Giant optical forces using an array of asymmetric split-ring plasmonic nanostructures

Abstract

We demonstrated optical trapping of 20 nm particles using a Fano-resonance-assisted plasmonic tweezers based on arrays of asymmetrical split nanoapertures on a 50-nm gold thin film. By transmission and reflection spectra measurements, the close-mode Fano-type excitation peak was estimated at 928 nm. We investigated the trapping performance through power- and wavelength-dependent characterization. We determined the trap stiffness using the transient time method and a linear dependence of the trap stiffness for low incident laser intensities under off-resonance conditions was observed. For the on-resonance condition, a large normalized trap stiffness of 8.65 fN/nm/mW was obtained which enables our system to improved motion control of the trapped nanoparticle. Furthermore, the trap stiffness on-resonance was enhanced by a factor of 63 compared to that of off-resonance. We conclude that this enhancement is due to the ultrasmall mode volume and a cavity effect contribution. Our approach opens new avenues for steady and dynamic optical trapping, making a variety of lab-on-chip applications possible.