HOLeYMETA

A novel class of low-profile metasurface lenses, “Holey Metasurfaces,” has been recently proposed in the scientific literature by the principal investigator and a colleague who is a partner of the present ANR. This project is devoted to the investigation of holey metasurfaces by developing a new analytic/numerical approach for their modeling and studying in detail their performances for the subsequent design of prototypes aimed at different applications. These new graded-index lenses are obtained by modulating an effective refractive index through a smooth size variation of periodic holes in a metallic plate. Thanks to specific periodic topologies and to the presence of two interacting surfaces, a dispersionless frequency behavior of the effective refractive index is achieved; as a result, these lenses are able for the first time to provide beam scanning in an ultra-wide band (UWB) frequency range without a significant degradation of the radiated beam. Furthermore, since in these new structures energy propagation occurs in the air, rather than in a dielectric as usual, resulting devices can exhibit low losses and high-power capabilities. All these features are of paramount importance to develop a new generation of devices for the connectivity of smart cities in the next 5G standard. The 5G connection needs can be met once a better and wider occupation of bandwidth, together with a reduced energy usage, is achieved. According to current researches in this subject, the required bandwidth demand can only be satisfied at millimeter waves (namely, preliminary studies confirm the feasibility of 28 and 38 GHz urban connections). While progresses have been done on channel characterization and on data processing strategies, at the moment no advantageous solutions are available for low-cost, light-weight and low-profile mm-wave antennas granting an UWB behavior. The new concept of “Holey Metasurfaces” described in this ANR proposal is at the moment the only answer to all these needs. The activities will be performed on two main topics: i) analytical/numerical modeling, ii) study and design of holey metasurfaces. Modeling activities will be aimed to develop of equivalent-circuit model for the single cell, dependent on the hole shape and size. The circuit parameters will be characterized with analytic formulas (in presence of holes of canonical shapes) or with numerical expressions. The complete holey metasurface will be then replaced by an array of equivalent circuits, whose parameters will be accordingly modulated. The full-wave electromagnetic problem will then be transformed into a much less demanding circuit problem. The presence of a concentrated source will be modeled through a rigorous approach (the "array scanning method"). The study of performances, potential and limitations of holey metasurfaces will be conducted with specific reference to the ultra-bandwidth behavior detected in the preliminary work, to the impact of conductor losses and of misalignment between plates, which is an important condition to maximize the available bandwidth. Different transformation-optics approaches (requiring either tensorial or isotropic metasurfaces) will be adopted to provide an arbitrary radiated field. These studies will be oriented to the subsequent design of different prototypes, each one aimed at a specific application, such as UWB, low-loss, wide bandgap, arbitrary patterns. A preliminary study will be performed on 2D beam switching by means of a stacked structure. The results of this project will be fundamental to establish a new paradigm for lens antennas at millimeter waves, having the potential to spread worldwide in the framework of next-generation 5G urban connections. Further European projects will be aimed to address other issues necessary for a commercialization and mass production: circular polarization, dual frequency operations, optimized 2D scanning, optimization of different kinds of feeds.