Transmission resonances in periodic U-shaped metallic nanostructures.

The spectral response of crescent-like metallic nanostructures, a sub-class of U-shaped split-ring resonators, on a glass substrate at normal incidence is studied numerically. Also, the interpretation of transmission resonances arising from periodic conventional standard split-ring resonators with rectangular edges (SSRR) at normal incidence is revisited. In particular, we focus on one specific transmission resonance which is present for nano-crescents (NC) but absent in the case of SSRRs used for metamaterials. It is proposed that for a U-shaped metallic structure of arbitrary geometry, coupling of plasmonic eigen modes at all the surfaces of the three-dimensional structure is essential to be considered. The manner in which the coupling takes place between plasmonic modes at all the surfaces of the three-dimensional structure is what completely characterizes transmission resonances, and it is unique for each given resonance.