Finite-Element Method Simulations of the Tunable Magnetic Anapole State in an All-Graphene Metasurface: Implications for Near-Field Sensing, Nanoscale Optical Trapping, and Cloaking

In this article, we have proposed an all-graphene metasurface where the magnetic anapole state can be dynamically tuned. The structure is composed of two layers of graphene separated by a dielectric spacer on an oxide substrate, forming a gap surface plasmons resonator structure in the long-wavelength infrared region. The top graphene layer is patterned into interconnected disks, while the interconnection is for electrical tuning purposes only. By multipolar analysis using the finite-element method (FEM) simulation, we have shown that at the particular dimension, the dominant electric dipole scattering can be minimized. This happens at the resonant point where along with the electric dipole suppression, the magnetic and magnetic toroidal dipoles destructively interfere, which gives rise to a magnetic anapole state. This resonant point can be tuned in two ways, either passively, which is the lithographical variation of structure parameters, or actively, by harnessing the chemical potential of the graphene. We have shown here both mechanisms whereby actively tuning we can shift the magnetic anapole to the far-infrared region. Through the interplay of multipole analysis, we have shown that by varying the angle of incidence, we can control the radiating channels, which can be termed an anapole switch. For the active tuning cases, by voltage mapping of the chemical potential, we have shown the effect of chemical potential on the components of the multipolar families. Our device concept and such implementations hold promise for the application in near-field sensing, nanoscale optical trapping, cloaking, etc.