Combining localized surface plasmons (LSPs) and diffractive surface waves (DSWs) in metallic nanoparticle gratings leads to the emergence of collective hybrid plasmonic-photonic modes known as surface lattice resonances (SLRs). These show reduced losses and therefore a higher Q factor with respect to pure LSPs, at the price of larger volumes. Thus, they can constitute a flexible and efficient platform for light-matter interaction. However, it remains an open question if there is, in terms of the Q/V ratio, a sizable gain with respect to the uncoupled LSPs or DSWs. This is a fundamental point to shed light upon if such modes want to be exploited, for instance, for cavity quantum electrodynamic effects. Here, using aluminum nanoparticle square gratings with unit cells consisting of narrow-gap disk dimers-a geometry featuring a very small modal volume-we demonstrate that an enhancement of the Q/V ratio with respect to the pure LSP and DSW is obtained for SLRs with a well-defined degree of plasmon hybridization. Simultaneously, we report a 5× increase of the Q/V ratio for the gap-coupled LSP with respect to that of the single nanoparticle. These outcomes are experimentally probed against the Rabi splitting, resulting from the coupling between the SLR and a J-aggregated molecular dye, showing an increase of 80% with respect to the DSW-like SLR sustained by the disk LSP of the dimer. The results of this work open the way toward more efficient applications for the exploitation of excitonic nonlinearities in hybrid plasmonic platforms.

The excitation of plasmonic dark modes via a radiative channel is a phenomenon strongly hindered in the subwavelength regime. Recently, for achieving this purpose it has been proposed to exploit near-field interactions between radiating (bright) modes and lossless dark modes. However, this approach unveils challenging difficulties related to the excitation of dark modes through the near-field coupling with a bright mode. Here, it is experimentally and numerically shown how symmetry breaking applied to a nanoantenna dimer can conversely induce the excitation of plasmonic resonances, which play a key role for the dark modes' activation in more complex nanoassemblies. On the basis of this study, a Tshaped nanoantenna trimer has been introduced as an elemental unit for the energy transfer between bright and dark modes in plasmonic nanostructures. Finally, we implemented an analytical perturbative model to further investigate the plasmonic hybridization of subwavelength systems.

The possibility to develop nanosystems with appreciable magnetic response at optical frequencies has been a matter of intense study in the past few years. This aim was strongly hindered by the saturation of the magnetic response of "natural" materials beyond the THz regime. Recently, in order to overcome such limitation, it has been considered to enhance the magnetic fields through the induction of displacement currents triggered by plasmonic resonances. Here we investigate a nanoassembly supporting the hybridization of an electric and magnetic plasmonic mode in Fano resonance conditions. Taking advantage of the enhancement properties owned by such interferential resonance, we have been able to generate an intense and localized magnetic hot-spot in the near-infrared spectral region.