Multiple Fano resonances in spoof localized surface plasmons.

We present the occurrence of bright modes and dark modes in spoof localized surface plasmons (LSPs) generated by ultrathin corrugated metallic disks. As two such disks with asymmetric geometries are placed in close proximity, we find that dark modes (in multipoles) of one disk emerge by coupling with the bright modes (in dipoles) of the other disk. Then we further observe multiple Fano resonances due to destructive interferences of dark modes with the overlapping and broadened bright modes. These Fano line-shapes clearly exhibit the strong polarization dependence. We design and fabricate the ultrathin corrugated bi-disk structure in the microwave frequency, and the measurement results show reasonable agreement with theoretical predictions and numerical simulations. Such multiple Fano resonances could be exploited for the plasmonic devices at lower frequencies.