Collective resonances in plasmonic crystals: size matters

Periodic arrays of metallic nanoparticles may sustain surface lattice resonances (SLRs), which are collectiveresonances associated with the diffractive coupling of localized surface plasmons resonances (LSPRs). By investigating a series of arrays with varying number of particles, we traced the evolution of SLRs to its origins. Polarization resolved extinction spectra of arrays formed by a few nanoparticles were measured, and found to be in very good agreement with calculations based on a coupled dipole model. Finite size effects on the optical properties of the arrays are observed, and our results provide insight into the characteristic length scales for collectiveplasmonic effects: for arrays smaller than ∼5×5 particles, the Q-factors of SLRs are lower than those of LSPRs; for arrays larger than ∼20×20 particles, the Q-factors of SLRs saturate at a much larger value than those of LSPRs; in between, the Q-factors of SLRs are an increasing function of the number of particles in the array.