Concentric Necklace Nanolenses for Optical Near-Field Focusing and Enhancement

In this paper, we design and analyze concentric necklace nanolenses (CNNLs) which consist of metal nanoparticle dimers placed in the center of one or more concentric rings of plasmonic necklaces. We use three-dimensional finite-difference time-domain simulations, electron-beam lithography fabrication, dark-field scattering analysis, and surface-enhanced Raman scattering (SERS) measurements to investigate the far-field scattering and near-field light localization properties of CNNLs. Using these methods, we show that CNNLs display far-field scattering properties that arise from coupling between the dimer and surrounding necklace(s), leading to two pronounced peaks in single-necklace CNNLs and three pronounced peaks in double-necklace CNNLs. In our near-field analysis, we find that the number of particles in the surrounding necklace is an important degree of freedom in the optimization of near-field intensity within the dimer hot-spot region. By using CNNLs where the necklace diameters have a diameter equal to an integer multiple of the resonance wavelength of the isolated dimer times a constant scaling factor, the intensity of near-fields can be optimized for all geometries over a broad-band wavelength range. Using optimized geometries, we perform SERS experiments on CNNLs coated with a pMA monolayer and demonstrate 7× Raman enhancement in the single-necklace CNNL and 18× enhancement in the double-necklace CNNL over the reference dimer antenna geometry, with an average Raman enhancement value of approximately 7 × 10(5).