Plasmonic nanoparticle on mirror antennas with sub-10 nm gaps have shown the great potential in nanophotonic applications because they offer tightly confined electric field in the gap and resultant large Purcell factors. However, in a nanosphere on mirror (NSoM) structure being studied experimentally, the degree of freedom of the antennas in terms of spectral and polarization control is limited. In this work, we report spectral shaping and polarization control of Purcell-enhanced fluorescence by the gap plasmon modes of an anisotropic gold (Au) nanorod on a mirror (NRoM) antenna. Systematic numerical calculations demonstrate the richer resonance behaviors of a NRoM antenna than a NSoM antenna due to the hybridization of the bright and dark modes. We fabricate a NRoM antenna by placing a Au NR on an ultraflat Au film via a mono-, double-, or quadruple-layers of light emitting quantum dots (QDs) (3 nm in diameter). The scattering spectra of single NRoM antennas coincide very well with those of the numerical simulations. We demonstrate large enhancement (>900-fold) and strong shaping of the luminescence from QDs in the gap due to the coupling with the hybridized mode of a NRoM antenna. We also show that the polarization property of the emission is controlled by that of the mode coupled.

A monolayer of silicon quantum dots (Si-QDs) 2.8 and 3.9 nm in diameter is placed in a gap between a gold (Au) thin film and a Au nanoparticle, and the photoluminescence (PL) properties are studied. By the metal nanoparticle over mirror (MNPoM) structure, the PL spectra of Si-QDs are strongly modified; the full width at half-maximum is reduced to ∼170 meV, which is less than half of that of Si-QDs on a silica substrate. The spectral shape coincides almost perfectly with that of the scattering spectrum of the MNPoM structure, indicating efficient coupling of the luminescence of Si-QDs with the gap surface plasmon modes. The luminescence intensity of Si-QDs in the gap is estimated to be enhanced about 700-fold compared to those on a Au film.

Colloidal silicon quantum dots (Si QDs) with a very broad photoluminescence (PL) band are proposed as a probe to monitor the Purcell enhancement in a plasmonic nanostructure. Si QDs placed on an arbitrary plasmonic nanostructure enable us to determine the Purcell enhancement factors in a broad spectral range (600–900 nm). As a proof-of-concept experiment, a layer of Si QDs is spin-coated on gold film-over nanosphere structures, and the Purcell enhancement is quantitatively determined from the analyses of the PL spectra and the decay rates. The method proposed in this work provides a facile approach to quantitatively measure the performance of plasmonic substrates for PL and Raman enhancements.