Spectral response and mapping of plasmonic resonances in nanometric-thick gold nanotriangles using a scanning tunneling microscope

The optical response and mapping of the electromagnetic Local Density of States (EMLDOS) of single gold triangle nano-platelets (GTNP) is studied using a scanning tunneling microscope (STM). The GTNP are chemically-synthesized and deposited on a 100nm-thick ITO layer on a glass cover-slip. As the STM tip scans the GTNP, the inelastic tunnel current (ITC) in the biased tip-sample cavity locally excites the sample and the emitted light is collected to generate a position-dependent intensity map. We theoretically describe this process by describing the ITC of the junction as an external point dipole located above the platelet which induces the excitation of its different plasmonic modes. By solving Maxwell's equations under the dipolar junction excitation, dark, bright, edge and breathing plasmonic modes are identified in the energy-filtered plasmonic maps, as well as in the spectral response of the light emission. By comparing theory and experiment, we attempt to demonstrate the effective excitation of edge plasmonic modes and the possibility of using photon mapping with the STM as a characterization tool of dark-modes hardly excitable by optical means. Our goal is to establish electrically driven light emission as an alternative way to access dark modes at the nanoscale.