Charge Transfer Plasmonics in Bespoke Graphene/α-RuCl₃ Cavities

Surface plasmon polaritons (SPPs) provide a window into the nano-optical, electrodynamic response of their host material and its dielectric environment. Graphene/α-RuCl₃ serves as an ideal model system for imaging SPPs since the large work function difference between these two layers facilitates charge transfer that hole dopes graphene with n ∼ 10¹³ cm^-2 free carriers. In this work, we study the emergent THz response of graphene/α-RuCl₃ heterostructures using our home-built cryogenic scanning near-field optical microscope. Using phase-resolved imaging, we clearly observe long wavelength, heavily damped THz SPPs in a series of variable-size graphene cavities. From this, we extract the plasmonic wavelength and scattering rate in the graphene/α-RuCl₃ heterostructures. We determine that the measured plasmon wavelength and electronic scattering rate match our heterostructures' theoretically predicted values. Our results demonstrate that shaping graphene into bespoke cavity structures enables observation and quantification of SPPs in heavily doped graphene that are largely not addressable with other experimental techniques. Moreover, the manifest lack of metallicity observed in the adjacent doped α-RuCl₃ layer provides significant constraints on the nature of the interfacial charge transfer in this 2D heterostructure.