Plasmon-assisted optical trapping and anti-trapping

The ability to manipulate small objects with focused laser beams has opened a venue for investigating dynamical phenomena relevant to both fundamental and applied science. Nanophotonic and plasmonic structures enable superior performance in optical trapping via highly confined near-fields. In this case, the interplay between the excitation field, re-scattered fields and the eigenmodes of a structure can lead to remarkable effects; one such effect, as reported here, is particle trapping by laser light in a vicinity of metal surface. Surface plasmon excitation at the metal substrate plays a key role in tailoring the optical forces acting on a nearby particle. Depending on whether the illuminating Gaussian beam is focused above or below the metal-dielectric interface, an order-of-magnitude enhancement or reduction of the trap stiffness is achieved compared with that of standard glass substrates. Furthermore, a novel plasmon-assisted anti-trapping effect (particle repulsion from the beam axis) is predicted and studied. A highly accurate particle sorting scheme based on the new anti-trapping effect is analyzed. The ability to distinguish and configure various electromagnetic channels through the developed analytical theory provides guidelines for designing auxiliary nanostructures and achieving ultimate control over mechanical motion at the micro- and nano-scales. Modeling of surface plasmon-assisted optical tweezers shows how focal spot positions may enhance, reduce or even invert trapping effects. Traditional optical traps rely on tightly focused lasers to manipulate microscopic objects, but are restricted by light’s distribution in a focus. An analytical tool from Aliaksandra Ivinskaya from ITMO University in Russia and colleagues has now identified regions where plasmon waves, generated on metal surfaces, can best overcome these classical limits. Using a rigorous theory to model the forces acting on sub-wavelength-sized particles, the team found that focusing lasers above a metal surface has a primary impact on the trap stiffness – it could be enhanced, reduced or become negative, depending on the focal position with respect to the surface. An unusual ‘anti-trapping’ regime that repels particles is predicted to benefit applications including optical sorting of biomolecules and nanomaterials.