  In this work, we use a site-selective functionalization strategy to decorate
plasmonic nanopores with one or more fluorescent dyes. Using an easy and robust
fabrication method, we manage to build single plasmonic rings on top of
dielectric nanotubes with different inner diameters. The modulation of the
dimension of the nanopores allows us to both tailor their field confinement and
their Purcell Factor in the visible spectral range. In order to investigate how
the changes in geometry influence the fluorescence emission efficiency,
thiol-conjugated dyes are anchored on the plasmonic ring, thus forming a
functional nanopore. We study the lifetime of ATTO 520 and ATTO 590 attached in
two different configurations: single dye, and FRET pair. For the single dye
configuration, we observe that the lifetime of both single dyes decreases as
the size of the nanopore is reduced. The smallest nanopores yield an
experimental Purcell Factor of 6. For the FRET pair configuration, we measure
two regimes. For large nanopore sizes, the FRET efficiency remains constant.
Whereas for smaller sizes, the FRET efficiency increases from 30 up to 45% with
a decrease of the nanopore size. These findings, which have been also supported
by numerical simulations, may open new pathways to engineer the energy transfer
in plasmonic nanopores with potential applications in photonics and biosensing,
in particular in single-molecule detection towards sequencing.
