Plasmonic Resonances in Self-Assembled Reduced Symmetry Gold Nanorod Structures

Self-assembled plasmonic Dolmen structures consisting of small gold nanorods (length = 50 nm and diameter = 20 nm) with a few nanometer gaps are observed to show coherent effects of super-radiance and characteristics of Fano resonance due to the significantly reduced symmetry of the structure. Relative to previous larger structures from top-down electron-beam lithography, the single crystallinity and atomically smooth surfaces of these self-assembled plasmonic structures result in 50% narrower resonances, and the small gaps with associated strong coupling enable observation of multiple dark and bright modes. By tilting the cap monomer with respect to the base dimer an order of magnitude increase in E-field enhancement at the Fano dip is obtained. In addition, a spectrally broad mode is observed indicating the strong impact of the geometry of the structure on the nature of coupled modes. The highly localized electric near-fields in the gaps will enable strong light matter interactions and the narrow resonances will be useful for improved figure of merits in inexpensive chemical and biosensing.