Mechanisms of Fano resonances in coupled plasmonic systems.

Fano resonances in hybridized systems formed from the interaction of bright modes only are reported. Despite precedent works, we demonstrate theoretically and experimentally that Fano resonances can be obtained by destructive interference between two bright dipolar modes out of phase. A simple oscillator model is provided to predict and fit the far-field scattering. The predictions are verified with numerical calculations using a surface integral equation method for a wide range of geometrical parameters. The validity of the model is then further demonstrated with experimental dark-field scattering measurements on actual nanostructures in the visible range. A remarkable set of properties like crossings, avoided crossings, inversion of subradiant and superradiant modes and a plasmonic equivalent of a bound state in the continuum are presented. The nanostructure, that takes advantage of the combination of Fano resonance and nanogap effects, also shows high tunability and strong near-field enhancement. Our study provides a general understanding of Fano resonances as well as a simple tool for engineering their spectral features.