Reversal of the optical force in a plasmonic trap.

We study in detail the optical forces generated by a plasmonic trap on a plasmonic nanoparticle. The permittivity of the trapped particle is tuned using a Drude model. The interplay between the plasmon resonances of the trap and of the particle can produce different regimes leading to attractive or repulsive forces. Hence a particle will be trapped or repulsed depending on its permittivity. Such a physical system should provide new functionalities for lab-on-the-chip applications.