  Work function-mediated charge transfer in graphene/$\alpha$-RuCl$_3$
heterostructures has been proposed as a strategy for generating highly-doped 2D
interfaces. In this geometry, graphene should become sufficiently doped to host
surface and edge plasmon-polaritons (SPPs and EPPs, respectively).
Characterization of the SPP and EPP behavior as a function of frequency and
temperature can be used to simultaneously probe the magnitude of interlayer
charge transfer while extracting the optical response of the interfacial doped
$\alpha$-RuCl$_3$. We accomplish this using scanning near-field optical
microscopy (SNOM) in conjunction with first-principles DFT calculations. This
reveals massive interlayer charge transfer (2.7 $\times$ 10$^{13}$ cm$^{-2}$)
and enhanced optical conductivity in $\alpha$-RuCl$_3$ as a result of
significant electron doping. Our results provide a general strategy for
generating highly-doped plasmonic interfaces in the 2D limit in a scanning
probe-accessible geometry without need of an electrostatic gate.
