Optical lateral forces and torques of a plasmonic nanorotator excited by focused radially and azimuthally polarized vector beams

As one of the fundamental elements of light-driven nanomachines, plasmonic nanorotators based on optical lateral forces have gained much attention in recent years. It is expected that steering plasmonic nanorotators by vector beams will be more versatile, but the relevant studies are still lacking. Here, we present a comprehensive theoretical and numerical study on the optical lateral force and optical torque of a plasmonic nanorotator consisting of four pairs of Au nanorods placed on the focal planes of radially and azimuthally polarized vector beams. Combining the Maxwell stress tensor and the electromagnetic multipole expansion, we disclose that the optical lateral force of the Au nanorod pair under the vector beam excitation includes both the extinction force and the recoil force. Specifically, we demonstrate that the nanorotator excited by the radially polarized vector beam is more efficient because all the nanorod pairs can be activated to provide the tangential force. As to the azimuthally polarized vector beam, we find that the rotation direction of the nanorotator can reverse due to the phase change of the out-of-plane magnetic dipole moment. We further investigate how the optical lateral force and the optical torque are affected by the size and position of the nanorotator. In particular, we show that the nanorotator can also generate the translation motion when the C4 rotation symmetry of the whole system is broken. Our results not only provide a fresh perspective to understand optical lateral forces in plasmonic nanostructures but also promote the development of optical micromachines and microrobots.