Real-space imaging of plasmon polaritons in quasi-freestanding gold monolayers

 

Gold is a model plasmonic metal with well-established optical, electronic, and chemical properties. Its behaviour at the monolayer limit, however, remains largely unexplored due to challenges in fabrication and characterisation. Here, we present an optical study of a stable, quasi-freestanding gold monolayer (ML-Au) [1], formed by intercalating gold atoms between graphene and a silicon carbide substrate [2]. Using scattering-type scanning near-field optical microscopy (s-SNOM) [3], we observe propagating plasmon polaritons in ML-Au, hybrid modes of light and collective electron oscillations, and determine their dispersion via polariton interferometry. In this approach, the near-field at the tip apex locally excites the modes, which propagate, reflect at edges, and interfere with the local field, producing spatial fringes in the measured signal. Analysis of these fringes provides direct access to the polariton dispersion. By modelling the dispersion with a two-dimensional Drude conductivity, we extract key transport parameters, including the Drude weight and the carrier scattering time. We find that the scattering time is comparable to that of bulk gold, while the Drude weight is nearly twice the bulk expectation, indicating a modified electronic response in the monolayer. Density functional theory calculations support the enhancement of the Drude weight. These results provide direct insights into the transport and optical properties of monolayer gold and establish it as a promising platform for fundamental studies and for future optoelectronic applications. A. Bylinkin et al. arXiv preprint, 2603.05030 (2026) S. Forti et al., Nat. Commun.  11, 2236 (2020) R. Hillenbrand et al., Nat. Rev. Matter. 10, 285 (2025)