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We introduce a new concept of three-states polarization converting metasurface (TS-PCM) based on p-i-n diodes. The proposed structure can offer three distinct wave manipulation functionalities: linear polarization to orthogonal linear polarization (LP-OLP), linear polarization to circular polarization (LP-CP), and linear polarization to full reflection (LP-FR) state. Unlike previous structures, the multiple conversion features are simultaneously achieved in different frequency ranges, depending on the operating state of diode (off or on). Finally, a sample prototype was fabricated and the experimental results were verified with the simulated ones.

The quest for increasing the scanning range of a phased array is a challenging task for antenna engineers, and its solution could lead to significant advances in different applicative scenarios, ranging from 5G and beyond 5G communications to radar and satellite systems. For this purpose, the use of a deflecting meta-dome is one of the most promising solutions recently proposed that, however, still presents some inherent limitations, such as the significant reduction of the broadside gain of the array, due to the diverging effect of the dome, as well as the complexity of the implementation due to the need of a continuous phase profile. In this framework, the paper aims at proposing some technical solutions for maximizing the meta-dome performance and relaxing the implementation complexity. In particular, by properly discretizing and engineering the phase profile along the dome and by taking into account the different angles of incidence onto the meta-cells, we show how it is possible designing realistic meta-domes with reduced insertion loss at broadside, improved steering capabilities, and a reduced profile. In addition, a complete design workflow for a realistic meta-dome based on cascaded metasurfaces is presented., (in press)

This paper presents a false target jamming method for linear frequency modulated (LFM) radar based on diverse frequency time-modulated metasurface (DF-MTS). The DF-MTS is composed by a set of elements, each of which is independently time-modulated to realize specific false targets in the high-resolution range profile (HRRP) of the radar system. Thanks to the control of the modulation scheme in each element of the metasurface and the combined response of the overall surface, we demonstrate the capability to realize several false target patterns consisting of single and multiple targets symmetrically, or non-symmetrically, distributed around the actual target. A general expression of one-dimension HRRP of DF-MTS at different observation angles is given to show the relation between the generated false targets and frequency components. Due to the flexible capacity of scattered field manipulation, DF-MTS has enhanced performance on control of false target jamming at different observation angles. Both simulated and measured results demonstrate the validity of false targets manipulation, demonstrating its effectiveness as electromagnetic countermeasure for achieving radar camouflaging and deception.

Wireless communications and sensing have notably advanced thanks to the recent developments in both software and hardware. Although various modulation schemes have been proposed to efficiently use the limited frequency resources by exploiting several degrees of freedom, antenna performance is essentially governed by frequency only. Here, we present a new antenna design concept based on metasurfaces to manipulate antenna performances in response to the time width of electromagnetic pulses. We numerically and experimentally show that by using a proper set of spatially arranged metasurfaces loaded with lumped circuits, ordinary omnidirectional antennas can be reconfigured by the incident pulse width to exhibit directional characteristics varying over hundreds of milliseconds or billions of cycles, far beyond conventional performance. We demonstrate that the proposed concept can be applied for sensing, selective reception under simultaneous incidence and mutual communications as the first step to expand existing frequency resources based on pulse width., 61 pages, 6 figures, 26 supplementary figures

Recently, the topological properties of composite vortices, i.e., the superposition between vortex modes of a different order, have proven effective in manipulating the radiation properties of patch antennas and making them suitable for new operative scenarios. In this paper, we exploit these properties to design a multi-antenna system exhibiting both pattern- and polarization-diversity. The low mutual coupling and the possibility to steer two orthogonal polarized fields in different directions, make this radiating element an optimal candidate for antenna diversity or MIMO systems and, thus, for increasing signal quality or channel capacity of a communication system.

In this contribution, we discuss the design of spatial filters realized through all-dielectric metasurfaces. Specifically, it is first discussed how the tangential polarizations of all-dielectric metasurfaces can be exploited to design wideband dielectric mirrors. Then, it is shown how it is possible engineering the normal polarization of these structures and use them to open a transmission window around a desired impinging angle. Several examples and realistic full-wave simulations are included to verify the analytical models and the physical discussions.

Filtering modules are essential devices of modern microwave systems given their capability to improve the signal-to-noise ratio of the received signal or to eliminate the unwanted interferences. For discriminating between different components, a filter exhibits a frequency-selective response that, however, is not able to distinguish between different signals whose spectrum falls within the passband of the filter itself. In this regard, some electromagnetic structures exhibiting, at the same frequency, different responses depending on the waveform of the incoming waves have been recently proposed. In this communication, we extend the aforementioned approach to the case of a standard waveguide filtering module. In particular, by loading a bandpass filtering iris with a proper lumped-element circuit, we design a waveguide component able to distinguish between different pulsed waves, even at the same frequency, depending on their pulsewidth. Moreover, by using this filter for capping an open-ended rectangular waveguide, a radiating element with both frequency- A nd time-domain selectivity properties is presented. The structures discussed in this communication may pave the way to a new class of microwave systems that, being both frequency selective and time selective, are less sensitive to noise and interferences.

Metasurfaces consisting of electrically thin and densely packed planar arrays of subwavelength elements enable an unprecedented control of the impinging electromagnetic fields. Spatially modulated metasurfaces can efficiently tailor the spatial distribution of these fields with great flexibility. Similarly, time-modulated metasurfaces can be successfully used to manipulate the frequency content and time variations in the impinging field. In this article, we present time-modulated reflective metasurfaces that cause a frequency shift to the impinging radiation, thus realizing an artificial Doppler effect in a nonmoving electrically thin structure. Starting from the theoretical analysis, we analytically derive the required time modulation of the surface admittance to achieve this effect and present a realistic time-varying structure, based on a properly designed and dynamically tuned high-impedance surface. It is analytically and numerically demonstrated that the field emerging from the metasurface is up-/down-converted in frequency according to the modulation profile of the metasurface. The proposed metasurface concept, enabling a frequency modulation of the electromagnetic field 'on-the-fly,' may find application in telecommunication, radar, and sensing scenarios.

Metagratings have recently shown promising features for wavefront manipulation, overcoming the efficiency limitations of gradient metasurfaces and their demanding fabrication requirements. Extreme functionalities, such as perfect anomalous reflection and refraction, focusing, and holography, have been proposed and tested. However, most of the developed analytical models concern the manipulation of the reflected field. In this article, we present a theoretical formulation that allows a complete manipulation of both reflected and transmitted fields through a metagrating consisting of arrays of capacitively loaded strips. In addition, the proposed solution enables the design of electronically reconfigurable metagratings for the dynamic control of the diffraction pattern at microwave frequencies. The theoretical formulation is numerically validated and a possible practical implementation of the metagrating is also discussed, as well as the effects of losses and parasitic reactances.

Temporal metamaterials have been recently exploited as a novel platform for conceiving several electromagnetic and optical devices based on the anomalous scattering response arising at a single or multiple sudden changes of the material properties. However, they are difficult to implement in realistic scenarios by switching the permittivity of a material in time, and new strategies to achieve time interfaces in a feasible manner must be identified. In this paper, we investigate the possibility to realize a temporal metamaterial without acting on the material properties, but rather on the effective refractive index and wave impedance perceived by the wave during the propagation in an empty guiding structure by varying the boundaries in time. We demonstrate analytically and through numerical experiments that suddenly changing the physical distance between the metallic plates of a parallel-plate waveguide will induce an effective temporal interface. In addition to the standard backward and forward scattered fields at different frequencies due to the temporal interface, we also identify the presence of a static field necessary to satisfy the continuity of the electromagnetic field across the interface. The proposed concept can be extended to temporally controlled metasurfaces, opening an easier path to the design and realization of novel devices based on time-varying metamaterials at microwave and optical frequencies.

Space-time-modulated metasurfaces can manipulate electromagnetic waves in space and frequency domain simultaneously. In this paper, an analytical design of space-time- modulated metasurfaces with modulation elements composed of two paths, In-phase (I) and Quadrature (Q), is proposed. The model is derived analytically, the space/frequency domain manipulations are achieved by designing the dimension and time sequence of I and Q paths. In the specular reflection direction, an objective frequency shift of the reflected first order harmonic can be obtained. While, in other directions, the opposite first order harmonic can be easily controlled by changing the dimension of I/Q paths and the objective first order harmonic remains unchanged. Furthermore, with a small dimension of I/Q paths, the first order harmonic can be used for beam scanning by pre-designing the start time of the modulation element. To realize the space-time-modulated metasurface with the required periodically time-varying responses, 2-bit unit-cells loaded with dynamically switchable pin diodes are employed as I/Q modulation. Both analytical and numerical results demonstrate that space and frequency domain manipulations of the reflected fields by the first order harmonics can be simultaneously obtained. The proposed designs have potential applications in wireless communications, radar camouflaging, and cloaking.

The papers in this special section focus on recent advancements in this field and provide an overview of the potential applications of this technology in the next generation antenna devices, with emphasis on metamaterial-based enhancements enabling real-time control of their radiation characteristics.

Next-generation of wireless communications and beyond-5G systems will require extreme performance in terms of low latency, data rates, reliability, just to name a few, and the possibility to tailor the communication channel on demand would be a key enabling technology for the so-called smart environment systems. Therefore, conventional antennas with fixed functionalities cannot be considered an efficient solution, and a new paradigm change in antenna design is thus required. In this frame, here, we show some of the most interesting possibilities enabled by functionalized conformal metasurfaces coating wired antennas for enabling unprecedented electromagnetic behaviours. In particular, we report the design of metasurface coats loaded with electronic elements allowing scattering, frequency, and radiation pattern tunability, in view of the designs of intelligent antennas whose properties can be tailored on demand.

So far, the environment has been considered as a source of fading, clutter, blockage, etc., with detrimental consequences for the efficiency and robustness of communication systems. However, the intense research developed toward beyond-5G communications is leading to a paradigm change, in which the environment is exploited as a new degree of freedom and plays an active role in achieving unprecedented system performances. For implementing this challenging paradigm, it has been recently proposed the use of intelligent surfaces able to control almost at will the propagation of electromagnetic waves. In this framework, metasurfaces have emerged as a promising solution, thanks to their field manipulation capabilities achieved through low-cost, lightweight, and planar structures. The aim of this paper is to review some recent applications of metasurfaces and cast them in the scenario of next-generation wireless systems. In particular, we show their potentialities in overcoming some detrimental effects presented by the environment in wireless communications, and discuss their crucial role towards the practical implementation of a smart electromagnetic environment.

Recently, great efforts have been made in the research of new enabling technologies for beyond 5G communication systems. In this framework, Reconfigurable Intelligent Surfaces (RISs) are considered as one of the most promising solutions and, thus, they have been the subject of several investigations by both the electromagnetic and the communication communities. In this contribution, we further discuss, from an electromagnetic point of view, about the synthesis of a RIS. In particular, by exploiting the topological properties of vortex modes, we present a possible solution to simplify the synthesis and the reconfigurability of a RIS composed of concentric annular rings.

In this Letter, we discuss a novel, to the best of our knowledge, approach for designing passive nanoantennas with maximum forward and almost-zero backward scattering. The proposed approach is based on the use of high-index dielectric spheres supporting dipolar magnetic resonances, which are coated by ultra-thin surface impedance coatings. It is shown that, by properly engineering the radius of the coat and its surface reactance, it is possible to introduce an additional electric dipolar resonance and to make this overlap with the magnetic one sustained by the high-index dielectric sphere. A realistic design that is based on graphene and works in the low-THz range is also proposed and verified with full-wave simulations. Compared to earlier techniques based on the combination of multipoles or on the use of ellipsoidal particles, the proposed one is quite robust toward realistic ohmic losses and preserves the isotropic behavior of the nanoantenna.

Space-time-modulated metasurfaces can manipulate electromagnetic waves in space and frequency domain simultaneously. In this paper, an analytical design of space-time-modulated metasurfaces with modulation elements composed of two paths, In-phase (I) and Quadrature (Q), is proposed. The model is derived analytically, the space/frequency domain manipulations are achieved by designing the dimension and time sequence of I and Q paths. In the specular reflection direction, an objective frequency shift of the reflected first order harmonic can be obtained. While, in other directions, the opposite first order harmonic can be easily controlled by changing the dimension of I/Q paths and the objective first order harmonic remains unchanged. Furthermore, with a small dimension of I/Q paths, the first order harmonic can be used for beam scanning by pre-designing the start time of the modulation element. To realize the space-time-modulated metasurface with the required periodically time-varying responses, 2-bit unit-cells loaded with dynamically switchable pin diodes are employed as I/Q modulation. Both analytical and numerical results demonstrate that space and frequency domain manipulations of the reflected fields by the first order harmonics can be simultaneously obtained. The proposed designs have potential applications in wireless communications, radar camouflaging, and cloaking.

The use of fifth-generation (5G) new radio (NR) spectrum around 26 GHz is currently raising the quest on its compatibility with the well-established Earth exploration-satellite service, which may be blinded by the spurious radiation emitted above the horizon (AtH) by base station (BS) antennas. Indeed, AtH grating lobes are often present during cell scanning due to the large interelement spacing in BS array antennas for achieving higher gains with a reduced number of RF chains. In this letter, we propose an approach based on an electrically thin metasurface-based dome for the reduction of AtH grating lobes in 5G-NR BS antennas. The proposed scanning range shifting approach exploits the natural lower amplitude of the grating lobes when the antenna array scans in an angular region closer to the broadside direction. The grating lobe reduction is here demonstrated considering a 1x4 phased linear antenna array operating under dual-liner +/- 45 degrees-slant polarization. A simple design procedure for designing the metasurface dome is reported, together with the antenna performances, evaluated through a proper set of numerical experiments. It is shown that the grating lobe radiation toward the satellite region is significantly reduced, whereas the overall insertion loss is moderate.

In this contribution, we look at some recent applications of metasurfaces and we highlight their potential in the scenario of next-generation wireless systems. We show how metasurfaces allow overcoming some limiting effects of the environment on wireless systems, and we discuss their fundamental role in the practical development of smart electromagnetic environments.

The aim of this contribution is to exploit composite vortex theory to manipulate the beam reflected by a single metasurface. In particular, by structuring the metasurface for reflecting both a vortex and a vortex-free component, its scattering pattern can be tailored according to the amplitude and phase of the two components. Full-wave numerical simulations confirm the effectiveness of the proposed approach, which could find applications for beam-shaping at different frequency ranges.

Energy confinement plays an important role in improving wave–matter interactions, enabling applications such as sensing, lasing, and filtering. One convenient solution for achieving large energy-confinement is based on embedded eigenstates, i.e., non-radiating eigenmodes supported by open structures. While the analysis of these modes is quite consolidated in electromagnetics and optics, their relevance in acoustics has been less explored, despite their wide application potential. In this work, we explore acoustic embedded scattering eigenstates within the radiation continuum enabled by resonant metasurface pairs. At resonance, each metasurface strongly reflects the input wave, but as their surface resonances interfere with each other through coupling with a longitudinal resonance, an embedded eigenstate emerges. Through both theoretical and numerical analysis, we show the conditions for such an exotic resonant state to emerge and its implication for acoustic systems.

In this contribution, we present our recent results on the study of the propagation and design of the scattering response exhibited by a temporal metamaterial, i.e. artificial material whose electromagnetic properties are suddenly and dynamically changed over time. We focus our attention on the temporal counterparts of conventional dielectric slabs and multilayered structures. The first is realized through two sudden media switching during the propagation of the wave, which enables a scattering response like the one exhibited by a conventional spatial dielectric slab. In case of more than two switchings, we realize the temporal counterpart of a multilayered structures. We demonstrate that by properly selecting the application time of each media realizing the temporal metamaterial, it is possible to design several types of optical and electromagnetic devices exploiting the temporal dimension instead of the spatial one.

In this contribution, we summarize some of our recent results about the design of dipolar and multipolar all-dielectric metasurfaces featuring advanced wave-manipulation effects. We focus our attention on both dipolar and multipolar metasurfaces and we show how it is possible to engineering their complex scattering response for designing several electromagnetic devices, such as angular filters, optical forward scatterers, high-Q passband filters and polarizers working for extreme angles of incidence.

In this contribution, we will present our investigation on design limits of the metasurfaces to be used for enabling virtual absorbing in metasurface-based virtual absorbers. To reach this goal, we analytically identify the conditions under which a properly modulated monochromatic plane wave can be virtually absorbed by the system, deriving the required metasurface values for a given cavity. The analysis is developed by modelling the system components, i.e., the metasurface reactance and the cavity input impedance, through an equivalent transmission line model, exploring the scattering zeros positions in the complex frequency plane and determining whether the metasurface-based system can support the virtual absorption or not by evaluating the time-constant from its equivalent circuit.

The aim of this contribution is to describe some of the potentialities of multipolar all-dielectric metasurfaces in tailoring the propagation and the polarization of electromagnetic waves. Specifically, we show how the complex dipolar and quadrupolar response of microwave dielectric metasurfaces can be engineered for designing high-Q band-pass filters to be applied, for instance, to self-filtering antenna systems. In addition, we also show that, despite their geometrical symmetry, the transmission window of these filtering metasurfaces is strongly polarization dependent and, as such, they can be also used as polarization-control devices working for extreme angles of incidence.

Temporal metamaterials are artificial materials whose electromagnetic properties vary over time. An abrupt change of the refractive index over time realizes the socalled temporal interface, that induces the generation of a reflected and a refracted wave. In this contribution, we investigate on the possibility to emulate temporal interfaces by acting on the boundary conditions rather than on the material properties. We demonstrate that by suddenly changing the geometrical properties of a waveguide, it is possible to have an equivalent temporal interface, without acting on the material properties. Such an approach opens the door to a simplified exploitation of the properties of temporal interfaces, relaxing the requirements of material modulation.

We provide a review of our results on the design of mantle cloaking devices for antenna applications with an emphasis on our recent investigations in designing advanced cloaking metasurfaces. It is shown how such cloaking devices allow increasing the degrees of freedom in the design of conventional antennas with a significant enhancement of their functionalities. Several examples of antennas whose conventional functionalities are enriched using cloaking metasurfaces loaded with electronic circuits are reported, and the most significant applications in view of a new generation of intelligent antennas are discussed.

Topology is a mathematical concept that has found numerous applications in physics. Among them, the topological nature of phase singularities of vortex fields has led to the definition of a new branch of optics known as singular optics [1] . One of the most intriguing results in this area is the possibility to structure almost at will the phase and amplitude distributions of an optical field, with possible applications in micromanipulation and trapping of microscopic particles [1] . This possibility is provided by the presence of phase singularity points (PSP) that are quite robust to external perturbations and involve the presence of corresponding amplitude nulls.

In this contribution, we exploit virtual perfect matching to achieve impedance matching of purely reactive loads in lossless transmission lines, by using complex frequency excitation, i.e. a signal having a specific temporal shape. The proposed system is lossless, since the load is purely reactive, energy is not dissipated, but stored in the load as long as the complex frequency excitation condition is satisfied, giving rise to an interesting singularity in the transient regime, also known as virtual absorption. Then, the accumulated energy is released by the load once the applied illumination changes.

Time-varying metamaterials and metasurfaces have unlocked a novel way for the electromagnetic waves to interact with artificial materials. In this contribution, we present an overview of the recent components and devices based on time modulation of artificial media, surfaces, and illumination signals for achieving time-domain devices, magnet-less non-reciprocity, doppler cloaking, frequency conversion, and virtual energy accumulation into open cavities, respectively.

In this contribution, we propose a quadratic-gradient metasurface dome operating in the near-field region of a dual-polarized phased array able enhancing its grating-lobe-free scanning range without affecting its other characteristics. The dome consists of an array of Huygens unit-cells exhibiting an extremely low insertion loss and a proper phase shift in transmission. The main characteristic of the proposed dome is the quadratic-gradient phase profile, which minimizes the beam divergence in broadside direction, whereas linearly increases the scanning region in off-broadside pointing angles. A proper set of numerical simulations have been performed showing the capability of the proposed dome to enhance the grating lobe free scanning region from -58° to -83°.

The term "electromagnetic cloaking" refers to a wide class of approaches whose aim is reducing the total scattering cross section of an object within a desired range of frequencies. In the last years, the use of electromagnetic cloaking for antennas have been widely investigated and several unprecedented effects have been proven. Just to cite a few examples, it has been shown that cloaking allows designing low-scattering receiving antennas [1] and overcoming the blockage and mutual coupling effects occurring between transmitting antennas in overcrowded telecommunication platforms [2] .

The aim of this Communication is to investigate and demonstrate the possibility to overcome the fundamental limitations of mantle cloaking for antennas by exploiting non-linear metasurfaces. First, we recap and give additional physical insights about the fundamental bounds that apply to the electric properties of an antenna that is made invisible at its own resonance frequency. Then, an innovative strategy is proposed to circumvent these limits through the introduction of non-linear elements that are able to dynamically transform the geometry of the cloaking metasurface depending on the power level of the impinging field. Different non-linear designs are discussed, and their effectiveness is assessed through the relevant benchmark example of a half-wavelength dipole antenna able to efficiently transmit high-power signals while being invisible to low power ones. In addition, the capability of such non-linear mantle cloaks to enable the shaping of the radiation pattern of an antenna array depending on the power level of the received/transmitted signal is demonstrated. These innovative cloaking devices may find applications in different radiating systems enabling unprecedented functionalities.