Multifold coupling enabled high quality factor toroidal resonances in metasurfaces

Abstract

The phenomena of toroidal resonances can lead to extraordinary strong confinement of electromagnetic energy inside a physical system. Very recently, toroidal resonances were observed in metamaterial systems too. Because of strong electromagnetic field confinements, associated radiative losses in toroidal modes can be tailored to extremely low values leading to simultaneous high quality (Q) factor and high figure of merit (FoM) resonances. Such metamaterial systems can be attractive for sensors, lasing spasers, nonlinear devices, narrow band filters, etc. However, demonstrating toroidal modes with high Q factors is a challenging task in a typical metamaterial system. In this work, we have experimentally demonstrated a strategy for achieving toroidal resonances with high Q factor and FoM in single resonator based planar metamaterials. At resonance, magnetic dipoles akin to toroidal topology are excited in these structures. The anti-aligned circulating surface currents are observed along the circumference of metal split rings which resulted in a head to tail arrangement of the magnetic dipoles across the unit cells. We attribute the observation of high-quality factor toroidal modes to the strong inter-unit cell near field coupling of the induced magnetic field lines. Such multifold toroidal mode leads to resonances with exceptionally high quality factors because of significant reduction in radiation losses. Our experiments demonstrate Q factor and FoM of 183 and 76, respectively, making these metasurfaces a strong candidate for practical applications where high Q resonances are crucial.