Lattice resonances, the collective modes supported by periodic arrays of metallic nanoparticles, give rise to very strong and spectrally narrow optical responses. Thanks to these properties, which emerge from the coherent multiple scattering enabled by the periodic ordering of the array, lattice resonances are used in a variety of applications such as nanoscale lasing and biosensing. Here, we investigate the lattice resonances supported by bipartite nanoparticle arrays. We find that, depending on the relative position of the two particles within the unit cell, these arrays can support lattice resonances with a super-or subradiant character. While the former result in large values of reflectance with broad lineshapes due to the increased radiative losses, the latter give rise to very small linewidths and maximum absorbance, consistent with a reduction of the radiative losses. Furthermore, by analyzing the response of arrays with finite dimensions, we demonstrate that the subradiant lattice resonances of bipartite arrays require a much smaller number of elements to reach a given quality factor than the lattice resonances of arrays with single-particle unit cells. The results of this work, in addition to advancing our knowledge of the optical response of periodic arrays of nanostructures, provide an efficient approach to obtain narrow lattice resonances that are robust to fabrication imperfections.

Metallic nanoparticles were shown to affect Förster energy transfer between fluorophore pairs. However, to date, the net plasmonic effect on FRET is still under dispute, with experiments showing efficiency enhancement and reduction. This controversy is due to the challenges involved in the precise positioning of FRET pairs in the near field of a metallic nanostructure, as well as in the accurate characterization of the plasmonic impact on the FRET mechanism. Here, we use the DNA origami technique to place a FRET pair 10 nm away from the surface of gold nanoparticles with sizes ranging from 5 to 20 nm. In this configuration, the fluorophores experience only moderate plasmonic quenching. We use the acceptor bleaching approach to extract the FRET rate constant and efficiency on immobilized single FRET pairs based solely on the donor lifetime. This technique does not require a posteriori correction factors neither a priori knowledge of the acceptor quantum yield, and importantly, it is performed in a single spectral channel. Our results allow us to conclude that, despite the plasmon-assisted Purcell enhancement experienced by donor and acceptor partners, the gold nanoparticles in our samples have a negligible effect on the FRET rate, which in turns yields a reduction of the transfer efficiency.

We investigate the impact that light-forbidden exciton transitions have in the near-field population dynamics and far-field scattering spectrum of hybrid plasmon-emitter systems. Specifically, we consider a V-type quantum emitter, sustaining one dipolar and one quadrupolar (dipole-inactive) excited states, placed at the nanometric gap of a particle-on-a-mirror metallic cavity. Our fully analytical description of plasmon-exciton coupling for both exciton transitions enables us to reveal the conditions in which the presence of the latter greatly alters the Purcell enhancement and Rabi splitting phenomenology in the system.The deeply sub-wavelength character of localized surface plasmons (SPs) provides new avenues for the control of light-matter interactions at the nanoscale, both in the weak [1,2] and strong coupling regimes [3,4]. Currently, hybrid systems comprising metal nanocavities and quantum emitters (QEs) are attracting much interest not only for their fundamental implications, but also for their technological prospects in areas such as photonics [5] and material science [6]. Lately, experimental reports have shown that the strong light confinement enabled by SPs can unveil features of microscopic light sources that remain hidden to propagating fields, such as mesoscopic effects in the electronic wavefunctions of quantum dots [7] or the fingerprint of individual chemical bonds in Raman molecules [8]. These advances indicate that in order to fully seize the potential of QE-SP devices their theoretical description [9] must combine the framework of macroscopic quantum electrodynamics, accounting for the lossy and open nature of SP quanta [10], and refined models for QEs, including ingredients such as rovibrational [11] or polarization degrees of freedom [12].In this Letter, we investigate the impact that lightforbidden exciton transitions have in QE-SP interactions at the single emitter level. We consider a V-type threelevel system with one dipolar and one quadrupolar (dipoleinactive) excited states. The latter are long-lived excitations which present radiative decay rates typically 5 orders of magnitude lower than dipolar ones [13,14]. They are effectively decoupled from propagating light, but recent theoretical predictions [14][15][16][17] suggest that the large evanescent field gradients associated to SPs may allow Purcell enhancing these transitions up to time scales comparable to lightallowed ones. We explore the influence of this phenomenon in QE-SP coupling and the formation of plasmon-excitonpolaritons (PEPs) in an archetypal nanoparticle-on-a-mirror (NPoM) cavity [4]. Using transformation optics [18], we describe in a fully analytical manner the near-and far-field characteristics of the SP modes supported by this structure. This provides deep physical insights into the population dynamics and the scattering spectrum of the hybrid QE-SP system, and allows us to reveal the conditions in which light-forbidden excitons yield a strong modification of the Purcell enhancement and Rabi splitting phenomena. FI...

We investigate plasmon-emitter interactions in a nanoparticle-on-a-mirror cavity. We consider two different sorts of emitters, those that sustain dipolar transitions, and those hosting only quadrupolar, dipole-inactive, excitons. By means of a fully analytical two-dimensional transformation optics approach, we calculate the lightmatter coupling strengths for the full plasmonic spectrum supported by the nanocavity. We reveal the impact of finite-size effects in the exciton charge distribution and describe the population dynamics in a spontaneous emission configuration. Pushing our model beyond the quasi-static approximation, we extract the plasmonic dipole moments, which enables us to calculate the far-field scattering spectrum of the hybrid plasmon-emitter system. Our findings, tested against fully numerical simulations, reveal the similarities and differences between the strong coupling phenomenology for bright and dark excitons in nanocavities.

Photobleaching is an effect terminating the photon output of fluorophores, limiting the duration of fluorescence-based experiments. Plasmonic nanoparticles (NPs) can increase the overall fluorophore photostability through an enhancement of the radiative rate. In this work, we use the DNA origami technique to arrange a single fluorophore in the 12-nm gap of a silver NP dimer and study the number of emitted photons at the single molecule level. Our findings yielded a 30× enhancement in the average number of photons emitted before photobleaching. Numerical simulations are employed to rationalize our results. They reveal the effect of silver oxidation on decreasing the radiative rate enhancement.