Tunable optical forces enhanced by plasmonic modes hybridization in optical trapping of gold nanorods with plasmonic nanocavity.

The optomechanical interaction between a plasmonic nanocavity and a gold nanorod through optical forces is demonstrated. It is revealed that strong localized plasmon resonance mode hybridization induced by a gold nanorod results in the resonance mode of the nanocavity splitting into two different plasmon resonance modes (bonding plasmon resonance mode and antibonding plasmon resonance mode). When the whole system (gold nanorod and gold nanocavity) is excited at the antibonding plasmon mode, the gold nanorod can receive an optical pushing force and be pushed away from the gold nanocavity. On the other hand, an optical pulling force acts on the gold nanorod and the gold nanorod can be trapped by the gold nanocavity when the plasmonic tweezers work at the bonding mode. The optical pulling force acting on the gold nanorod can be enhanced by two orders of magnitude larger than that of the same sized dielectric nanorod, which benefits from the strong resonant nearfield interaction between the gold nanorod and the gold nanocavity. More importantly, the shape and the position of the optical potential can be tuned by tailoring the wavelength of the laser used in the optical trapping, which can be used to manipulate the gold nanorod within a nanoscale region. Our findings have important implications for optical trapping, manipulation, sorting, and sieving of plasmonic nanoparticles with plasmonic tweezers.