Collective Resonances in Nanoparticle Oligomers

Abstract: The study of nanostructured artificial media for optics has expanded rapidly over the last few decades, coupled with improvements of fabrication technology that have enabled investigation of previously unrealisable optical scattering systems. Such development is complemented by renewed impetus to understand the physics of optical scattering from complex subwavelength geometry and nanoparticle systems. Here I investigate specifically the optical properties of closely packed arrangements of nanoparticles, known as nanoparticle oligomers, which provide an intuitive platform for analytical and numerical study on the formation and interplay of collective resonances. I consider both plasmonic nanoparticles, and also high-refractive-index dielectric nanoparticles that support Mie-type electric and magnetic dipole resonances. Specific outcomes of this study are listed as follows. (i) A new model is presented for optical Fano resonances, which is based on interference between nonorthogonal eigenmodes of the associated scattering object. This is demonstrated to correctly describe Fano resonances in both plasmonic and high-refractive-index dielectric nanoparticle oligomers. (ii) Polarisation-independent scattering and absorption losses are shown to be enforced by n-fold discrete rotational symmetry, Cn (n \geq 3), and the reciprocal degeneracy of eigenmodes. (iii) A form of circular dichroism that emerges from interaction of nonorthogonal resonances is presented, and specifically impacts the ratio of radiative scattering loss to dissipative absorption loss experienced by reciprocal plane waves. The sequence of general theoretical conclusions (i)-(iii) serve to build the understanding of optical scattering from nanoparticle systems while removing existing ambiguities.