Radius-dependent binding or repelling forces exerted on metal nano-sphere clusters by infrared-induced plasmonic resonance

The force exerted on metal nano-particles is investigated numerically when the particles are in contact to form a cluster and are illuminated by a light of their fundamental plasmonic resonance frequency. For touching spheres, the mode of resonance changes drastically from those of separate spheres, and the resonance typically appears in the infrared wavelength range. For silver and gold two-sphere clusters, i.e. homo-dimers, of various radii, the magnitudes and the directions of the exerted forces are analyzed. It is found that the field enhancement in the gap region between the spheres depends largely on the sphere radius, and the force on the fundamental resonance exhibits binding only for small spheres; the greatest binding force is observed for radii of 40–60 nm; and the force turns to repelling when the radius becomes greater than approximately 100–150 nm as the field in the gap region becomes weaker.