Light emission from the gap cavity formed by the tip of a scanning tunneling microscope (STM) and a flat metallic sample allows to probe the dielectric response of metals at the atomic scale. The underlying phenomenon is confirmed by a full-wave optical simulations of the tip-substrate system using JCMsuite.
K. Edelmann, et al. Influence of Co bilayers and trilayers on the plasmon-driven light emission from Cu(111) in a scanning tunneling microscope. Phys. Rev. B 101, 205405 (2020).