  Controlled long-range transport of micro- and nano-scale objects is a key
requirement in lab-on-a-chip and microfluidic applications, enabling the
efficient capture, concentration, manipulation, and detection of analytes.
Traditional methods such as microfluidic pumps and optical trapping face
challenges including high power consumption and limited range of action. This
study introduces a plasmonic Brownian ratchet designed for the directed
transport of dielectric nanometer-sized particles at low optical powers.
Through numerical simulations, the ratchet geometry was optimized to enhance
electric fields, optical forces, and trapping potentials. Experimentally, the
plasmonic ratchet demonstrated the ability to rectify random thermal motion of
40 nm polysterene spheres over extended distances in a specific direction,
achieving velocities up to 2.4 $\mu$m/s at excitation powers as low as 0.785
kW/cm^2. This plasmonic ratchet offers a robust and efficient solution for the
targeted delivery and concentration of nanoscale analytes on chips, with
significant implications for advancing applications in the life sciences.
