Photonic-Plasmonic Coupling and Near Field Engineering in Nanoparticle Necklaces

Particle clusters with different degrees of rotational symmetry consisting of circular loops of gold nanoparticles, dubbed nanoplasmonic necklaces, are proposed as a novel, reproducible platform for elastic and inelastic optical sensors with polarization insensitive behavior. Engineering of the necklaces allows for full control of the plasmonic hot-spot locations and near-field strength by coupling photonic resonances to the circular resonator structure. The polarization insensitivity of necklaces guarantees that the plasmonic hot-spots remain excited within the necklaces irrespective of the incident polarization of the excitation field, which is a significant advantage compared to hot-spots in dimer configurations. Near-fields can be further enhanced using radiative coupling in concentric necklaces having integer multiple diameters. Engineering design rules are determined for hot-spot formation, polarization insensitivity, and intensity distribution in necklaces using 3-dimensional Finite-Difference Time-Domain simulations. Plasmonic necklaces of different rotational axes were fabricated using electron-beam lithography and electron-beam deposition of gold films. Surface enhanced Raman scattering measurements were used to experimentally validate our near-field calculations.