Strong Fano Resonance Excited in an Array of Nanoparticle-in-Ring Nanostructures for Dual Plasmonic Sensor Applications

A large-area (over 1 cm(2)) nanoparticle-in-ring array (NPIRA) with strong Fano resonance is fabricated by a low-cost shadow sphere lithography (SSL) method. Controlled narrow nanogaps of 8-20 nm are formed between the nanoparticles (NPs) and rings. Quadrupole modes are excited in the nanogaps and interact with the dipole mode of the NPs and ring, resulting in two Fano resonances. The two Fano resonances are in the visible and near-infrared region, respectively, and show a maximum refractive index sensitivity of 425 nm/RIU and a figure of merit of 4.7-7.1 RIU-1. In addition, the NPIRAs show enhanced Raman intensity due to the strong electric fields confined in the nanogaps. The structure has great potential for the dual plasmonic sensor applications, i.e., surface-enhanced Raman scattering and refractive index sensing, and provides a robust sensing performance for each.