Clarification of surface modes of a periodic nanopatch metasurface.

We study the angle-dependent optical reflectance spectrum of a metasurface consisting of a periodic array of film-coupled plasmonic nanopatch particles. The nanopatch metasurface exhibits a strong, angle-independent absorption resonance at a wavelength defined by the nanopatch geometry and relative density. When the nanopatches are arranged in a regular lattice, a second, sharp absorption dip is present that varies strongly as a function of the incidence angle. This second resonance is a collective effect involving the excitation of surface plasmon modes and relates to a Wood's anomaly. Using an analytical model, we compute the surface modes of the structure and confirm details about the various mechanisms that contribute to the reflection spectra. The measured reflectance spectra are in excellent agreement with both analytical calculations and full-wave numerical simulations.