Plasmonic Brownian Ratchets for Directed Transport of Analytes

Plasmonic nanostructures provide strong optical near-fields for trapping and manipulating nanosized particles, but converting these interactions into robust directional transport has remained challenging. Here we demonstrate a plasmonic Brownian ratchet that rectifies colloidal diffusion using an asymmetric gold nanoarray under continuous-wave illumination. Finite-element simulations reveal anisotropic near-field distributions that bias optical forces, and experiments confirm directed motion for 40-200 nm nanoparticles of various compositions (dielectric, semiconducting and metallic). We show that, under periodic light modulation, nanoparticles undergo unidirectional lateral transport with velocities up to 2.4 μm/s at incident intensities below 1 kW/cm². These results establish plasmonic ratcheting as an efficient route to bias transport of nanosized analytes, achieving markedly higher speeds and lower operating powers than previous optical ratchets, and opening opportunities for integration into nanophotonic and lab-on-chip systems.