Electric Dipole-Magnetic Quadrupole Resonant Coupling for Broadband Operating Metasurfaces

In this work, we have applied the multipole expansion method to analyze the transmission/reflection properties of all-metallic metasurfaces operating in the microwave regime. This analytical technique provides enhanced physical insights into the electromagnetic properties of these metasurfaces. We show that the sharp electric dipole (ED) resonance in a metasurface composed of metallic squares can be resonantly coupled with the sharp magnetic quadrupole (MQ) resonance from a metasurface made of metallic circles. This coupling yields a significantly broadened operating band for the metasurface. Specifically, the metallic-square-based metasurface operates under a fractional bandwidth of around 61%, while the operational bandwidth of a metallic-circle-based metasurface is ca. 22%. In contrast, the resonantly coupled ED-MQ metasurface exhibits a fractional bandwidth of approximately 96% operating at the central frequency 5.7 GHz. The feasibility of the design was proved by fabricating metasurfaces over an FR4 substrate, whose electromagnetic properties were as predicted qualitatively using the analytical multipole expansion method.