THz nano-imaging of long wavelength graphene plasmons

Propagating plasmon polaritons in gate tunable graphene devices have been extensively studied throughout the mid-IR frequency range using scanning near-field optical microscopy (SNOM). Recent works have shown that graphene can be doped to exceedingly high carrier concentrations when in contact with certain materials via a charge transfer process (CT). A candidate quantum spin liquid material, a-RuCl₃ is a prime example of this work function mediated CT, hole doping graphene with ~10¹³ cm^-2 carriers. In this work, we study graphene/a-RuCl₃ heterostructures with THz light using our home-built, cryogenic THz-SNOM. Using phase resolved imaging techniques, we can clearly observe long wavelength, heavily damped THz plasmon polaritons. These observations allow us to extract the plasmonic wavelength, λ, and quality factor, Q, in graphene/a-RuCl₃ heterostructures, which together fully define the complex conductivity of the heterostructures in the THz range. We find that the plasmonic wavelength matches well with expected values for n ~10¹³ cm^-2, while the plasmonic scattering rate is almost four times larger than in pristine graphene. Thus, we can conclude that the dielectric environment of the samples leads to much higher plasmonic damping. One possible interpretation of this result is that the a-RuCl₃ is doped sufficiently, via the same CT process, to support free carriers that screen the plasmonic response in graphene.