Plasmonic coupling in graphene nanoribbon dimers

Abstract

Plasmonics in graphene nanostructures has shown great promise for terahertz and mid-infrared applications. In order to achieve multiple functionalities and broad operating bandwidth, the nanostructures are usually very compact, in which plasmonic coupling is inevitable and even plays an important role in performance improvement. Here, we investigate plasmonic coupling in graphene nanoribbon (GNR) dimers theoretically through full wave simulation and coupled dipole approximation. In practice, we treat GNR dimers as a pair of Lorentz type point dipoles, whose coupling is described by a phenomenological parameter (coupling strength). We show the resonance line-shape of GNR dimers can be engineered by tuning the distance, resonance frequency difference, damping rates of two GNRs. However, in both horizontally aligned dimer (HD) and vertically aligned dimer (VD), the coupling strength only depends on the distance, and exhibits an inverse relation of cube root, but due to different symmetries, they are opposite in sign. When two GNRs are placed with an increasing oblique angle, the coupling strength varies continuously from the value of HD to that of VD, and satisfies a sinusoidal function of the angle. We further discuss a general scheme to engineer broadband optical response through tuning damping rates of two GNRs, namely, larger damping rate for lower frequency resonance in HD, while on the contrary in VD. Our results provide a basic understanding of plasmonic coupling in graphene nanostructures, and will stimulate graphene plasmonic applications at terahertz and mid-infrared frequency range.