Multiple reversals of optical binding force in plasmonic disk-ring nanostructures with dipole-multipole Fano resonances

We study the optical far-field and near-field characteristics, and the optical force effects of plasmonic disk-ring nanostructures. There are multiple Fano features resulting from the scattering interferences of the hybridized modes from the disk's dipole and the ring's higher-order modes. In particular, it is found that the optical binding force between the disk and the ring shows multiple sign reversals spectrally, from the dipole-quadrupole regime up to the dipole-decapole regime. We show that the zero-force points can be categorized into two types: the positive-to-negative ones resulting from the Fano dip and the negative-to-positive ones associated with the transitions between dipole-multipole modes. The multiple sign reversals of the optical forces are tunable by the geometrical size and gap of the disk and ring. Such characters make it possible to organize unusual optical matters from individual plasmonic nanoparticles.