Digital coding metasurfaces are aimed at simplifying the design and optimization procedures, and manipulating electromagnetic waves in digital manner. In this paper, a multilayered anisotropic coding metasurface is designed to realize multiple independent functionalities by changing the polarization and direction of incident waves. As a proof of concept, the beam deflection, diffuse scattering, and vortex beam generation are realized by using only a single transmission‐reflection‐integrated (TRI) coding metasurface. This design can achieve three different functionalities and simultaneous controls of transmitted and reflected wavefronts on a shared aperture with the TRI coding scheme. Both numerical and measured results verify the excellent performance of the multifunctional digital coding metasurface, which provides a simple way to extend the functionality of high‐efficiency metadevices.
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...
The programmable and digital metamaterials or metasurfaces presented recently have huge potentials in designing real-time-controlled electromagnetic devices. Here, we propose the first transmission-type 2-bit programmable coding metasurface for single-sensor and single- frequency imaging in the microwave frequency. Compared with the existing single-sensor imagers composed of active spatial modulators with their units controlled independently, we introduce randomly programmable metasurface to transform the masks of modulators, in which their rows and columns are controlled simultaneously so that the complexity and cost of the imaging system can be reduced drastically. Different from the single-sensor approach using the frequency agility, the proposed imaging system makes use of variable modulators under single frequency, which can avoid the object dispersion. In order to realize the transmission-type 2-bit programmable metasurface, we propose a two-layer binary coding unit, which is convenient for changing the voltages in rows and columns to switch the diodes in the top and bottom layers, respectively. In our imaging measurements, we generate the random codes by computer to achieve different transmission patterns, which can support enough multiple modes to solve the inverse-scattering problem in the single-sensor imaging. Simple experimental results are presented in the microwave frequency, validating our new single-sensor and single-frequency imaging system.
Recently, 2D versions of metamaterials, metasurfaces, have attracted more attention, [11] due to their advantages of low cost, low profile, and strong abilities to manipulate spatial and surface waves. Novel generalized sheet transition condition method [12] and transverse resonance method [13] were first presented to analyze the EM performance of metasurfaces. Then generalized Snell's law [14] was proposed to introduce the concept of abrupt phase when designing metasurfaces. By changing the size, shape, or orientation of unit cells, the abrupt phase provided by the metasurface can be tailored accordingly, and the outgoing EM waves are engineered arbitrarily. Metasurfaces have offered more convenience and freedom for manipulating EM wavefronts, and have been widely applied in the microwave, [15][16][17][18][19][20] terahertz, [21][22][23][24] visible, [25][26][27][28] and even acoustic [29,30] frequencies.Metamaterials and metasurfaces described by continuously effective medium parameters and phase distributions have powerful capabilities in controlling EM waves, but in static ways. That is to say, once a metamaterial or metasurface is fabricated, its function will be fixed. In order to reach real-time controls to EM waves, digital coding characterization has been proposed to describe metamaterial, resulting in the concepts of coding, digital, and programmable metamaterials. [31] The binary 1-bit digital codes "0" and "1" are adopted to indicate the reflection phases of 0° and 180°, from which one can manipulate EM waves using different coding sequences. The digital codes have been extended to 2-bit and more to bring more freedom for controlling scattering beams. The digital states "0." "1," "2," and "3" represent the reflection phases of 0°, 90°, 180°, and 270°, respectively.By designing a unit cell controllable by a diode to achieve either "0" or "1" state, the digital and programmable metamaterials have been realized to reach real-time manipulations to EM waves. [31] The digital coding representation links the traditional metamaterials to information theory, giving us an opportunity to control EM performance through discrete digital states. Based on these concepts, many kinds of functions such as beam steering [31][32][33] and reduction of radar crosssections [34] have been achieved by switching coding sequences on coding metamaterials in microwave and terahertz regions. Recently, the concept of anisotropic coding metamaterials has been demonstrated, which can achieve two independent coding behaviors for different polarizations. [35] Furthermore, convolution operations on coding metasurfaces were presented to Coding representation of metamaterials builds up a bridge between the physical world and the digital world, making it possible to manipulate electromagnetic (EM) waves by digital coding sequences and reach field-programmable metamaterials. Here, the coding space is extended to complex domain and proposed complex digital codes to provide closer essence of EM-wave propagation. Based on the analytic geometr...
Digital coding metasurfaces have attracted attention due to their advantages compared to metamaterials. Many devices have been proposed by encoding the phase responses on metasurfaces such as orbit angular momentum generation, prefect absorbers, and holography. For complete manipulation for propagation of electromagnetic waves, it would be beneficial to control both phase and amplitude responses. Here, we propose a metasurface with independent control of phase and amplitude profiles, which is composed of four unit cells, and the amplitude responses of all unit cells range from 0.3 to 0.7. The four units have phase responses of 0, π/2, π, and 3π/2 separately to mimic the “00,” “01,” “10,” and “11” digital elements. The direction of the reflected beam from the metasurface can be manipulated by different sequences of digital elements, and meanwhile, the intensity of the reflected beam can be modulated through changing different amplitude responses in the same direction. We show that the distributions of both phase and amplitude responses of the metasurface will contribute to scattering reduction. We have used indium tin oxide to design the patterns of four units. The experiments and simulations confirm the physical phenomena mentioned above. The separate control of phase and amplitude responses suggests potential applications in high quality holography and mathematical operations of metasurfaces.
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