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 simultaneously scanning and measuring the light emitted by a single Er3þ/Yb3þ doped nanocrystal glued at the end of an atomic force microscope tip. The excitation of the nanocrystal was performed at k ¼ 975 nm via upconversion, and fluorescence was detected in the visible part of the spectrum at k ¼ 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 sizable separation distance (385 nm) illuminated with the polarization along the interparticle axis, we observe a negative effect of the coupling with a slight decrease in fluorescence in the gap. For smaller gap values (195, 95, and 55 nm), a strong increase in 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 nearfield 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.The authors thank the support from the DIM Nano-K program from “Region Ile de France,” from the Idex Paris Sciences & Lettres through Grant No. ANR-10-IDEX-0001-02 PSL from the CNRS and the CSIC through the Spanish-French program PICS (Grant Nos. SolarNano PICS07687 and PIC2016FR2), and from the Spanish Ministerio de Ciencia e Innovacion through Grant No. PID2019-109905GA-C22.Peer reviewe