Reconfigurable meta-mirror for wavefronts control: applications to microwave antennas

Low-coupling reflective metasurfaces for accurate near-field focusing

Metasurface is a 2D metamaterial which manipulates electromagnetic wavefront by carefully designing the transmissive or reflective responses of the planar subwavelength structures. Despite various emerging new functionalities, one of the limitations of metasurfaces in practical applications is the lack of control accuracy of its local amplitude and phase responses. This is in part caused by the discrepancy between the periodic EM simulated responses of unit cells and the actual non-periodic metasurfaces when functionality-determined amplitude and phase gradients are introduced. Under such conditions, the focusing capabilities of metasurfaces can be greatly affected. In this paper, we address this problem by introducing a slotted ground plane metasurface design which reduces the above-mentioned discrepancy by limiting the mutual coupling effects of a unit cell with its surrounding elements. An offset-fed near-field focusing reflective metasurface is designed and fabricated to verify the performance of the proposed design. Both the EM simulated and measurement results demonstrate the advantages of the proposed design in terms of sidelobe level and transfer efficiency.

Metasurfaces and Blinking Jamming: Convergent Study, Comparative Analysis, and Challenges

Blinking jamming is an active self-screening technique performed by at least two aircraft to tackle monopulse radars and all complexity related thereto. Nowadays, the technique can be performed with digital radiofrequency memories (DRFMs), which are cumbersome, complex, expensive, need a dedicated compartment and antenna, and introduce spurs in the signals. In this paper, we propose an alternative to the implementation of blinking jamming with DRFMs, namely with reconfigurable metasurfaces. By covering the aircraft parts that most contribute to the radar cross-section (RCS), reconfigurable metasurfaces can interchangeably absorb or amplify impinging waves, making the aircraft 'blink' from the radar perspective. To validate the feasibility, simulations accounting for realistic phenomena are conducted. It is seen that, if the aircraft RCS can be varied in a ratio of 10:1, either with absorptive or power-amplifying metasurfaces, a performance similar to that of the DRFM is achieved. Furthermore, a ratio of 2:1 is sufficient to make the radar antenna system movements exceed the angular range of the formation. We also anticipate our work to be a starting point for completely new ways of countering radars, e.g., with countless small drones performing passive or active stand-off blinking jamming.

Taming Fabry-Pérot resonances in a dual-metasurface multiband antenna with beam steering in one of the bands

Metasurfaces are artificial materials that can provide properties not readily available in nature for the interaction with acoustic, elastodynamic, or electromagnetic waves. In Electromagnetics, metasurfaces allow particular functionalities to antennas, which in turn lately have been increasingly pushed to a multiband operation. To fully exploit metasurfaces' capabilities, the use of a metasurface reflector and a metasurface superstrate surrounding a radiating element in multiband antennas is interesting. However, such topology generally creates multiple reflections inside the formed cavity, known as Fabry-Pérot resonances. Here we show that one should tame this phenomenon to use two parallel metasurfaces surrounding a planar radiating element. We present the conditions to obtain directive, multiband antennas under such circumstances. The concepts are validated with a compact device for 5G/4G/Wi-Fi 2.4/5/6E performing a beam steering in the 5G without disturbing the radiation patterns of the other bands. This device demonstrates that the functionalities of two metasurfaces may be exploited in a single design if the presented conditions are respected. We also anticipate our work to be a starting point for other studies in the wave domain. For example, compact, multiband, beam-steerable microphones or sonar transducers with two parallel metasurfaces could be investigated in the future.

Electronically reconfigurable transmitarray for fully independent beam manipulation in two divided frequency bands

Electronically reconfigurable transmitarray (ERTA) combines the advantages of optic theory and coding metasurface mechanism with the characteristic of low-loss spatial feed and real-time beam manipulation. Designing a dual-band ERTA is challenging due to multiple factors, including large mutual coupling generated by dual-band operation and separate phase control in each band. In this paper, a dual-band ERTA is demonstrated with the capability of fully independent beam manipulation in two divided bands. This dual-band ERTA is constructed by two kinds of orthogonally polarized reconfigurable elements which share the aperture in an interleaved way. The low coupling is achieved by utilizing polarization isolation and a backed cavity connected to the ground. To separately control the 1-bit phase in each band, a hierarchical bias method is elaborately presented. As proof of concept, a dual-band ERTA prototype composed of 15 × 15 upper-band elements and 16 × 16 lower-band elements is designed, fabricated, and measured. Experimental results verify that fully independent beam manipulation with orthogonal polarization is implemented in 8.2-8.8 GHz and 11.1-11.4 GHz. The proposed dual-band ERTA may be a suitable candidate for space-based synthetic aperture radar imaging.

Recent Advances in Reconfigurable Metasurfaces: Principle and Applications

Metasurfaces have shown their great capability to manipulate electromagnetic waves. As a new concept, reconfigurable metasurfaces attract researchers' attention. There are many kinds of reconfigurable components, devices and materials that can be loaded on metasurfaces. When cooperating with reconfigurable structures, dynamic control of the responses of metasurfaces are realized under external excitations, offering new opportunities to manipulate electromagnetic waves dynamically. This review introduces some common methods to design reconfigurable metasurfaces classified by the techniques they use, such as special materials, semiconductor components and mechanical devices. Specifically, this review provides a comparison among all the methods mentioned and discusses their pros and cons. Finally, based on the unsolved problems in the designs and applications, the challenges and possible developments in the future are discussed.

Full-space beam scanning based on transmission reflection switchable quadratic phase metasurface

Beam scanning based on metasurfaces is widely discussed in recent years owing to its high integration, lightweight, and low cost. However, most of the reported beam scanning metasurfaces operate in either transmission or reflection mode. Here, we propose a full-space beam scanning metasurface based on transmission reflection switchable meta-atom and the quadratic phase distribution. As a validation, a metasurface array with 400 units (20 × 20) was experimentally demonstrated. Beam scanning of ± 35 ° was achieved in both transmission mode and reflection mode. A larger scanning angle (± 45 °) was further verified simulatively with a 900-units (30 × 30) array. The method provides an avenue for expanding the space of electromagnetic wave manipulation and may have great potential in wireless communication and radar detection.

Electronic Beam-Scanning Antenna Based on a Reconfigurable Phase-Modulated Metasurface

Metasurfaces (MSs) have enabled the emergence of new ideas and solutions in the design of antennas and for the control of electromagnetic waves. In this work, we propose to design a directional high-gain reconfigurable planar antenna based on a phase-modulated metasurface. Reconfigurability is achieved by integrating varactor diodes into the elementary meta-atoms composing the metasurface. As a proof of concept, a metasurface prototype that operates around 5 GHz is designed and fabricated to be tested in an antenna configuration. The metasurface is flexibly controlled by different bias voltages applied to the varactor diodes, thus allowing the user to control its phase characteristics. By assigning judiciously calculated phase profiles to the metasurface illuminated by a feeding primary source, different scenarios of far-field patterns can be considered. Different phase profiles are tested, allowing us to, firstly, achieve a highly directive boresight radiation and, secondly, to steer the main radiated beam towards an off-normal direction. The whole design process is verified by numerical simulations and is validated experimentally by far-field antenna measurements. The proposed metasurface enables the design of directive flat antennas with beam-scanning characteristics without complex feeding systems and power-consuming phase shifters, and thus provides potential interests for next generation antenna hardware.

A reconfigurable intelligent surface with integrated sensing capability

Reconfigurable reflective surfaces can alter the propagation environment to improve wireless communication and power transfer. Paramount to this operation-which has attracted much attention recently-is the assumption that the reflective surface has prior knowledge of the propagation environment, for example, the direction/location of the transmitter and the intended receiver(s). To address this need, we propose a reconfigurable reflective metasurface with integrated sensing capabilities. By modifying the tunable meta-atoms constituting the metasurface, we couple small portions of the incident wave to an array of sensing waveguides. As an illustrative example, we demonstrate the ability to use the sampled incident wave to detect its angle of arrival. In addition, we propose and numerically demonstrate the possibility to reduce the required sensors, i.e., the number of radio frequency (RF) chains needed to acquire the sensed signals, by leveraging the inherent metasurface's tunable multiplexing capability. A reconfigurable reflective metasurface with integrated sensing capabilities can benefit wireless communications, wireless power transfer, RF sensing, and smart sensors.

Fourier Convolution Operation on Metasurface-Based Hologram in Microwave Region

In this paper, a 0.1λ0-thick 1-bit coding metasurface is proposed to achieve a polarization-insensitive hologram under oblique incidence, utilizing compact ground-backed patch unit cells. Fourier convolution theory in a digital signal processing system is added to the hologram calculation of the improved weighted Gerchberg–Saxton (GSW) algorithm to achieve control of the scattered pattern in the microwave region. As a proof of concept, a prototype operating at 15 GHz is designed to verify the validity of our proposed approach. The measured performances show good imaging quality under different incident polarizations, providing potential applications in imaging processing and information storage.

Threshold conditions of electric field enhancement and energy confinement in the low-index core of nanoscale waveguides

Threshold conditions to realize electric field enhancement and energy confinement in the low-refractive-index core of nanoscale waveguides are studied by solving the field function. When the incident lightwave meets the relation of special thresholds, we observe the enhanced electric field and a concentrated light energy in the core. The electric field enhancement and the confined light power are highly dependent on the light wavelength. When the core width is 30 nm, for a wavelength of 1.55 µm, we achieve a power confinement factor above 40%. As the basis for a growing number of potential applications, the threshold conditions discovered in this work will find significant applications in many fields, such as optical sensors and optical communication components.

Helicity-switched hologram utilizing a polarization-free multi-bit coding metasurface

In this work, a polarization-free coding metasurface is proposed to manipulate circularly polarized waves. Compared to a Pancharatnam-Berry phase metasurface, the proposed design not only allows for overcoming anti-symmetrical response characteristics between orthogonal circularly polarized states to enable achieving identical functionality under both right-handed and left-handed circularly polarized wave illuminations and avoiding polarization-conversion losses but also offers additional degree of freedom in the control of handedness. As a proof-of-concept demonstration, a polarization-free multi-bit coding metasurface is designed to realize helicity-switched holograms in the microwave region. Experimental measurements performed on a fabricated prototype reveal outstanding imaging quality with extremely high imaging efficiency above 76% for arbitrary polarizations at 10 GHz. Our proposed method expands the route in manipulating circularly polarized waves and can be applied over the whole electromagnetic spectrum for wavefront manipulation.

High-purity polarized multi-beams from polarization-twisting meta-surface Cassegrain systems

Bi-functional meta-surfaces capable of simultaneously controlling polarization states and wave-fronts of electromagnetic fields are introduced into the design of Cassegrain system for the synthesis of multi-beams. More specifically, electromagnetic fields reflected by the secondary meta-surface with tailored diverged wave-fronts would be collimated by the primary meta-surface into multi-beams with transformed polarization states that can directly go through the secondary meta-surface without any blockage. Especially, we show that such a polarization-twisting meta-surface Cassegrain system can possess much more compact configuration by properly devising the phase distribution over the secondary meta-surface, and can also achieve high-purity polarized multiple radiations when we enlarge the secondary meta-surface as a radome. The present approach of integrating two bi-functional meta-surfaces into the design of Cassegrain system for the generation of multi-beams should pave the way for building up more advanced meta-surface based architectures with specific characteristics.

Cavity-enhanced metallic metalens with improved Efficiency

Metasurfaces are made of subwavelength nanoantennas with a flat, ultrathin architecture, and strong capability in manipulating the propagation of light by flexible modulations on its phase, amplitude, and polarization. Conventional metallic metalenses always suffer from its low efficiencies due to large intrinsic loss. Here, we demonstrate a cavity enhanced bilayer metalens composed of aluminum nanobars and its complementary structures. The focusing and imaging experiments definitely show an improved efficiency of such kind of bilayer metalens compared with its single layer counterpart. Detailed theoretical analyses based on full-wave simulations are carried out with respect to different cavity lengthes and working wavelengths, which reveals that the improvement rightly attributes to enhanced cavity mode. Our design will not only improve the working efficiency for metalens with simplified manufacturing procedure, but also indicates more possibilities by employing the metal as electrodes.

Electronically-engineered metasurface for directional beaming of electromagnetic waves through a subwavelength aperture

Controlling diffracted waves has attracted extensive research interests these last years particularly for the potential application of beaming functionality. In this paper, we propose to realize on-axis beaming of diffracted electromagnetic waves by using a phase- gradient metasurface. The structure is optimally designed in order to transform surface waves to propagating waves and to enhance transmission through a subwavelength aperture. Both numerical simulations and near-field measurements are performed at microwave frequencies to validate the proposed concept. Furthermore, the metasurface is frequency-tunable and can be controlled by an external DC bias voltage. Consequently, by adjusting the electromagnetic response of each unit cell through the bias voltage, different phase gradients can be tailored, enabling broadband operation spanning from 9 GHz to 12 GHz.

Multi-focus hologram utilizing Pancharatnam-Berry phase elements based metamirror

An ultrathin reflection-type metamirror is proposed for multi-focusing with any desired focusing fashion including focal number and location. The metamirror is composed of reflection-type Pancharatnam-Berry (P-B) phase elements, which are able to provide full reflection phase of 2π, together with near-unity reflection efficiency by judiciously engineering the rotation angle of each latter element. A holographic algorithm is utilized to calculate the phase distribution at the interface of the metamirror to achieve the desired multi-focus spots. Experimental demonstrations performed in microwave region show good imaging quality with high reflection efficiency and imaging efficiency. The proposed metamirror provides a high-performance solution for low-cost and lightweight beam-shaping and beam-focusing devices.

Anomalous refraction and reflection characteristics of bend V-shaped antenna metasurfaces

Stabilization issue of anomalous refraction and reflection in V-shaped antenna metasurfaces are investigated. Specifically, when a V-shaped metasurface is artificially tilted, the induced refraction and reflection are theoretically analyzed. Detailed numerical and experimental study is then performed for the upward and downward bending metasurfaces. Our results show that although the anomalous reflection is sensitive to the deformation of metasurface geometry; the anomalous refraction is, surprisingly, barely affected by relatively small-angle tilting and able to support perfect beam orienting. Since in real-world applications, the optical objects are often affected by multiple uncertain factors, such as deformation, vibration, non-standard surface, non-perfect planar, etc., the stabilization of optical functionality has therefore been a long-standing design challenge for optical engineering. We believe our findings can shed new light on this stability issue.

Near-zero metamaterial inspired UHF antenna for nanosatellite communication system

Epsilon-and-mu-near-zero (EMNZ) metamaterial structure inspired UHF antenna for nanosatellite has been proposed in this paper. The antenna consists of 3 × 2-unit cell array on the ground plane and a meander line radiating patch. Coaxial probe feeding technique has been obtained to excite the antenna. The meander line enables the antenna to resonate at lower UHF band and the metamaterial array is used to make the resonant frequency stable by reducing the coupling effect with metallic nanosatellite structure. The metamaterial structure exhibits EMNZ characteristics from 385 MHz to 488.5 MHz, which facilitates stable resonant frequency and higher antenna efficiency when embedded with nanosatellite structure. The proposed EMNZ inspired antenna has achieved measured impedance bandwidth (S₁₁ < −10 dB) of 14.92 MHz (391 MHz–405.92 MHz). The perceptible novelty of this paper is the development of EMNZ metamaterial that significantly improves the UHF antenna’s operating frequency stability as well as efficiency for low earth orbit nanosatellite communications.

Transparent transmission-selective radar-infrared bi-stealth structure

We report a multifunctional metamaterial composite structure that not only provides the broadband radar and thermal infrared bi-stealth function but also possesses an in-band microwave transmission window and high optical transparency. It is composed of four metasurface layers made of indium tin oxide (ITO) films with different surface resistances, which are specifically designed to sequentially control the infrared emission, microwave absorption and transmission. The fabricated sample exhibits a low reflectivity less than 10% in 1.5-9 GHz and a transmission peak of 50% around 3.8 GHz up to the incident angle of 30 degrees. In the infrared atmosphere window, a low thermal emissivity of about 0.52 is achieved. Meanwhile, it keeps good optical transparency by the use of the ITO films. The optically transparent, low-infrared-emissivity, radar-reflectionless and frequency-selective-transmission properties will enable the promising application of communication-compatible multispectral stealth technology.