Resonance conditions for a substrate-superstrate printed antenna geometry which allow for large antenna gain are presented. Asymptotic formulas for gain, beamwidth, and bandwidth are given, and the bandwidth limitation of the method is discussed. The method is extended to produce narrow patterns about the horizon, and directive patterns at two different angles.

Gain enhancement methods for printed circuit antennas

During the past few years, metasurfaces have been used to demonstrate optical elements and systems with capabilities that surpass those of conventional diffractive optics. Here, we review some of these recent developments, with a focus on dielectric structures for shaping optical wavefronts. We discuss the mechanisms for achieving steep phase gradients with high efficiency, simultaneous polarization and phase control, controlling the chromatic dispersion, and controlling the angular response. Then, we review applications in imaging, conformal optics, tunable devices, and optical systems. We conclude with an outlook on future potentials and challenges that need to be overcome.

https://www.degruyter.com/document/doi/10.1515/nanoph-2017-0129/pdf

A review of dielectric optical metasurfaces for wavefront control

Metasurfaces are a topic of significant research and are used in various applications due to their unique ability to manipulate electromagnetic waves in microwave and optical frequencies. These artificial sheet materials, which are usually composed of metallic patches or dielectric etchings in planar or multi-layer configurations with subwavelength thickness, have the advantages of light weight, ease of fabrication, and ability to control wave propagation both on the surface and in the surrounding free space. Recent progress in the field has been classified by application and reviewed in this article. Starting with the development of frequency-selective surfaces and metamaterials, the unique capabilities of different kinds of metasurfaces have been highlighted. Surface impedance can be varied and manipulated by patterning the metasurface unit cells, which has broad applications in surface wave absorbers and surface waveguides. They also enable beam shaping in both transmission and reflection. Another important application is to radiate in a leaky wave mode as an antenna. Other applications of metasurfaces include cloaking, polarizers, and modulators. The controllable surface refractive index provided by metasurfaces can also be applied to lenses. When active and non-linear components are added to traditional metasurfaces, exceptional tunability and switching ability are enabled. Finally, metasurfaces allow applications in new forms of imaging.

/pdf/metasurfaces-and-their-applications-4m6u3i243c.pdf

Metasurfaces and their applications

Metasurfaces, ultrathin metamaterials constructed by planar meta-atoms with tailored electromagnetic (EM) responses, have attracted tremendous attention due to their exotic abilities to freely control EM waves. With active elements incorporated into metasurface designs, one can realize tunable and/or reconfigurable metadevices with functionalities controlled by external stimuli, opening a new platform to dynamically manipulate EM waves. In this article, we briefly review recent progress on tunable/reconfigurable metasurfaces, focusing on their working mechanisms and practical applications. We first describe available approaches, categorized into different classes based on external stimuli applied, to realize homogeneous tunable/reconfigurable metasurfaces, which can offer uniform manipulations on EM waves. We next summarize recent achievements on inhomogeneous tunable/reconfigurable metasurfaces with constitutional meta-atoms locally tuned by external knobs, which can dynamically control the wave-fronts of EM waves. We conclude this review by presenting our own perspectives on possible future directions and existing challenges in this fast developing field.

/pdf/tunable-reconfigurable-metasurfaces-physics-and-applications-54devaoe26.pdf

Tunable/Reconfigurable Metasurfaces: Physics and Applications

Metasurfaces are thin two-dimensional metamaterial layers that allow or inhibit the propagation of electromagnetic waves in desired directions. For example, metasurfaces have been demonstrated to p ...

/pdf/roadmap-on-metasurfaces-3hqumgmul7.pdf

Roadmap on metasurfaces

We demonstrate a series of nonlinear, active, and tunable metasurfaces for a variety of electromagnetic applications. The metasurfaces have achieved a range of exotic properties by populating nonlinear or active circuit components on a periodically patterned metallic surface. The circuit components such as diodes, varactors, transistors, and other devices can be controlled manually, actively, or self-adaptively. This allows nonlinear metasurfaces to have active tuning, power-dependent behavior, self-focusing, reconfigurable surface topology, or frequency self-tuning capabilities. The power-dependent metasurfaces can be applied to active RF absorbers that only absorb high-power surface waves to prevent malfunction or damages to sensitive devices. The rectifier-based waveform-dependent metasurface absorber can be specifically designed to absorb either high power pulsed waves or continuous waves. The transistor-based surface wave metasurface absorber provides another degree of freedom in that it can be manually switched to tune the absorber, or it can be tuned using computer controlled feedback. The self-focusing effect has been demonstrated for the first time at RF frequencies to automatically collimate high-power surface waves. The reconfigurable and self-tuning metamaterial surfaces can be implemented to support a broadband reconfigurable antenna system or to adapt to a wide range of incoming frequencies. In this paper, the concepts of nonlinear and active tunable metasurfaces are discussed, including results of full-wave simulation analysis, EM/circuit co-simulation, and experimental results in waveguides, using a near-field scanner, as well as far-field measurements in an anechoic chamber.

/pdf/nonlinear-active-and-tunable-metasurfaces-for-advanced-2w89o2wamt.pdf

Nonlinear, Active, and Tunable Metasurfaces for Advanced Electromagnetics Applications

An electrically tunable metasurface that absorbs continuous electromagnetic (EM) surface waves is proposed by taking advantage of varactor diodes embedded in the surface. On the one hand, the varactors perform as the main dissipating components due to their parasitic series resistance; on the other hand, they function as the tuning elements because the dissipation is highly dependent on their capacitance. Therefore, the absorption of the surface can be tuned by the direct current biasing voltage across the varactors, which is validated numerically and experimentally in this letter. This absorbing mechanism of the surface differs from prior surface-wave absorbers and can lead to greater flexibility for absorbing metasurfaces. In this work, a power-dependent absorbing performance is achieved by loading microwave power sensors. If incorporated with other types of sensors, the absorption could potentially be controlled by corresponding physical variables such as light, pressure, or temperature, thus giving rise to various absorbing applications in a complex EM environment.

Electrically tunable metasurface absorber based on dissipating behavior of embedded varactors

Impedance surfaces are artificially designed periodic structures that support surface waves. This paper presents a novel concept of a nonlinear impedance surface, whose impedance increases with the surface wave power. The proposed surface consists of a lattice of modified slotted mushroom-like cells loaded with varactor diodes and lumped circuits. The circuits are designed to sense the incoming surface wave power and feed back the signal to control the bias of the varactors, which eventually tunes the surface impedance. Such proposed power-dependent impedance surface has been demonstrated by a prototype consisting of $4 \times 10$ cells. It is observed from the experiments that as the power varies from 16 to 27 dBm, the surface exhibits a surface impedance range as much as j385 to j710 $\Omega$ . This characteristic enables the surface to be potentially useful in self-trapping surface-wave waveguide, power-dependent beam-steering antenna reflector, and other nonlinear applications.

Nonlinear Power-Dependent Impedance Surface

Aiming at exploring the basic functional mechanism of superstrates in the resonant cavity antenna (RCA), two popular superstrates, i.e., the homogeneous dielectric superstrate and the periodic microstrip superstrate are investigated. Their reflection coefficients to normal and oblique plane wave incidences are studied by using mathematical derivation and simulations. Based on it, their effects on performances of the RCA are analyzed through simulations and measurements. Results show that, despite of different configurations, the superstrates can be purposely engineered to have the same reflection coefficients, which results in the same improving effect on the RCAs broadside gain, radiation patterns and, more interestingly, broadening effect on the return loss bandwidth. The research demonstrates the key role of their effective reflective characteristics in the RCA, and can inspire more advanced superstrates for RCA designs.

Effective Reflective Characteristics of Superstrates and Their Effects on the Resonant Cavity Antenna

The self-focusing effect of optical beams has been a popular topic of study for quite a while, but such a nonlinear phenomenon at microwave frequencies has never been realized, partially due to the underdevelopment of nonlinear material. In this research, self-focused electromagnetic (EM) surface waves are demonstrated on a circuit-based, power-dependent impedance surface. The formation of a self-focused beam is investigated using a series of discrete-time simulations, and the result is further validated in measurement. It is experimentally observed that, in contrast to the normal scattering of low-power surface waves, high-power waves propagate through the surface while maintaining narrow beam width, and even converge extremely tightly to create a hot spot with higher power. The result is essentially a nonlinear effect of the surface that compensates for the natural tendency of surface waves to diffract. This intriguing experiment can be extended to various potential EM applications such as power-dependent beam steering antennas and nonlinear microwave propagation or dissipation.

Zhangjie Luo

Papers

Nonlinear, Active, and Tunable Metasurfaces for Advanced Electromagnetics Applications

Electrically tunable metasurface absorber based on dissipating behavior of embedded varactors

Nonlinear Power-Dependent Impedance Surface

Effective Reflective Characteristics of Superstrates and Their Effects on the Resonant Cavity Antenna

Self-focusing of electromagnetic surface waves on a nonlinear impedance surface