Abstract

Tunable Fano resonances in the gold rod-ring plasmonic nanocavities with strong coherent coupling are demonstrated by finite-difference time-domain (FDTD) method. For one rod-one ring nanocavity, symmetry breaking activates the high-order plasmon modes in the ring and causes a Fano resonance with a dip in the extinction spectrum of the cavity by the coherent coupling between the bright rod mode and dark ring mode. The addition of a non-resonant rod introduces a second dark mode, which could be successively excited by the first dark ring mode, and produces double Fano resonances. By adjusting the rod number and configuration of the nanocavity, the extinction line shape can be controlled and the near-field distribution can be dramatically modified. An intriguing phenomenon of superposition beats in the plasmon damping process is revealed, which induces an energy swap between the rod and ring. The antenna effect of the resonant rod as well as the energy transfer and distribution in the nanocavities are also discussed.

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