Decoupling absorption and emission processes in super-resolution localisation of emitters in a plasmonic hotspot

Abstract

The absorption process of an emitter close to a plasmonic antenna is enhanced due to strong local electromagnetic (EM) fields. The emission process, if resonant with the plasmonic system, re-radiates to the far-field by coupling with the antenna due to the availability of plasmonic states. This increases the local density of states (LDOS), effectively providing more, or alternate, pathways for emission. Through the mapping of localized emission events from single molecules close to plasmonic antennas – performed using far-field data – one gains combined information on both the local EM field strength and the LDOS available. The localization from these emission-coupled events generally do not, therefore, report the real position of the molecules, nor the EM enhancement distribution at the illuminating wavelength. Here we propose the use of a fluorescent molecule with a large Stokes shift in order to spectrally decouple the emission process of the dye from the plasmonic system, leaving only the absorption strongly in resonance with the enhanced EM field in the antenna’s vicinity. We demonstrate that this technique provides an effective way of exploring either the EM field or the LDOS with nanometre spatial resolution.