Investigation of void plasmon ring resonators
Here, using EFTEM and EELS, we investigate the possible modes of void hexamer and heptamer plasmon resonators, focusing on their symmetries and topologies. The hexamer resonator has 6 holes of 70 nm diameter and rim-to-rim spacing of 30 nm, drilled into a 100 nm thick silver film. The heptamer resonator has 7 holes, one at the center and 6 in a ring. Each hole has a diameter of 60 nm and the rim-to-rim spacing is 50 nm. The structures have similar symmetry. However, they differ in the number of holes and the topology.
To investigate the spatial distribution of localized plasmon resonance (LPR) modes, a peak-finding algorithm  is used. Four distinct modes are observed experimentally. These LPR modes can be classified into those modes related to topology, such as toroidal plasmonic modes [2,3], and those only related to symmetries, such as radially and azimuthally polarized modes . To investigate the modes related to symmetries more systematically, FDTD calculations are performed, providing modal decomposition analysis (see figure) with selective excitation of different resonances, and hence yielding their eigen-energies and spatial distributions.
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 N. Talebi et al. Appl. Phys. A, accepted for publication (2014)
This research has received funding from the European Union Seventh Framework Programme [FP7/2007-2013] under grant no. 312483 (ESTEEM2).