Plasmonic response of STM tips

We present theoretical studies of the plasmonic response of STM tips near substrates, with emphasis on the influence of tip shape and topology on its plasmonic resonances. The calculations employ COMSOL software. We find, for tips made of Ag, a plasmonic resonance in the 600-700 nm wavelength regime, which is well below the spectral range where one encounters surface and tip plasmons in electrostatic theory for Ag based structures. A similar low-energy resonance is found for Au tips. We present calculations of light emission spectra, wherein the light emission is stimulated by the noise necessarily present on a dc tunneling current. The calculated spectra are similar to those observed in recent experiments. We also discuss excitation of the low-frequency resonance by means of an external laser beam, and for both laser excitation and excitation by current noise we explore the relative strength of the low-frequency resonance and the classical tip/substrate plasmons in the 3.5 eV spectral range. The results suggest future experimental studies.