves are coupled into surface waves propagating along opposite directions. A proof-of-concept prototype is simulated and measured to validate the effectiveness of our methodology. The results indicate that the broadband surface waves coupler shows low infrared emissivity less than 0.28 from 3 to 14 µm and has microwave reflection reduction larger than 10 dB in 7.3-9.5 GHz. The exceptional performances of the proposed broadband surface waves coupler make us believe that our design offers an alternative strategy for multispectral stealth and multifunctional application.

Simultaneous reduction of microwave reflection and infrared emission enabled by a phase gradient metasurface.

Metasurfaces have shown promising applications in radar-infrared compatible stealth because of its superior electromagnetic wave control capabilities, but, to date, the majority of designs still suffer from the defects of large thickness, limited working bandwidth, relatively high infrared emissivity and so on. Here, an exotic phase gradient metasurface (PGM) is proposed to achieve low microwave reflection and low infrared emission concurrently, which has a small thickness of about 0.10λ0. The microwave reflection reduction larger than 10 dB in 14-20 GHz is attributed to the anomalous reflection for arbitrary LP incident waves, and the infrared emissivity less than 0.28 from 3 to 14 µm is due to the indium-tin-oxide (ITO) with low infrared emissivity and high filling ratio. Also, the designed PGM can also realize beam deflection for orthogonal CP waves because of the meta-atoms' isotropic characteristics. Our methodology is fully verified by numerous simulations and experiments and may open a new avenue for radar-infrared compatible stealth research.

Multifunctional full-space metasurface controlled by frequency, polarization and incidence angle.

Multifunctional metasurfaces have exhibited considerable abilities of manipulating electromagnetic (EM) waves, especially in full-space manipulation. However, most works are implemented with functions controlled by polarization or frequency and seldom involve the incidence angle. Herein, we propose a multifunctional full-space metasurface controlled by frequency, polarization and incidence angle. A meta-atom is firstly designed. When EM waves illumine normally in the C-band, it possesses the characteristic of asymmetric transmission with high-efficient polarization conversion. In the Ku-band, both x- and y-polarized EM waves along both sides will be reflected and achieve broadband and high-efficient cross-polarization conversion. Also, when illumined obliquely, both sides can achieve efficient retroreflection at a certain frequency. As a proof of concept, a metasurface consisting of the above meta-atoms is configured as a dual orbital angular momentum (OAM) vortex beam generator and different beam deflector when illumined normally. Meanwhile, it acts as a multi-channel retroreflector when illumined obliquely. Both the simulated and measured results show excellent performances. Our findings provide a new degree of freedom to design multifunctional metasurfaces that can further promote applications.

Planar multi-angle retro-reflectors based on the wave-vector-reversion of spoof surface plasmon polaritons.

Spoof surface plasmon polariton (SSPP) is kind of sub-wavelength electromagnetic (EM) mode, which is favorable for miniaturization and thinning of EM devices. In this paper, we propose a method of designing thin planar retro-reflector that can operate under multiple incidence angles at the same frequency. The retro-reflector is composed of a transmissive phase gradient metasurface (TPGM) placed above a metallic patch array (MPA), where the former couples and decouples SSPPs while the latter supports eigen-mode propagation of SSPPs. Under oblique incident angles, the TGPM can impart 0 and π Pancharatnam-Berry (P-B) phases alternatively, producing P-B phase gradients along its surface. Incident waves can be coupled as SSPPs propagating on the MPA which will be reflected at the borders of the MPA, thus the wave-vector of SSPPs is reversed. In this way, retro-reflection can be realized under the two incidence angles θ=±45.0°. Moreover, due to mode mismatch between the TPGM and MPA under normal incidence, the retro-reflector acts like a planar metallic plate under θ=0°. To verify this method, a prototype was designed, fabricated and measured. Both the simulation and measurement results verify significant backscattering enhancement under θ=±45.0° and 0° at 10.0 GHz. This work provides an alternative method of designing planar retro-reflectors and may find applications in wireless communication, target tracking, etc.