Electromagnetic response of nanosphere pairs: Collective plasmon resonances, enhanced fields, and laser-induced forces

Abstract

We present theoretical studies of the electromagnetic response of metallic nanosphere pairs, with emphasis on the role of their collective plasmon resonances in enhancing electromagnetic fields in their near vicinity, and also in enhancing laser-induced forces between the particles. We emphasize effects encountered when the two particles have dissimilar radii or are fabricated from dissimilar materials. The calculations explore the response of the particles to electric fields polarized parallel and perpendicular to the line between their centers. We find, compared to two identical spheres, that on resonance, both the maximum enhancement in the electric fields and the laser-induced force are increased when the radii differ. Also, there is a breakdown of the selection rule associated with reflection symmetry through the plane which passes through the midpoint of the line between centers. A result is that all collective modes are dipole active when the spheres are dissimilar, for electric fields both parallel to and perpendicular to the line between the sphere centers. Our analysis exploits the electrostatic approximation appropriate to nanoscale objects but is exact within this framework without resort to a dipole approximation.