Plasmonic nanobilliards: controlling nanoparticle movement using forces induced by swift electrons

Manipulation of nanoscale objects to build useful structures requires a detailed understanding and control of forces that guide nanoscale motion. We report here observation of electromagnetic forces in groups of nanoscale metal particles, derived from the plasmonic response to the passage of a swift electron beam. At moderate impact parameters, the forces are attractive, toward the electron beam, in agreement with simple image charge arguments. For smaller impact parameters, however, the forces are repulsive, driving the nanoparticle away from the passing electron. Particle pairs are most often pulled together by coupled plasmon modes having bonding symmetry. However, placement of the electron beam between a particle pair pushes the two particles apart by exciting antibonding plasmonic modes. We suggest how the repulsive force could be used to create a nanometer-sized trap for moving and orienting molecular-sized objects.