Digital Metasurface Based on Graphene: An Application to Beam Steering in Terahertz Plasmonic Antennas

Abstract

Metasurfaces, the two-dimensional counterpart of metamaterials, have caught great attention thanks to their powerful capabilities on manipulation of electromagnetic waves. Recent times have seen the emergence of a variety of metasurfaces exhibiting not only countless functionalities, but also a reconfigurable response. Additionally, digital or coding metasurfaces have revolutionized the field by describing the device as a matrix of discrete building block states, thus drawing clear parallelisms with information theory and opening new ways to model, compose, and (re)program advanced metasurfaces. This paper joins the reconfigurable and digital approaches, and presents a metasurface that leverages the tunability of graphene to perform beam steering at terahertz frequencies. A comprehensive design methodology is presented encompassing technological, unit cell design, digital metamaterial synthesis, and programmability aspects. By setting up and dynamically adjusting a phase gradient along the metasurface plane, the resulting device achieves beam steering at all practical directions. The proposed design is studied through analytical models and validated numerically, showing beam widths and steering errors well below 10 o and 5% in most cases. Finally, design guidelines are extracted through a scalability analysis involving the metasurface size and number of unit cell states.

Error Analysis of Programmable Metasurfaces for Beam Steering

“…Determining the characteristics of the unit cell that will lead to the desired behavior is typically performed analytically, through methods such as impedance matching [22]. A large subset of designs has been based on the generalization of the Snell's laws of reflection and refraction [6], [50]- [52], which provides fundamental understanding on how to achieve certain functionalities through the drawing of specific phase gradients. When analytical methods are not practical, computational optimization methods [53], [54] or even machine learning approaches can be employed [55]- [57].…”

“…As such, the metasurface can be accurately modeled as a compact array following the Huygens principle [68]. This method has been validated in several works via extensive simulations [52]. Considering each unit cell as an element of the array, the far field of the metasurface can be obtained as…”

Scalability Analysis of Programmable Metasurfaces for Beam Steering

“…The subwavelength granularity of these unit cells confers MSs with exceptional control of electromagnetic (EM) waves as demonstrated in a variety of works [22]- [36]. The actual response of the MS is derived from the aggregated response of all unit cells, which need to be modified individually.…”

An Overview of Signal Processing Techniques for Terahertz Communications

“…Furthermore, graphene-based metasurfaces can control the chemical potential of reflecting elements via electrostatic biasing, which varies the complex conductivity to achieve phase control [293]. Graphene-based digital metasurfaces combining both reconfigurable and digital approaches are studied in [294], where beamsteering is achieved by dynamically adjusting a phase gradient along the metasurface plane. A graphene-based metasurface is also proposed in [295], where a two-dimensional periodic array of graphene meta-atoms is shown to guarantee a wideband perfect-absorption polarization-insensitive reconfigurable behaviour at THz frequencies.…”