On the scaling behavior of dipole and quadrupole modes in coupled plasmonic nanoparticle pairs

Abstract

We investigate shifts of localized surface plasmon resonance (LSPR) caused by coupling between Ag nanospheres in close proximity to one another by using three-dimensional (3D) finite-difference time-domain (FDTD) simulations and exact Mie solutions. Our findings agree well with previous reports of universal scaling in coupled nanostructures where the relative fractional shift in dipole plasmon resonance wavelength decays over an inter-particle gap with the same universal trend independent of particle size, shape and material composition. To expand upon this, we investigate universal scaling of the dipole mode in coupled particle pairs greater than 100 nm in diameter where higher-order modes of resonance (i.e. both dipole and quadrupole modes of resonance) are present. It is shown that fractional shifts of the quadrupole mode in coupled sphere-pairs do not follow a universal scaling trend independent of particle size. Rather, the fractional shifts are dependent on a predetermined set of particle sizes defined by the particle-pair spacing at which the onset of shifts follow bands that are dependent on the center-to-center particle distance.