FSS-embedded substrates: a facile method of augmenting functions of metasurfaces

Metasurfaces, usually composed of 2D arrays of sub-wavelength metallic patterns (also called meta-atoms), have provided unprecedented freedoms for manipulating electromagnetic (EM) waves. In practice, meta-atoms are usually patterned on a certain substrate, so the EM properties of substrates directly affect the performances of metasurfaces. Therefore, it is desirable that the substrates can also be engineered with a high degree of freedom (DOF) so as to augment the functions of the metasurfaces. With this in mind, in this paper, we propose a method of designing functional metasurfaces by engineering the EM properties of substrates. By embedding frequency selective surface (FSS) within dielectric substrates, the electrical thickness of the substrates can be tailored according to the specific operating frequencies. In this way, functions of the metasurfaces can be augmented, either by better performances or by additional functions. As an example, we designed a multi-functional metasurface and applied it to a low-RCS reflector antenna. A circular patch was inserted into the dielectric substrate beneath the meta-atom. Due to the band-stop property of the metallic patch, two polarization conversion (PC) bands, together with a polarization-keeping band in between them, were created. Using this meta-atom, the metasurface is further designed, which can simultaneously achieve in-band focusing and out-of-band PC. A prototype was fabricated and measured. Both the simulated and measured results verified the good performance. Empowered by the metasurface, the reflector antenna achieved high radiation gain in band and significant RCS reduction out of band. This work provides a facile method of designing multi-functional metasurfaces and can be readily extended to terahertz, optical and other frequency regimes.