Directional double Fano resonances in plasmonic hetero-oligomers.

We experimentally demonstrate for the first time a very compact plasmonic hetero-oligomer structure where the multiple radiant and subradiant modes can be tailored independently. Unlike previous approaches based on collective excitations in complex plasmonic systems, we show precise engineering of resonances leading to simultaneous spectral overlap of multiple plasmonic modes with opposite radiative character. This asymmetric behavior combined with inherent spatial features of the structure leads to directional double Fano resonances as shown with numerical analysis. A model based on temporal coupled mode theory is also provided to describe the double Fano behavior.