Controlled manipulation and trapping of submicron-size particles has many applications in different research fields, including those in the general areas of biology and soft-condensed matter physics. Optical tweezers that make use of strongly focused laser beams are widely used for this purpose. However, their trapping abilities are substantially limited by diffraction and a lack of scalability. To overcome this, nanostructures made of plasmonic materials have attracted significant attention, as their ability to concentrate energy to very small dimensions can be exploited to generate optical traps capable of acting on nanometer-size objects. Furthermore, and when compared to conventional systems, these plasmonic traps also provide large field enhancements that allow for lower input powers. Despite such advantages, these techniques still lack the ability to provide the controlled manipulation of the trapped objects over long distances.
In this work we present a Brownian ratchet, based on plasmonic interactions, which can optically trap and manipulate dielectric nanometer-sized beads over long distances. For this purpose, the geometries of the plasmonic ratchets and the respective electric fields were modelled with COMSOL Multiphysics, and the optical forces experienced by the beads were calculated with COMSOL Multiphysics and processed with MATLAB®. Additionally, we experimentally demonstrate the rectification of the random thermal motion of subwavelength dielectric beads into one specific direction by periodically turning on and off a laser beam that illuminates the plasmonic nanostructure array and exploiting the asymmetries in the system.