Optical binding force of gold nanorod dimers coupled to a metallic slab

By full-wave 3D electromagnetic simulations and using the Maxwell's stress tensor integration technique, we calculate the optical forces on a gold nanorod dimer placed head-to-tail near a metallic slab. It is found that the plasmonic resonance wavelength λres of the system shifts obviously as the nanorod gap g and the dimer–slab spacing h are changed. Meanwhile, the optical forces under plane wave illumination crucially depend on the geometrical parameters and can be enhanced dramatically comparing to the bare dimer case. Moreover, numerical results show that when both g and h are far less than the resonant wavelength λres, the resonant binding force between the two nanorods decreases, while the force acting on each nanorod toward the slab increases, both monotonically as h is reduced. On the contrary, as the dimer–slab spacing h increases, there is a local maximum force at h≈λres/4 for various g. Both the strong near-field coupling and the standing wave formed by incident and reflected waves need to be accounted for the observed characteristics in the optical force spectra. Our results are useful for selective optical manipulation on particles over a substrate platform.