Robust Plasmonic Fano Resonances in π-Shaped Nanostructures

Plasmonic Fano resonances hold immense potential for biosensors and optical information processing due to their sharp spectral response. A majority of nanogap-involved complex metallic nanostructures have been demonstrated to support Fano resonances, but the gap distance between the constituents are very difficult to control without the use of sophisticated microfabrication techniques. Here, we propose a simple pi-shaped metallic nanostructure that is free from the particular requirement of nanogaps to generate a strong plasmonic Fano resonance. The plasmonic Fano resonance is demonstrated to be a result of the interference between a broad magnetic dipolar mode and a narrow electric quadrupolar mode. We realize the pi-shaped nanostructure experimentally by employing the angle-resolved nanosphere lithography to produce partially overlapped double metallic nanotriangles. The effect of the geometry parameters including the length of the base and the size of pi-shaped nanostructure on the Fano resonance is investigated in detail. As an application example, we further show that the pi-shaped double metallic nanotriangles could be used as a Fano-based sensor for refractive index sensing, in which a threefold enhancement of sensitivity is achieved as compared with the single nanotriangle-based sensor.