Observation of the Fano resonance in gold nanorods supported on high-dielectric-constant substrates.

Fano resonances in plasmonic nanostructures, characterized by their asymmetric resonance spectral profile, are currently attracting much interest due to their potential applications in biological sensing, metamaterials, photoswitching, and nonlinear optical devices. In this study, we report on the observation of the Fano resonance in Au nanorods induced by their coupling with the supporting substrate. For Au nanorods having a large size and deposited on a substrate with a large dielectric constant, the strong nanorod-substrate coupling gives rise to a Fano line shape on the far-field scattering spectrum. Electrodynamic calculations reveal that the Fano resonance originates from the interference of a broad octupolar and a narrow quadrupolar plasmon mode of the nanorod. Such an interaction is enabled by the strong image charges induced by substrates with high dielectric constants. Moreover, the Fano resonance is very sensitive to the nanorod-substrate spacing. When the spacing is experimentally increased to be larger than ∼8 nm, the Fano resonance disappears. These results will be important not only for understanding the interference of different plasmon modes in plasmonic systems but also for developing a number of plasmon-based optical and optoelectronic devices.