Left-handed optical torque on dipolar plasmonic nanoparticles induced by Fano-like resonance

We theoretically and numerically demonstrate that Fano-like resonance can induce a left-handed optical torque on a dipolar plasmonic core-shell nanoparticle in the interference optical field composed of two linearly polarized plane waves. It is shown that the optical torque on the dipolar plasmonic nanoparticle is significantly enhanced at the Fano-like resonance, and its direction is opposite to that of the angular momentum of the incident field, termed Fano-like resonance-induced left-handed optical torque. The extinction spectra exhibit that the Fano-like resonance stems from the coupling between a narrow electric quadrupole dark mode and a broad electric dipole bright mode. In addition, such Fano-like resonance-induced left-handed optical torque can flexibly be tailored by the particle morphology. To further trace the physical origin of the left-handed optical torque, we derive an analytical expression of optical torques up to electric quadrupole in generic monochromatic optical fields based on the multipole expansion theory. The results obtained from our analytical expression show that the left-handed optical torque comes completely from the electric quadrupole terms while other terms from the electric dipole make no contribution. Our results may open a new avenue for tailoring optical torques on plasmonic structures.

Automated summary

We theoretically and numerically demonstrate that Fano-like resonance can induce a left-handed optical torque on a dipolar plasmonic core-shell nanoparticle in the interference optical field composed of two linearly polarized plane waves. This torque can be flexibly tailored by the particle morphology.