Direct imaging of fluorescence enhancement in the gap between two gold nanodisks

Abstract

We present an analysis of the optical coupling between two gold nanodisks by near-field fluorescence microscopy. This is achieved by scanning and measuring simultaneously the light emitted by a single Er 3+ /Yb 3+ doped nanocrystal glued at the end of an atomic force microscope tip. The excitation of the nanocrystal was performed at  = 975 nm, via upconversion, and fluorescence was detected in the visible part of the spectrum at  = 550 nm.For an isolated nanodisk, the near-field presents a two-lobe pattern oriented along the direction of the incident polarization. For two nanodisks with a sizeable separation distance (385 nm), illuminated with the polarization along the interparticle axis, we observe a negative effect of the coupling with a slight decrease of fluorescence in the gap. For smaller gap values (195 nm, 95 nm and 55 nm) a strong increase of fluorescence is observed as well as a reduced spatial localization of the field as the distance decreases. Finally, when the disks touch each other (0 nm), the dipolar-dipolar interaction between them disappears, and no fluorescence enhancement occurs. A new plasmon mode is created, at another wavelength. Our experimental results are in good agreement with numerical simulations of the near-field intensity distribution at the excitation wavelength on the surface of the structures. Combining fluorescence mapping and far-field scattering spectroscopy should be of strong interest to develop bio-chemical sensors based on field enhancement effects.