Coding Metasurfaces for Diffuse Scattering: Scaling Laws, Bounds, and Suboptimal Design

Abstract

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...

Error Analysis of Programmable Metasurfaces for Beam Steering

“…Among the monotonically increasing cases, clustered distributions seem to have a higher impact in this metric, whereas independent and out of state errors are the least relevant. Since random coding [51] leads to random scattering, we argue that uncorrelated random errors generate scattering that does not accumulate as a large secondary lobe. On the other hand, clustered and deterministic errors, which tend to group unit cells together and to apply a uniform state, lead to large secondary lobes.…”

Effects of deterministic disorder at deeply subwavelength scales in multilayered dielectric metamaterials

“…Accordingly, alternative terms such as "flat-spectrum aperiodic order" or "pseudo-randomness" are also utilized in the topical literature to define this type of geometries. So far, in electromagnetics and optical engineering, GRS sequences have been explored in connection with antenna arrays [27], spread-spectrum communications and encryption [28], diffuse scattering [29,30], nanoplasmonic arrays [31][32][33], and photonic crystals [34][35][36][37][38][39].…”

Machine‐Learning Designs of Anisotropic Digital Coding Metasurfaces

“…[1] In recent years, a wide variety of metasurfaces have been presented to design radiation and scattering electromagnetic DOI: 10.1002/adts.201800132 (EM) beams by arranging gradient variation phases of elements, such as the anomalous reflections and refractions governed by the generalized Snell's law, [2][3][4] transformations between spatial modes and surface modes, [5] photonic spin Hall effect, [6][7][8] reflect/transmit arrays with multiple functions, [9][10][11] and optical-vortex generations. [14][15][16][17][18][19][20][21][22][23][24][25][26][27] There are many forms of coding elements for coding metasurfaces, such as relative phase information with fixed phase difference, relative amplitude information with fixed amplitude difference, and relative time information with fixed time difference for providing more convenience and methods. [14][15][16][17][18][19][20][21][22][23][24][25][26][27] There are many forms of coding elements for coding metasurfaces, such as relative phase information with fixed phase difference, relative amplitude information with fixed amplitude difference, and relative time information with fixed time difference for providing more convenience and methods.…”