Magnetoplasmonic coupling in graphene nanodisk dimers: An extended coupled-dipole model for circularly polarized states

Abstract

Plasmonic coupling is one of the most important effects in compact plasmonic systems, and it has been studied intensively. In contrast, magnetoplasmonic coupling (MPC) is rarely mentioned, even in graphene nanostructures supporting the strong magneto-optic effect. Here, we theoretically investigate MPC in graphene nanodisk dimers in the presence of either parallel (case I) or antiparallel (case II) magnetic fields. We find the hybridized modes always appear for two states with same chirality, while their excitations depend on incident polarization. Moreover, two antisymmetric modes are dark in case I, but all four modes are bright in case II. To provide better insight, an extended coupled-dipole model is presented, in which the fundamental circularly polarized magnetoplasmons are decomposed into two linear and orthogonal dipoles, with a $\ensuremath{\pi}/2$ phase difference, and then the coupling is described by two linear dipoles along the two orthogonal directions separately. The parameters for the magneto-optic effect and coupling strengths are independent and can be easily extracted from their individual simulations. The eigenvalues and wave functions obtained from the model can describe well the resonance frequency and excitation strength of each hybridized mode. We finally discuss MPC in touching graphene nanodisks, where the charge transfer plasmon is immune to the magneto-optic effect, and in case II, a circular resonance state will be replaced by a linear one with the incident electric field along the touching direction. In this paper, we provide a general framework for investigating mode coupling of two circular states and pave the way to magneto-optic and plasmonic applications.