3D Optical Vortex Trapping of Plasmonic Nanostructure

3D optical vortex trapping upon a polystyrene nanoparticle (NP) by a 1D gold dimer array is studied theoretically. The optical force field shows that the trapping mode can be contact or non-contact. For the former, the NP is attracted toward a corresponding dimer. For the latter, it is trapped toward a stagnation point of zero force with a 3D spiral trajectory, revealing optical vortex. Additionally the optical torque causes the NP to transversely spin, even though the system is irradiated by a linearly polarized light. The transverse spin-orbit interaction is manifested from the opposite helicities of the spin and spiral orbit. Along with the growth and decline of optical vortices the trapped NP performs a step-like motion, as the array continuously moves. Our results, in agreement with the previous experiment, identify the role of optical vortex in the near-field trapping of plasmonic nanostructure.