Reversal of optical binding force by Fano resonance in plasmonic nanorod heterodimer.

We present calculations of the optical force on heterodimer of two gold nanorods aligned head-to-tail, under plane wave illumination that is polarized along the dimer axis. It is found that near the dipole-quadrupole Fano resonance, the optical binding force between the nanorods reverses, indicating an attractive to repulsive transition. This is in contrast to homodimer which in similar configuration shows no negative binding force. Moreover, the force spectrum features asymmetric line shape and shifts accordingly when the Fano resonance is tuned by varying the nanorods length or their gap. We show that the force reversal is associated with the strong phase variation between the hybridized dipole and quadrupole modes near the Fano dip. The numerical results may be demonstrated by a near-field optical tweezer and shall be useful for studying "optical matters" in plasmonics.