Low-power nano-optical vortex trapping via plasmonic diabolo nanoantennas.

Optical vortex trapping can allow the capture and manipulation of micro- and nanometre-sized objects such as damageable biological particles or particles with a refractive index lower than the surrounding material. However, the quest for nanometric optical vortex trapping that overcomes the diffraction limit remains. Here we demonstrate the first experimental implementation of low-power nano-optical vortex trapping using plasmonic resonance in gold diabolo nanoantennas. The vortex trapping potential was formed with a minimum at 170 nm from the central local maximum, and allowed polystyrene nanoparticles in water to be trapped strongly at the boundary of the nanoantenna. Furthermore, a large radial trapping stiffness, ~0.69 pN nm(-1) W(-1), was measured at the position of the minimum potential, showing good agreement with numerical simulations. This subwavelength-scale nanoantenna system capable of low-power trapping represents a significant step toward versatile, efficient nano-optical manipulations in lab-on-a-chip devices.