Enhancing near-field optical tweezers by spin-to-orbital angular momentum conversion

Abstract

Near-field patterns of light provide a way to optically trap, deliver, and sort single nanoscopic particles in a wide variety of applications in nanophotonics, microbiology, and nanotechnology. Using rigorous electromagnetic theory, we investigate the forces and trapping performance of near-field optical tweezers carrying spin and orbital angular momenta. The trapping field is assumed to be generated by a total internal reflection microscope objective at a glass–water interface in conditions where most of the transmitted light is evanescent. We discover aspects of these tweezers, including the possibility to rotate and stably trap nanoscopic beads. More importantly, we show that, under near-field conditions, the contributions of spin and orbital angular momenta to the rotation of small particles are almost equivalent, opening the possibility to cancel each other when they have an opposite sign. We show that these conditions result in optimal optical trapping, giving rise to extremely effective optical tweezers for nanomanipulation, with both circular symmetry and relatively weak rotation.