Silver nanoring structures were chosen for three reasons. First, the plasmon resonances in nanorings are prototype examples of hybridized resonances where the plasmons at the inner and outer surfaces couple strongly. Second, only the dipolar resonances are present and high-ordered multipolar modes remain dark for incident light normal to the nanorings, which can minimize the influence of multipolar modes on the dipolar anti-bonding mode. Finally, silver is a suitable metal. Its interband transitions start at a relatively higher energy region (~3.8 eV), which decreases the damping of anti-bonding resonance and makes it observable within our experimentally accessible UV-vis-NIR spectral range.

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In the framework of the plasmon hybridization model, the nanorings can be considered as a composite of nanodiscs and nanoholes. The interaction of nanodiscs and nanoholes leads to an energy splitting of resonance modes, resulting in a low-energy bonding mode and a high-energy anti-bonding mode for nanorings. The researchers have experimentally detected the bonding and anti-bonding resonances in the extinction spectrum of silver nanorings. The numerical simulations of the electric field and surface charge profiles confirm the presence of the bonding and anti-bonding modes as well.

Silver nanorings are fabricated using a nanosphere-lithography technique by sputtering a 30 nm thick metal layer on the top of 100 nm sized polystyrene sulphate latex beads, followed by an ion milling process. Gold nanorings are constructed for comparison, and in this case only the dipolar bonding modes appear. The dipolar anti-bonding modes are absent due to the strong damping by the interband transitions in gold.

The researchers presented their work in Nanotechnology.