Fano resonances in nanohole oligomers in a gold film

The optical properties of nanohole oligomers in a gold film have been investigated theoretically and experimentally. Helium focused ion beam milling was used to create arrays of close-packed heptamer-arranged nanoholes (HNH) in a gold film on a fused silica substrate. The holes are slightly elliptical about a nominal diameter of 100 nm and are separated by 15 nm Au regions (“bridges”) to form close-packed heptamer arrangements. Optical responses are reported as transmittance spectra, showing various Fano resonances due to different excitations on the structure: surface plasmon polaritons localized to the HNH, symmetric and asymmetric surface plasmon polaritons propagating along with the film, and surface waves associated with Wood’s anomaly. All resonances observed involve the coupling (bonding and anti-bonding) of magnetic dipoles within the holes of HNH and in some cases of magnetic hexapoles within the central hole. The sensing performance was investigated by measuring the bulk sensitivity of the resonances, anticipating application to biosensing, yielding 380 nm RIU−1 in the best case. Surface-enhanced Raman scattering measurements were also performed on HNH, where adventitious carbon was detected directly through the identification of D and G bands (spectra taken from nearby un-milled Au areas were essentially featureless).