Enhanced Optical Forces in Plasmonic Microstructures

Micromanipulation of dielectric objects, from polystyrene spheres to living cells, is achieved when radiation pressure forces create stable trapping by highly focused laser beams through microscopes. However, the impressive history of optical trapping is shadowed by the light diffraction limit, as research currently has focused on materials below the micron scale, requiring stronger optical confinement and higher intensities than can be provided by the conventional optical tweezers. Recently, plasmonic nanostructures have entered the field, either to assist or enhance it. In this study, we present experimental results on using localized fields of metallic structures for efficient trapping, with various patterns (dots, fringes and squares). The patterns were produced by laser interferometry on almost continuous Ag or Au films on glass and glass covered by an amorphous Al2O3 layer (10 nm thick) respectively. We have calculated the optical forces by measuring the particle's escape velocity. The results show that the effective quality factor Q in the patterned metal film is enhanced by a factor > 10, with respect to the unpatterned metal film and a factor > 100, with respect to an uncoated glass. In addition, mathematical simulation of plasmonic fields is investigated to confirm and explain theoretically, the experimentally observed plasmonic enhancement.