Charge Transfer\nPlasmonics in Bespoke Graphene/α-RuCl<sub>3</sub> Cavities

Surface plasmon polaritons (SPPs) provide a window into\nthe nano-optical,\nelectrodynamic response of their host material and its dielectric\nenvironment. Graphene/α-RuCl<sub>3</sub> serves as an ideal\nmodel system for imaging SPPs since the large work function difference\nbetween these two layers facilitates charge transfer that hole dopes\ngraphene with <i>n</i> ∼ 10<sup>13</sup> cm<sup>–2</sup> free carriers. In this work, we study the emergent THz response\nof graphene/α-RuCl<sub>3</sub> heterostructures using our home-built\ncryogenic scanning near-field optical microscope. Using phase-resolved\nimaging, we clearly observe long wavelength, heavily damped THz SPPs\nin a series of variable-size graphene cavities. From this, we extract\nthe plasmonic wavelength and scattering rate in the graphene/α-RuCl<sub>3</sub> heterostructures. We determine that the measured plasmon\nwavelength and electronic scattering rate match our heterostructures’\ntheoretically predicted values. Our results demonstrate that shaping\ngraphene into bespoke cavity structures enables observation and quantification\nof SPPs in heavily doped graphene that are largely not addressable\nwith other experimental techniques. Moreover, the manifest lack of\nmetallicity observed in the adjacent doped α-RuCl<sub>3</sub> layer provides significant constraints on the nature of the interfacial\ncharge transfer in this 2D heterostructure.