Probing the radiative electromagnetic local density of states in nanostructures with a scanning tunneling microscope

A novel technique for the investigation of the radiative contribution to the electromagnetic local density of states is presented. The inelastic tunneling current from a scanning tunneling microscope (STM) is used to locally and electrically excite the plasmonic modes of a triangular gold platelet. The radiative decay of these modes is detected through the transparent substrate in the far field. Emission spectra, which depend on the position of the STM excitation, as well as energy-filtered emission maps for particular spectral windows are acquired using this technique. The STM-nanosource spectroscopy and microscopy results are compared to those obtained from spatially resolved electron energy loss spectroscopy (EELS) maps on similar platelets. While EELS is known to be related to the total projected electromagnetic local density of states, the light emission from the STM-nanosource is shown here to select the radiative contribution. Full electromagnetic calculations are carried out to explain the experimental STM data and provide valuable insight into the radiative nature of the different contributions of the breathing and edge plasmon modes of the nanoparticles. Our results introduce the STM-nanosource as a tool to investigate and engineer light emission at the nanoscale.S.C. acknowledges the financial support of the China Scholarship Council (CSC; No. 201304910386). M.Z.-H. acknowledges the financial support of the European Union under the Project H2020 FETOPEN-2016-2017 “FEMTOTERABYTE” (Project No. 737093) and the “Colombian Administrative Department of Science, Technology and Innovation” - COLCIENCIAS under its “Estancias Postdoctorales 784-2017” call. He also acknowledges the hospitality of the Institute des Sciences Moleculaires d’Orsay and Dr. Nuno de Sousa for his guidance and support using COMSOL Multiphysics. S.M. was supported by the HAPPLE grant (French National Research Agency: ANR-12-BS10-0016). This work is partially funded by the Conseil Regional, Ile-deFrance (DIM Nano-K). J.A. acknowledges Project FIS2016-80174-P of the Spanish MICINN and Project IT1164-19 of the Basque Government.Peer reviewe