Finite-difference analysis of plasmon-induced forces of metal nano-clusters by the Lorentz force formulation

We analyze light-induced forces on metal nano-spheres by using the three-dimensional finite-difference time-domain method with the Lorentz force formulation. Convergent analysis of the force on metal nano-particle clusters has been achieved by integrating the Lorentz and the Coulomb forces over the volume of the metal particles. Comparison to the Mie theory of radiation pressure on metal spheres under a plane wave illumination has verified rigorously the accuracy of the numerical method. We also analyze separate two metal spheres in close proximity and the results of the induced forces are compared to those in previous publications. The present method allows analysis of forces on various irregular structures; we apply the method to touching metal spheres, forming a simple cluster with a slight deformation at the contact point, to analyze the forces induced by the plasmonic resonance of the clusters. We show that the fundamental resonance modes, which newly appear in an infrared range when spheres are touching, exhibit strong binding forces within the clusters. Based on the numerical analyses we identify the resonance modes and evaluate quantitatively the infrared-induced forces on metal nano-sphere clusters.