Laser nanotrapping and manipulation of nanoscale objects using subwavelength apertured plasmonic media

We study the net radiation force on dielectric nanowires above a planar plasmonic medium consisting of a nanostructured gold film on a glass substrate. The film has a subwavelength aperture and the medium is illuminated by a continuous plane wave. We compute the steady-state field distribution by using two-dimensional finite element analysis, and we evaluate the force on the nanowires above the aperture by using the Maxwell stress tensor. We show that the vertical position of a nanowire can be controlled by either tuning the wavelength of the laser source or tapering the aperture. We experimentally check the conclusions of the theory by realizing laser nanotrapping of dielectric beads near the surface of nanostructured metal. Our results demonstrate the potential of using nanostructured plasmonic media for the controlled optical manipulation of nanowires and nanoparticles, including their application as "plasmonic sieve" for separation based on size and/or refractive index.