and/or polarization) via the engineering of metallic or dielectric resonating elements suitably arranged on a 2D surface. Indeed, their inherent 2D character has played a major catalyzing role, by considerably simplifying the fabrication process, as opposed to "bulk" 3D metamaterials. [4] The reader is referred to refs. [5-9] (and references therein) for recent reviews on the modeling, design, and attainable physical effects, as well as the abundant applications, ranging from wavefront shaping and beam-forming to chemical and biological sensing.Of specific interest for the present study is the concept of "coding and digital" metasurfaces, recently put forward by Cui et al. [10] (see also ref. [11] for an analogous concept applied to bulk metamaterials), which relies on the exploitation of a limited number of element-types (unit cells). In its simplest form, only two elementtypes (labeled as "1" and "0") are employed, so that the metasurface design can be effectively associated with a 2D binary coding. This can be viewed, in a sense, as an evolution of the "checkerboard-surface" concept originally conceived by Paquay et al. [12] As implied by the name, the basic idea underlying a checkerboard surface is to alternate two types of unit cells (e.g., metallic and artificial-magnetic-conducting, at microwave frequencies) characterized by out-of-phase reflection coefficients, so as to suppress the specular reflection in view of the inherent cancellation effects. With suitable extensions and modifications of the unit cells as well as the spatial arrangement, this basic concept has been exploited in several subsequent studies [13][14][15][16][17][18] in order to attain broadband and wide-angle reduction of the radar cross-section (RCS) of planar surfaces.Within this framework, the digital-metasurface concept [10] introduces further levels of sophistication. First, the spatial arrangement (described by a coding) of the unit cells is far more general and flexible. Further versatility can be introduced by employing more than two unit cells, corresponding to multibit coding. Most important, by exploiting reconfigurable unit cells (whose response can be switched, e.g., by means of a biased diode or a microelectromechanical system), the coding is no longer irreversibly bound to the structure design, but can be controlled, e.g., via a field-programmable gate array. To date, this represents one of the first working examples of a programmable metamaterial platform for field manipulation and Coding metasurfaces, based on the combination of two basic unit cells with out-of-phase responses, have been the subject of many recent studies aimed at achieving diffuse scattering, with potential applications to diverse fields ranging from radar-signature control to computational imaging. Here, via a theoretical study of the relevant scaling-laws, the physical mechanism underlying the scattering-signature reduction is elucidated, and some absolute and realistic bounds are analytically derived. Moreover, a simple, deterministic suboptimal desi...

Spatial tailoring of the material constitutive properties is a well-known strategy to mold the local flow of given observables in different physical domains. Coordinate-transformation-based methods (e.g., transformation optics) offer a powerful and systematic approach to design anisotropic, spatially-inhomogeneous artificial materials ("metamaterials") capable of precisely manipulating wave-based (electromagnetic, acoustic, elastic) as well as diffusion-based (heat) phenomena in a desired fashion. However versatile these approaches have been, most designs have so far been limited to serving single-target functionalities in a given physical domain. Here we present a step towards a "transformation multiphysics" framework that allows independent and simultaneous manipulation of multiple physical phenomena. As a proof of principle of this new scheme, we design and synthesize (in terms of realistic material constituents) a metamaterial shell that simultaneously behaves as a thermal concentrator and an electrical "invisibility cloak".Our numerical results open up intriguing possibilities in the largely unexplored phase space of multi-functional metadevices, with a wide variety of potential applications to electrical, magnetic, acoustic, and thermal scenarios.

Digital programmable metasurfaces provide a very powerful and versatile platform for implementing spatio‐temporal modulation schemes that are of great interest within the emerging framework of space–time metastructures. In particular, space–time‐coding digital metasurfaces have been successfully applied to advanced wavefront‐manipulations in both the spatial and spectral domains. However, conventional space–time‐coding schemes do not allow the joint syntheses of the transmission/scattering angular responses at multiple frequencies, which are potentially useful in a variety of applications of practical interest. Here, a strategy is put forward to lift this limitation, thereby enabling joint multi‐frequency beam shaping and steering, that is, the independent and simultaneous syntheses of prescribed scattering patterns at given harmonic frequencies. The proposed approach relies on a more sophisticated space–time coding, with suitably designed, and temporally intertwined coding sub‐sequences, which effectively disentangles the joint multi‐frequency syntheses. The power and versatility of the approach are illustrated via a series of representative application examples, including multi‐beam, diffuse‐scattering, and orbital‐angular‐momentum patterns. Theoretical predictions are experimentally validated by means of microwave measurements. The outcomes of this study hold promising potentials for applications to future imaging, information, and mobile‐communication systems.

Planar junctions between reactive surface impedances with dual character (capacitive/inductive) can sustain line waves localized both in-plane and out-of-plane around the discontinuity, which propagate unattenuated along one-dimensional paths. Due to their attractive properties, these waves are of potential interest in applications ranging from integrated photonics to optical sensing. Here, we introduce and explore a non-Hermitian platform supporting these exotic modes based on parity−time symmetry. Specifically, we show that line waves can occur in the presence of a uniform surface reactance (either capacitive or inductive) and a symmetric resistance discontinuity from negative to positive values (i.e., gain and loss). We study analytically and numerically the propagation properties of these waves and the underlying physical mechanisms and also illustrate their intriguing properties in terms of confinement, reconfigurability, spin-momentum locking and lasing. Finally, we address possible practical implementations based on photoexcited graphene. Our results hold intriguing potentials for applications in flat optics and reconfigurable photonics.

Coding metasurfaces, composed of only two types of elements arranged according to a binary code, are attracting a steadily increasing interest in many application scenarios. In this study, we apply this concept to attain diffuse scattering at THz frequencies. Building up on previously derived theoretical results, we carry out a suboptimal metasurface design based on a simple, deterministic and computationally inexpensive algorithm that can be applied to arbitrarily large structures. For experimental validation, we fabricate and characterize three prototypes working at 1 THz, which, in accordance with numerical predictions, exhibit significant reductions of the radar cross-section, with reasonably good frequency and angular stability. Besides the radar-signature control, our results may also find potentially interesting applications to diffusive imaging, computational imaging, and (scaled to optical wavelengths) photovoltaics.