Fast and efficient nanoparticle trapping using plasmonic connected nanoring apertures

The manipulation of microparticles using optical forces has led to many applications in the life and physical sciences. To extend optical trapping towards the nano-regime, in this work we demonstrate trapping of single nanoparticles in arrays of plasmonic coaxial nano-apertures with various inner disk sizes and theoretically estimate the associated forces. A high normalized experimental trap stiffness of 3.50 fN nm^-1 mW^-1 μm^-2 for 20 nm polystyrene particles is observed for an optimum design of 149 nm for the nanodisk diameter at a trapping wavelength of 980 nm. Theoretical simulations are used to interpret the enhancement of the observed trap stiffness. A quick particle trapping time of less than 8  s is obtained at a concentration of 14 × 10¹¹ particles ml^-1 with low incident laser intensity of 0.59 mW μm^-2. This good trapping performance with fast delivery of nanoparticles to multiple trapping sites emerges from a combination of the enhanced electromagnetic near-field and spatial temperature increase. This work has applications in nanoparticle delivery and trapping with high accuracy, and bridges the gap between optical manipulation and nanofluidics.