This paper presents a two-dimensional Fabry-Pérot cavity antenna (FPCA) based on a bianisotropic Huygens' metasurface (BHMS) acting as an enhanced version of the usual partially reflective surface. The BHMS wavefront transformation capabilities enable achieving broadside pencil-beam radiation without relying on the conventional resonance condition that constraints classic FPCA designs regarding excitation and maximum directivity. By rigorously stipulating the guided and radiated field, the BHMS is able to implement the required transmission phase shift across the radiating aperture while guaranteeing proper wave propagation inside the cavity without impedance mismatches. Hence, the proposed approach allows achieving higher directivity without the guided modes getting closer to cutoff nor the beam becoming conical. This concept is validated through a design example showcasing a directive broadside beam with a preliminarily stable frequency behavior in simulation.

We present a high-efficiency wireless powering architecture designed for subcortical medical implants. Conventional inductive links struggle to bridge the mid-field gap, where the power density decays rapidly with depth. To overcome this, we propose a 13.56-MHz system enhancing a coupled-matching topology with a passive hexagonal metasurface (MS) relay. This MS configuration boosts evanescent fields to create a magnetic funnel, while the matching network enables intrinsic implant auto-localization. Complementing the link, we report a 65-nm CMOS hybrid rectifier utilizing a bootstrapped scheme with a dynamic auxiliary biasing path to achieve >70% efficiency. Experimental validation confirms a 1.7X power boost at 30-mm depth, delivering 250 mu W, and improved misalignment tolerance, establishing a robust framework for batteryless deep-brain neuromodulation.

We propose a portable, single-frequency microwave permittivity sensor based on radio frequency identification (RFID) backscattering that addresses three open challenges in wireless dielectric characterization: nonlinear sensor response, dependence on bulky laboratory instrumentation, and sensitivity to reader-tag misalignment. Unlike conventional vector network analyzer (VNA)-based frequency-swept methodologies requiring broadband spectral acquisition, the proposed architecture monitors amplitude at a single industrial, scientific, and medical (ISM)-band frequency (2.45 GHz), translating permittivity-induced resonance shifts into direct voltage readout. The platform integrates four codesigned elements. A dual-layer high-gain antenna (9.9 dBi) extends the wireless interrogation range to 10 cm. A Koch-fractal complementary split-ring resonator (CSSR, 30 & times;30 mm(2) patch and 0.024 lambda(2) (g) footprint) converts dielectric loading into amplitude modulation. A custom millimeter-scale 65-nm complementary metal-oxide-semiconductor (CMOS) rectifier (74.7% peak efficiency) replaces larger laboratory power meters; its nonlinear transfer function compensates for the compressive response of the resonator tag, yielding a linear voltage-permittivity relationship. A retrodirective Van Atta (VA) array is developed to re-radiate the backscattered energy in cross-polarization toward the interrogating source regardless of the incidence angle, restricting amplitude fluctuations to <4.7 dB over +/- 60 degrees angular range and +/- 40-mm lateral displacement, compared with 14 dB for the conventional single-element tag. Experimental validation confirms linear operation across solid-dielectric substrates (epsilon(r) is an element of [2.2, 10.2], 90-mV/unit sensitivity) and microfluidic isopropyl alcohol (IPA)-water mixtures (0%-100% and 9-mV/% resolution). By codesigning the electromagnetic tag response with the CMOS rectifier nonlinearity and embedding the sensing function within a retrodirective array, this work delivers a spatially robust, field-deployable, VNA-free readout sensing platform suitable for mobile interrogation scenarios such as drone-mounted inspection and handheld field diagnostics.

Electromagnetic metasurfaces (MTSs) have been widely utilized for systematic wavefront transformation to enhance wireless communication through transmission and reflection. By embedding active radiating sources into the MTS, a more compact and integrated antenna system can be realized. In this work, we utilize a design framework based on the method of moments and optimization, for implementing impedance MTSs with embedded sources. This framework is utilized to develop a physical realization of the required impedance sheet and embedded line source at millimeter-wave (mmWave) frequencies. The incident power, which is initially concentrated near the source, is efficiently redistributed across the entire surface with the assistance of auxiliary surface waves. We demonstrate the effectiveness of the proposed approach through numerical examples that achieve beamforming up to 60 degrees and a slightly over 100% aperture illumination efficiency. The proposed approach is further validated with experimental results at mmWave frequencies. The fabricated MTS antenna operates at 25.65 GHz, demonstrates beam pointing at 30 degrees, and achieves an illumination efficiency of 65%.

Periodic surfaces made of capacitive patches or mushroom elements over a metal groundplane have been used for years as engineered reflectors. These surfaces have been studied extensively in this capacity, but more recently their utility as radiating elements has been demonstrated. When designed to support surface wave propagation, careful dispersion engineering of the surface elements can create high-efficiency, wideband antennas. This work presents a novel antenna that uses a dipole embedded in-plane with the elements of a mushroom surface to simultaneously excite orthogonally propagating TM and TE surface waves. The surface is tuned and sized such that each surface wave mode resonates independently but radiates in the same polarization, achieving a wide impedance bandwidth and high radiation efficiency while only requiring a simple two-layer PCB process and inexpensive lossy FR-4 substrate for fabrication. Two antennas of different surface dimensions are fabricated and compared to verify the radiation mechanism. The optimal fabricated antenna has an impedance bandwidth of 33%, peak total efficiency of 89%, and broadside radiation with a realized gain between 4.6 and 6.6 dBi, with an overall size of 0.82λ0×0.82λ0×0.048λ0.

Superoscillatory (SO) functions offer a promising pathway to sub-diffraction optical imaging that is far-field, single-shot, and label-free, requiring only a passive optical element. However, the design of SO point spread functions (PSFs) remains an open problem—existing design techniques are guided by PSF-space heuristics rather than by the quality of the resulting image, and current SO PSF parameterizations are not directly amenable to gradient-based optimization. Specifically, previous work creates SO PSFs using basis functions that are either not physically-realizable in optics, or do not reliably yield SO behavior when optimized. As such, prior work has relied on limited SO PSF designs to explore their potential for sub-diffraction imaging. In this work, we introduce a first-principles approach to designing SO PSFs computationally and the first learning-based method for optimizing their imaging performance. Our framework provides direct control over where superoscillations appear in the spatial domain while preserving an explicit analytic mapping to the Fourier-domain pupil. This enables the SO design space to be systematically characterized for the first time: SO constraints are enforced at every iteration, guaranteeing a valid SO solution, and each design corresponds to a physically realizable optical element. We also present an end-to-end optimization pipeline and evaluate our approach in simulation on greyscale objects and dense structured patterns at varying spatial frequencies. Our results show that our framework produces SO PSFs that resolve features at 3× below the diffraction limit on standard targets (Ronchi grating) and significantly outperform existing methods on widely used image quality metrics. Finally, we characterize the noise regimes in which SO PSF engineering outperforms diffraction-limited imaging. By providing a computational roadmap to SO design and optimization, we hope to encourage further exploration by the computational imaging community of superoscillations as a tool for passive sub-diffraction imaging. Code is available at https://so-psfs.github.io.

Reflective electromagnetic metasurfaces (MTSs) provide a passive platform for manipulating the reflection of incident electromagnetic waves, offering opportunities for beamforming and polarization control in wireless communication systems. However, systematic 3D design methods that account for anisotropic unit cells and full-wave interactions remain limited. In this paper, a 3D full-wave design procedure for reflective MTSs based on the integral equation method and tensor impedance boundary conditions is presented. The MTS consists of impedance sheets placed on top of a grounded dielectric slab. Based on a method of moments formulation that explicitly incorporates coupling between unit cells, the impedance values are optimized via gradient descent to achieve beamforming and polarization control of the reflected field. A numerical example demonstrates the beamforming capability of the proposed method, where precise amplitude and phase control through auxiliary surface waves allows the synthesis of a target sector pattern. For practical implementation, a regression neural network is employed to establish an inverse mapping between the optimized impedance tensors and the geometric parameters of anisotropic unit cells featuring three degrees of freedom, enabling efficient realization of spatially varying anisotropic impedance distributions. Two MTSs are designed and experimentally validated at 10 GHz based on an introduced novel physical unit cell. The first, with 576 unit cells spanning four wavelengths by four wavelengths, performs anomalous reflection and converts linear to right-hand circular polarization (RHCP), achieving 13.0 dB cross-polar discrimination and 21.8 dB directivity, with an instantaneous 3-dB fractional gain bandwidth of 6.2%. The second, with 1296 unit cells, splits the reflected wave into RHCP and LHCP beams in different directions, with measured directivities of 23.9 dB and 23.7 dB, respectively, and a bandwidth of 7.6%. These high directivities are achieved by accounting for mutual coupling and by exploiting auxiliary surface waves that redistribute incident power across the MTS.

Reconfigurable metasurfaces are composed of unit cells capable of beamsteering an incoming wavefront to enable auxiliary connections in complex communication scenarios. In this paper, we first use a reconfigurable metasurface that can beamsteer in a single plane to localize a single source in the [−60°, 60°] range using multiple phase gradient profiles. A coarse to fine localization scheme is employed to reduce the scanning time. The localization experiment is followed by a transmission experiment, where different phase profiles are evaluated using the Method of Moments and the one that produces patterns with the lowest sidelobe levels is selected. Software defined radio (SDR) units are used to transmit an FM modulated signal at 5 GHz from a monopole source antenna to a receiving horn antenna for both the localization and transmission experiments.

A reconfigurable intelligent surface (RIS) is composed of low-cost elements that manipulate the propagation environment from a transmitter by applying phase shifts to incoming signals before they are reflected. This paper explores a unipolarized RIS with linear shape aimed at transmitting a common signal to multiple user equipments (UEs) spread across a wide angular region. To achieve uniform coverage, the uni-polarized RIS is designed to emit a broad and spectrally-efficient beam featuring a spatially flat-like array factor, diverging from the conventional narrow beam approach. To achieve this objective, we start by deriving probabilistic lower and upper bounds for the average spectral efficiency (SE) delivered to the UEs. Leveraging the insights from the lower bound, we focus on optimizing the minimum value of the power domain array factor (PDAF) across a range of azimuth angles from −π/2 to π/2 . We employ the continuous genetic algorithm (CGA) for this optimization task, aiming to improve the SE delivered to the UEs while also creating a wide beam. Extensive simulation experiments are carried out to assess the performance of the proposed code, focusing on key metrics such as the minimum and average values of the PDAF and the SE delivered to the UEs. Our findings demonstrate that the proposed code enhances the minimum SE delivered to the UEs while maintaining the desired attribute of a broad beam. This performance is notably superior to that of established codes, including the Barker, Frank, Golay, and Chu codes. Moreover, the effectiveness of the proposed code in generating a broad beam is experimentally validated using a RIS prototype.

Using metasurface (MTS) antennas in wireless communication networks offers a promising way to achieve both high-quality connections and low power usage. In this paper, we study a setup where a transmissive MTS is integrated into the basestation antenna. We explore how the type of electromagnetic wave used to illuminate the MTS affects communication performance, assuming all other system parameters remain fixed. Our results show that using a cylindrical wave improves downlink performance compared to a plane wave. Although a plane wave might seem ideal due to its uniform illumination, our findings highlight the benefit of a richer set of wavevectors in the illuminating field.

Recent advances in metasurfaces will be presented for achieving electromagnetic-wave beam-steering and beam-shaping in the far zone. In particular, we will present a paradigm shift on how to implement reflecting surfaces that can provide complete control of the reflected wave, thus enabling both beam-steering and beam-shaping, without the explicit need of loss or gain [1] . The key enabling feature is the utilization of surface waves to both change the phase of the reflected waves but also allow for amplitude control through power redistribution. We will showcase several such surface-wave assisted metasurfaces, including beam-redirection surfaces, multi-beam surfaces, surfaces with shaped reflected beams, electrically-thin reflectors with low-beam squinting vs. frequency, and electronically reconfigurable ones.

Electromagnetic metasurfaces (MTSs) are typically modeled as passive, lossless, and purely reactive homogenized impedance sheets on dielectric substrates. In this paper, we present an overview of designing reflecting MTSs using the method of moments (MoM) with gradient-based optimization. The proposed MTSs consist of printed metallic patterns placed over a grounded dielectric slab, with the patterns modeled by impedance sheets, and the impedance values optimized through gradient-descent to achieve specific far-field (FF) power patterns or beamforming objectives. This beamforming/beamshaping capability is enabled utilizing auxiliary surface waves, without the need of any explicit gain or loss. Full-wave simulations of realistic copper structures are presented to validate the proposed design procedure, with measurements preformed for an one dimensional (1D) MTS. An example of a 2D MTS featuring polarization control is also presented.

We present a comprehensive tutorial on the finite-difference time-domain (FDTD) modeling of space, time, and space-time-varying media, building upon our previous review by offering a detailed, step-by-step guide for numerical simulations. This manuscript emphasizes practical computational methodologies and examines the dynamic behavior of both TE and TM electromagnetic fields under oblique incidence on space-time-modulated gratings, in which electrical permittivity, magnetic permeability, and conductivity vary in both space and time. The article details the derivation of the FDTD schemes, including the Courant-Friedrichs-Lewy (CFL) stability condition, explicit formulations for TE and TM wave illuminations, and the implementation of appropriate boundary conditions. In addition, we extend the FDTD framework to incorporate nonlinear effects, thereby broadening its applicability to advanced applications in wireless communications, quantum technologies, and radars. Engaging illustrative examples are provided to demonstrate the versatility and practical relevance of the approach.

Metasurface anomalous reflectors can enhance future wireless communication systems due to their ability to redirect electromagnetic waves and establish metasurface-assisted communication links. However, metasurface reflectors typically suffer from chromatic aberrations with the reflected angle shifting as the frequency varies. Herein, we design achromatic metasurfaces for anomalous reflection by using an integral-equation framework that fully accounts for the frequency dispersion of the individual scatterers (unit cells). Rather than engineering the dispersion of each metasurface cell locally, we harness the emerging near-field interactions (evanescent waves) between the scatterers to realize anomalous reflection at a fixed angle over a wide frequency range. Three anomalous reflectors are designed and experimentally verified at 10GHz, demonstrating high performance metrics compared to traditional achromatic reflectors, while employing simple, low-profile unit cells. The presented results pave an effective way to realize achromatic wave functionalities, such as anomalous reflection, refraction or lensing, over a wide bandwidth with low-profile and highly efficient metasurfaces.

We hereby propose a dual-polarized beam-deflecting metasurface that can double the scan-angle range of a phased-array antenna without compromising the directivity across all scan angles, including broadside. In particular, our research focuses on expanding the scanning range of a dual-polarized phased-array antenna capable of independently steering TE and TM waves. This is achieved by placing a dual-polarized phase-gradient Huygens' metasurface in front of the antenna. The Huygens' metasurface employs crossed meander lines in four impedance layers that are suitably optimized to independently control the local electric and magnetic responses for maximizing the transmission for all incident beams. We validate our approach through theoretical analysis, full-wave simulations, and experimental verification. It is demonstrated that the beam-deflecting HMS achieves effective scan range expansion to -30(degrees)similar to 0(degrees) and 0(degrees)similar to 30(degrees) for TE and TM beams, respectively, using a dual-polarized phased array antenna source that scans from -15 degrees and 15 degrees.

This paper presents a non-linear programmable metasurface (PMSF) for smart wireless environment applications. The proposed PMSF demonstrates simultaneous frequency conversion and beam steering capabilities. This is achieved through an innovative unit cell design which incorporates single sideband (SSB) mixers. The unit cell operates at 3.5 GHz (5G, sub-6GHz band) and 5.5 GHz (WiFi), as validated through full-wave and circuit simulations. The functionality of the PMSF is demonstrated with a 1 x 7 unit cell simulation, where phase control is provided at the intermediate frequency (IF), enabling beam steering. The results show significant suppression of undesired harmonics and efficient frequency conversion. The presented non-linear PMSF design could find applications in telecommunications to enhance coverage, energy efficiency, and security.

Herein, a metamaterial cover layer is presented to enhance the scanning range of patch-antenna phased arrays in both principal planes while preserving directivity. The main novelty resides in the anisotropic characteristics of the cover layer, which inhibit the excitation of the fundamental surface-wave (SW) mode within the desired angular range. This suppression approach is infeasible with a standard dielectric-slab wide-angle impedance matching (WAIM) layer. The process for suppressing surface waves in an anisotropic slab is examined through the transverse resonance technique, resulting in design equations for practical application. The metamaterial cover uniquely integrates a wire medium (WM) slab with an artificial dielectric layer. This combination mitigates two significant limitations of wide-angle scanning phased arrays: scan blindness and mutual coupling. The proposed cover layer is utilized for two recently reported phased arrays and improvements in the scan range are demonstrated. In both cases, it is demonstrated that the porposed metamaterial cover layer outperforms a conventional dielectric-slab WAIM sheet.

Multifunctional electromagnetic composites are highly important in various wave propagation applications. Here, we propose an all-metallic metagrating composed of resonant scatterers that is capable of performing multiple wave transformations, including isolation, polarization control, beam deflection, and retroreflection, for impinging transverse magnetic (TM) plane waves at a discrete set of incident angles. The operation relies on the efficient coupling between different propagating Floquet modes. The structure is linear, timeinvariant, passive, reciprocal, and low-loss. It is shown that the far-field behavior of the designed metagrating resembles that of spatially dispersive (nonlocal) homogeneous bianisotropic slabs and results from the tailored resonances of the constituent scatterers.