  The nonclassical modification of plasmon-assisted fluorescence enhancement is
theoretically explored by placing two-level dipole emitters at the narrow gaps
encountered in canonical plasmonic architectures, namely dimers and trimers of
different metallic nanoparticles. Through detailed simulations, in comparison
with appropriate analytical modelling, it is shown that within classical
electrodynamics, and for the reduced separations explored here, fluorescence
enhancement factors of the order of $10^{5}$ can be achieved, with a divergent
behaviour as the particle touching regime is approached. This remarkable
prediction is mainly governed by the dramatic increase in excitation rate
triggered by the corresponding field enhancement inside the gaps. Nevertheless,
once nonclassical corrections are included, the amplification factors decrease
by up to two orders of magnitude and a saturation regime for narrower gaps is
reached. These nonclassical limitations are demonstrated by simulations based
on the generalised nonlocal optical response theory, which accounts in an
efficient way not only for nonlocal screening, but also for the enhanced Landau
damping near the metal surface. A simple strategy to introduce nonlocal
corrections to the analytic solutions is also proposed. It is therefore shown
that the nonlocal optical response of the metal imposes more realistic, finite
upper bounds to the enhancement feasible with ultrasmall plasmonic cavities,
thus providing a theoretical description closer to state of the art
experiments.
