Switch ON and OFF Dipolar Plasmon Dark Mode in Four-Fold Triangle Nanoclusters

In this letter, we experimentally and theoretically investigate the optical properties of four-fold symmetric triangle nanoclusters (TNCs) in nanoholes. Multipolar Fano resonances are observed in the presented TNCs, while dipolar Fano resonance is switched OFF. A plasmon hybridization model is proposed to investigate the underlying reasons of the absence of dipolar Fano resonance through decomposing the optical properties of TNCs into plasmon couplings between the horizontal and vertical bow-ties. Near-field charge density distributions show that the dipolar bonding mode (dipolar dark mode) between the two bow-ties is forbidden due to the four-fold symmetry of TNCs. Moreover, we demonstrate that the dipolar plasmon dark mode can be switched ON by breaking the structural symmetry. The presented nanostructures are helpful to investigate the optical properties of rotational symmetry nanostructures.