Strong coupling of light to flat metals via a buried nanovoid lattice: the interplay of localized and free plasmons.

We study the optical plasmonic properties of metal surfaces which have a periodic lattice of voids buried immediately beneath their flat upper surface. Light reflection spectra calculated in the framework of a self-consistent electromagnetic multiple-scattering layer-KKR approach exhibit two types of plasmon resonances originating from the excitation of different plasmon modes: surface plasmon-polaritons propagating on the planar surface of metal and Mie plasmons localized in the buried voids. Coupling between these two types of plasma oscillation leads to an enhancement of the surface plasmon-polariton resonances even for close-packed void lattices. Our theoretical model quantitatively agrees with experimental results, demonstrating that planar surfaces can exhibit strong plasmonic field enhancements.