The experimental demonstration of narrow resonances in arrays of metallic nanoparticles was more elusive due to limitations in the quality of samples and the use of focused beams. Kravets et al. reported ultranarrow plasmonic resonances in asymmetric (different refractive indexes in the upper and lower media) arrays of Au nanoparticles. Shortly after, Auguié and Barnes and Chu et al. reported narrow resonances in symmetric arrays. The potential of narrow plasmonic resonances in arrays of nanoparticles for modifying the emission of fluorophores was also demonstrated. The origin of the narrow resonances, which are known as surface lattice resonances (SLRs), is the diffractive coupling of LSPRs through in-plane diffraction orders in symmetric media or evanescent
diffraction orders—the so-called Rayleigh anomalies (RAs)—in asymmetric media. SLRs can be described as a driven damped coupled oscillator system in which one oscillator has the natural frequency of the LSPR while the other has the frequency of the diffraction order. Nanoparticle arrays are open cavities that are easy to fabricate and offer the possibility of integration with thin films or planar structures. The remarkable properties of SLRs have led to improved surface-enhanced Raman scattering, sensitive bio/chemical sensing, plasmonic band-edge lasing, strong light–matter coupling, Bose–Einstein condensation, and optoelectronic devices. The multidisciplinary impact of SLRs has stimulated the quest toward modes with the highest possible quality (Q) factor. One strategy to obtain narrow linewidths with SLRs is by coupling multipolar resonances with different diffraction orders. However, Q-factors by these approaches vary significantly over momentum space.
We have demonstrated high quality factor plasmonic resonances in arrays of Ag nanoparticles (Q > 300). These resonances, known as surface lattice resonances, emerge from the coupling of localized surface plasmon polaritons to diffraction orders in the plane of the array. The quadratic dispersion of SLRs leads to a nearly constant Q-factor over a wide range
of wave vectors or angles of incidence. We have investigated the role of the intrinsic quality of the metal in the Q-factor of SLRs. We have also iscussed the effect of the adhesion layer used between the substrate and the metal on the SLRs. The suppression of this layer can lead to SLRs with Q-factors larger than 1500. These extremely high Q-factors render arrays of metallic nanoparticles very interesting systems for plasmonic applications such in sensors, for enhanced light–matter interaction and nonlinear phenomena.
For more information: DOI: 10.1002/adom.201801451