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A point-spread function (PSF) that locally oscillates faster than its highest Fourier component can reconstruct the sub-diffraction information of objects in the far field without any near-field placements to break the diffraction limit. However, the spatial capacity of such super-oscillatory (SO) PSFs for carrying sub-diffraction information is restricted by high sidelobes surrounding a desired region of interest (ROI). Here, we propose generalized periodic SO masks without optimization to push the ROI borders away for imaging extended objects. Our imaging experiments without any image post-processing demonstrate single-shot extended-object SO imaging with extended ROIs more than ten times the size of some typical ROIs (around 2λ/NA) and a sub-wavelength resolution of 0.49λ. The SO sub-wavelength resolution (0.7 times the diffraction limit) remains robust to additive noise with a signal-to-noise ratio above 13 dB. Our method is applicable to lifting the ROI size restrictions for various SO applications such as high-density data storage, acoustic SO imaging, super-narrow frequency conversion, and temporal SO pulses.

Over the past decades, many radome designs to extend the angular scan range of phased-array antennas have been devised by utilizing dielectric materials and metamaterials. More recently, metasurface technology such as planar lenses and beam deflectors have been applied to phased arrays, enabling scan-angle enhancers to have a low profile. In this work, a physical Huygens’ metasurface (HMS) two-lens system for scan-angle doubling of a phased array is presented. For the HMS unit cells, the wire-loop topology is deployed to achieve high transmission for the required phase-angle shift. The proposed two-lens system is analyzed by full-wave simulations and experiments. The simulation results demonstrate that the scan angle doubles when the incident angle is below 15° in accordance to the design specification. Furthermore, the directivity degradation of the refracted beams by the two-HMS lenses is in good agreement with theory. Finally, a fabricated two-lens system with two $15\lambda $ long by $15\lambda $ wide metasurface lenses and a $16\times 16$ -element patch antenna array as a source is experimentally verified at ${\mathrm {10 \text {GHz}}}$ . The experimental results are in good agreement with the simulated results by showing angle-doubling performance with ±2° scan errors.

In this work, we present a study on the existence of Dirac type dispersion in the simplest of periodic metallic waveguide structures. It is shown that periodic repetitions of two dissimilar waveguides (WGs) can be properly designed to lead to a Dirac type dispersion. A simple theory using circuit modeling is presented to find the condition for Dirac point operation. In addition, mode-matching followed by full-wave simulations validate that the band structure matches that of the theory and shows that a Dirac dispersion can be realized. A Dirac Leaky-Wave Antenna (DLWA) is then implemented using this simple arrangement in substrate-integrated-waveguide (SIW) technology. This DLWA has the closed broadside stopband feature, leading to continuous frequency beam scanning through broadside and a simple feeding network. Phase and leakage constants are adjusted to yield a directive fan-beam featuring continuous scanning in a wide range of angles. The presented DLWA has a wide impedance bandwidth with high efficiency and operates with peak gains of about 12.5 dBi without any significant gain variation throughout the frequency range from 13.5 GHz to 16.5 GHz. Measured results show excellent agreement with the simulated results, validating the proposed concepts.

In this paper, we describe the design and implementation of a frequency-scanned slotted waveguide antenna with a closed stopband which has a low sensitivity to the fabrication errors. The antenna is implemented as a slotted slow-wave waveguide by loading a standard WR22 waveguide with elliptical posts. A systematic design approach for scanning through broadside and closing the stopband with a reduced sensitivity to fabrication tolerances is discussed in the paper. This approach allows for fabricating the antenna using CNC machining with a tolerance of +/−0.13 mm for the waveguide and a tolerance of +/−0.05 mm for the posts. The antenna beam can be steered from −38° to +27° by changing the frequency from 27 GHz to 34.7 GHz. The effective radiating length of the antenna is $27.6\lambda $ corresponding to a 1.75° half-power beamwidth while its radiation efficiency changes between 54% and 90% throughout the steering range.

We present the end-to-end design of compact passive and lossless metasurface antennas with integrated feeds. The complete low-profile system consists of a single-layered reactive impedance metasurface on top of a grounded dielectric substrate, and is fed by sources which are embedded inside the substrate. The top impedance layer is implemented with an array of printed metallic wires, each of which is periodically loaded with subwavelength reactive elements (e.g. printed capacitors). An accurate and efficient volume-surface integral equation-based model of the device is developed, and used as the basis for the rapid optimization of the wire impedances, with the goal of producing the desired radiation characteristics. It is found that the optimized designs leverage tailored surface waves to facilitate the realization of extreme field transformations. In particular, we present surfaces capable of wide-angle beamforming up to 60° off-broadside with nearly 100% aperture efficiency. We also demonstrate a multi-input, multi-output, antenna with two embedded sources emitting independent beams at ±20°. The output beams each exhibits an aperture efficiency of around 90%, despite sharing the same physical aperture. Our design framework is supplemented by several feasibility-related constraints, which can significantly enhance the power efficiency as well as the bandwidth of the metasurface antennas when they are implemented in practice. Utilizing these constraints, a Chebyshev pattern antenna with a side lobe level of −20 dB is designed with realistic loaded wires and validated with full-wave simulations. The obtained radiation pattern confirms the ability of the developed framework for arbitrary beam-shaping. The realized power efficiency (limited by copper and dielectric losses) is over 93% and the 3-dB directivity bandwidth slightly over 6%.

We present a simple reconfigurable leaky-wave antenna (LWA) capable of fixed-frequency continuous beam-scanning, along with an efficient optimization procedure with which its radiation pattern can be shaped. To construct the LWA, an array of varactor-loaded unit-cells are connected in series over a grounded dielectric substrate, forming a reconfigurable leaky microstrip. The varactors are individually addressed, with a biasing scheme determined via optimization. To ensure the practicality of the converged solutions, several feasibility-based optimization constraints are derived. In contrast with conventional LWAs, the proposed device is synthesized based on an aperiodic non-local model which rigorously accounts for the mutual coupling between all radiating elements. As a result, it can be optimized to meet very demanding requirements, such as wide-angle continuous beam-scanning with strong side lobe suppression. Importantly, the antenna input impedance can be adaptively tuned to match that of the source, without the need for a complicated matching network. Furthermore, the optimization procedure can accommodate different end terminations, even when they cause strong backward reflections. These features of the proposed LWA are confirmed through numerical simulations. Measurements of a fabricated prototype working at 10 GHz demonstrated a 130° continuous scan range, and a peak gain of 8.9 dBi.

Two-dimensional antennas that can create and steer narrow beams are of interest for modern communication systems. Here, the authors demonstrate a scalable antenna array design that performs these functions with fewer elements than standard approaches and excited only with peripheral sources.

Metasurfaces promise complex functionalities in electromagnetic wave control. Here the authors present an architecture for full-duplex reflective beamsteering based on series-connected radiative patches and nonreciprocal phase shifters.

Abstract Optical prisms are made of glass and map temporal frequencies into spatial frequencies by decomposing incident white light into its constituent colors and refract them into different directions. Conventional prisms suffer from their volumetric bulky and heavy structure and their material parameters are dictated by the Lorentz reciprocity theorem. Considering various applications of prisms in wave engineering and their growing applications in the invisible spectrum and antenna applications, there is a demand for compact apparatuses that are capable of providing prism functionality in a reconfigurable manner, with a nonreciprocal/reciprocal response. Here, we propose a nonreciprocal metasurface-based prism constituted of an array of phase- and amplitude-gradient frequency-dependent spatially variant radiating super-cells. In conventional optical prisms, nonreciprocal devices and metamaterials, the spatial decomposition and nonreciprocity functions are fixed and noneditable. Here, we present a programmable metasurface integrated with amplifiers to realize controllable nonreciprocal spatial decomposition, where each frequency component of the incident polychromatic wave can be transmitted under an arbitrary and programmable angle of transmission with a desired transmission gain. Such a polychromatic metasurface prism is constituted of frequency-dependent spatially variant transistor-based phase shifters and amplifiers for the spatial decomposition of the wave components. Interesting features include three-dimensional prism functionality with programmable angles of refraction, power amplification, and directive and diverse radiation beams. Furthermore, the metasurface prism can be digitally controlled via a field- programmable gate array (FPGA), making the metasurface a suitable solution for radars, holography applications, and wireless telecommunication systems.

In this paper, we describe the development of a new beam-steerable antenna array using a reconfigurable waveguide with tunable impedance side walls. The array has 36 radiating elements placed on the top wall of the waveguide and a common biasing voltage for tuning the impedance walls. The antenna operates at 9.3 GHz and its beam can be steered from 22° to 65° by changing the bias voltage from 16 to 7 volts. The gain of the antenna remains within the 3 dB limit of the maximum gain of 11.8 dB throughout the steering range. The effective radiating length of the antenna is $13\lambda $ and its half-power beamwidth is 5°. For this frequency range and electrical length, this antenna has a high radiation efficiency. This is due to the phase-shifting mechanism used which results in the varactor loss being less than all other sources of loss throughout most of the steering range. This makes the antenna suitable for applications where the varactors should operate at high frequencies with relatively low quality factors.

In this paper we present the theory, design, and implementation of a continuously tunable phase shifter using a surface-wave structure. The surface-wave travels on a reconfigurable impedance surface which is implemented by arranging dipoles on a single-layer PCB where a varactor is placed in the gap in the middle of each dipole. The proposed phase shifter operates at X-band from 8.5 to 10 GHz and it can provide 464o phase shift with a figure of merit of 110o/dB at 9.16 GHz. In the designed phase shifter, the varactor loss is less than all other loss mechanisms which makes it possible to achieve a high figure of merit at a high frequency range. The proposed phase shifter can be used in the design of phased arrays that require a high radiation efficiency at a relatively high frequency compared to the self-resonance frequency of the tunable electronic components.

In this article, we review the topic of Huygens' metasurfaces with an emphasis on existing and emerging applications at microwave frequencies. Huygens' metasurfaces have demonstrated unprecedented capabilities of controlling electromagnetic wavefronts by means of electric and magnetic dipole moments arranged in a thin sheet. We present the fundamental principles of Huygens' metasurfaces based on the boundary conditions governing their operation. Then, we discuss the aspect of practical realization of Huygens' metasurfaces and the different types of constituent subwavelength scatterers (unit cells). Moreover, we summarize recent developments in several areas related to metasurfaces, such as perfect anomalous refraction, polarization control, antenna beamforming and reconfigurable metasurfaces. Lastly, we provide a brief outlook on emerging metasurface-based microwave technologies that are expected to further grow in the future.

In this paper we describe the design, theory, implementation, and measurement results of a novel frequency-scanned slow-wave slotted waveguide antenna. The slow-wave antenna is fabricated by periodically loading a standard WR22 waveguide using cylindrical posts. The antenna has a backward radiation and can steer the beam from -72° to -12° by changing the frequency from 32.5 GHz to 37.4 GHz. The gain of the antenna remains within a 3 dB range from the maximum gain of 17 dBi throughout the steering range. The antenna is 29λ in length and 0.69λ in width. The antenna radiation efficiency is between 74% and 92% throughout its frequency range which allows for further extension of its length in order to achieve a higher gain and a smaller beamwidth. Moreover, the small lateral width of the antenna allows placing several of them side-by-side to either narrow or facilitate scanning the beam in the transverse plane. The antenna has a fast and constant scanning rate of 4.3° over a 1% bandwidth and a return loss of less than -10 dB throughout its operating frequency range.

In this paper, different reconfigurability mechanisms are proposed to scan the pointing direction at a fixed frequency of recently-proposed leaky-wave antennas based on omega-bianisotropic Huygens' metasurfaces. The tunability of both the waveguide height and the dielectric permittivity of the substrate filling the waveguide are studied, extracting theoretical expressions to predict the dependence of the beam pointing direction with the design parameters. It is shown that it is possible to control the scanning rate, a challenging task for conventional leaky-wave antennas, leading to different performances: high scanning, radiation pattern stability and backward-to-forward scanning. Additionally, the control of the scanning rate is exploited to reduce the beam squinting when varying the frequency. Simulation results of several designs with both impedance sheets and physical realization to implement the metasurfaces demonstrate the validity of the theoretical predictions and the reconfigurability potential of such a structure.

Abstract We propose a new family of impedance-matched chiral metasurfaces that offer arbitrary polarization control at two different frequencies. To this end, two main problems are addressed: (1) determination of the required surface impedances for a certain user-defined chiral functionality at two frequencies and (2) their physical realization at microwaves. The first milestone is achieved through a proposed synthesis method that combines a semi-analytical method and a nonlinear optimization technique. In particular, the impedances are computed such that the devised chiral metasurface is also impedance-matched to a terminating medium. The chiral metasurfaces are then physically realized at microwaves by cascading layers of rotated arrays of multiple concentric rectangular copper rings. We establish that these proposed unit cells offer distinct dual-resonances that can be arbitrarily and independently tuned for two orthogonal linear polarizations at each of the two operating frequencies. This allows simultaneous physical mapping of the required surface impedances at two frequencies. The versatility and generality of the proposed numerical and physical solutions are demonstrated through two design examples: A dual-band circular polarization selective surface (CPSS) and a dual-band polarization rotator (PR). The dual-band CPSS is further confirmed experimentally at 20 GHz and 30 GHz based on a free-space quasi-optical system.

Recently, the super-oscillation phenomenon has attracted attention because of its ability to super-resolve unlabelled objects in the far-field. Previous synthesis of super-oscillatory point-spread functions used the Chebyshev patterns where all sidelobes are equal. In this work, an approach is introduced to generate super-oscillatory Taylor-like point-spread functions that have tapered sidelobes. The proposed method is based on the Schelkunoff’s super-directive antenna theory. This approach enables the super-resolution, the first sidelobe level and the tapering rate of the sidelobes to be controlled. Finally, we present the design of several imaging experiments using a spatial light modulator as an advanced programmable grating to form the Taylor-like super-oscillatory point-spread functions and demonstrate their superiority over the Chebyshev ones in resolving the objects of two apertures and of a mask with the letter E.

The realization of highly-directive beams using low-profile devices has been a long-standing problem. Here, Epstein et al.demonstrate a cavity-excited Huygens’ metasurface antenna, where the single-source-fed cavity is designed to optimize aperture illumination, while the metasurface facilitates phase purity.

Considerable research has been devoted to the development of imaging apparatus capable of high-resolution imaging beyond the diffraction limit. Innovative subdiffraction imaging systems have been proposed and successfully demonstrated. These systems either focus the electromagnetic near-field more tightly than a Hertzian dipole or resolve the electromagnetic far-field beyond Abbé's diffraction limit and, hence, can be termed subdiffraction imaging systems. This paper reviews major advances in the field of subdiffraction imaging in the year 2011, along the research fronts of superlenses, hyperlenses, metascreens, and superoscillations.

In this paper, we propose a new metasurface that is able to reflect a known incoming electromagnetic wave into an arbitrary direction, with perfect power efficiency. This seemingly simple task, which we hereafter call perfect anomalous reflection, is actually highly nontrivial because of the differing wave impedances and complex interference between the incident and reflected waves. Heretofore, proposed metasurfaces that achieve perfect anomalous reflection require complicated, deeply subwavelength and/or multilayer element structures, which allow them to couple to and from leaky and/or evanescent waves. In contrast, we demonstrate that using a bipartite Huygens’ metasurface (BHM)—a passive and lossless metasurface with only two cells per period—perfect anomalous reflection can be achieved over a wide angular and frequency range. Through simulations and experiments at 24 GHz, we show that a properly designed BHM can anomalously reflect an incident electromagnetic wave from θ_{i}=50° to θ_{r}=-22.5°, with perfect power efficiency to within experimental precision.

Implementation of abrupt phase discontinuities along a surface has been the theme of recent research on electromagnetic metasurfaces. Simple functionalities such as reflecting, refracting, or focusing plane waves have been demonstrated with devices featuring phase discontinuities, but optical surfaces allowing independent magnitude and phase control on the scattered waves have yet to emerge. In this paper, we propose the first true optical Huygens’ surface, which explicitly utilizes orthogonal electric and magnetic responses to realize total control on an optical surface’s local reflection coefficients. This extends the functionality of metasurfaces to an unprecedented level. We first demonstrate that a nanorod gap-surface plasmon resonator can act as a Huygens’ source. Thereafter, by properly tuning and rotating these resonators, we realize arbitrary reflection optical metasurfaces—surfaces for which the local reflection coefficients can be independently tailored in both magnitude and phase. We demonstrate the versatility of this approach through designs of a metasurface that asymmetrically reflects two copolarized beams and a Dolph-Tschebyscheff optical reflectarray.

Active electromagnetic cloaking uses an array of elementary sources to cancel the scattered fields created by an object. An active interior cloak does this by placing the sources along the boundary of the object. This process can be thought of as introducing a discontinuity in the field to cancel out the scattered field by the object. Here, an experimental version of a thin active cloak at microwave frequencies is demonstrated for an aluminum cylinder with a radius of 0.56λ. The cloak consists of a 12-element magnetic-dipole array. By controlling the weights of the current on each element of the array, the scattering off of the cylinder is reduced in the backward and forward directions. The ability to disguise the aluminum cylinder as another object by varying the weights of the dipole array is also demonstrated. Finally, potential ways of overcoming the constraint of requiring a priori knowledge of the incident field leading to camouflaging-type behavior are discussed.