Nanoparticle optical properties: Far- and near-field electrodynamic coupling in a chain of silver spherical nanoparticles

The extinction spectra of arrays of nanoparticles provides a potentially powerful platform for chemical and biological sensing based on variation in plasmon wavelength excitation with analyte index of refraction. In this paper we use exact electrodynamic theory to study the radiative electromagnetic coupling between spherical silver nanoparticles equidistantly arranged in a linear chain. Two distinct coupling regimes are observed depending on the distance between the particles in the chain, and on the polarization of the incident light. Near-field coupling of the particles occurs when the interparticle distance is smaller than twice the particle diameter. This leads to pronounced red shifts in the plasmon resonance wavelength and increasing widths as the particle separation decreases for p-polarization. Far-field coupling leads to non-monotonic shift and broadening at larger distances, with more important effects being associated with s-polarization. A region where the plasmon width is significantly smaller than the isolated particle width is identified in the far-field regime that could be important in biological sensing applications. We also develop quasi-static analytical models to study the transition from near to far-field coupling.