Fano-enhanced pulling and pushing optical force on active plasmonic nanoparticles

We demonstrate tunable pulling and pushing optical forces on plasmonic nanostructures at plasmon singularity and Fano resonance. The plasmonic nanostructure containing a spherical core with optical gain and a metallic shell shows much larger optical pulling force than a pure gain sphere. When the size of the nanostructure is beyond the quasistatic limit, one can obtain large field enhancement and giant pulling force at the emerged quadrupole mode. The introduction of an optical pump compensates the dissipative loss from the metal shell, thus enabling strong coupling between a narrow quadrupole mode and a board dipole mode, giving rise to Fano resonance. The giant negative forces originate from the reversal of the electric field at Fano resonance, which leads to pulling forces on bound currents and charges. Meanwhile, the separation of the Lorentz force helps to reveal the nature of the pulling forces in the gain system. We have shown that by applying the Lorentz force density formula, it is possible to obtain the correct value of the force inside our complex inhomogeneous structure made up of dispersive and lossy metamaterial irrespective of the electromagnetic momentum density. Our results provide a practical way to manipulate nanoparticles and give deep insight into light-matter interaction.