LAUSR

Plasmonic metallic tapers with conical shapes are one of the most common and simple structures, with concomitant capabilities of nanofocusing and strong field enhancement. Such tapers can serve either as waveguides or nanoantennas in near-field scanning optical microscopy and as a ultrafast photoemission point source in electron microscopy [1]. Electron energy-loss spectroscopy (EELS) is exploited to comprehensively study the plasmonic response of mesoscopic tapers in a wide energy range. However, the interpretation of EELS signatures is intricate by the fact that different resonance mechanisms are active when relativistic electrons interact with mesoscopic plasmonic tapers, namely phase matching [2] and reflection [3] (Fig.1a). It is of fundamental importance to clarify the reasons behind the EELS resonances explicitly.