Origin of the super-enhanced light transmission through a 2-D metallic annular aperture array: a study of photonic bands

Recently a new structure showing a super-enhanced transmission [Optics Commun. 209, 17--22 (2002); Phys. Rev. B, 67, 155314, (2003)] has been proposed. The origin of this phenomenon was not clearly explained. In this paper, by using a numerical Order-N FDTD spectral method, we study the eigenmodes, the band structure and the dispersion curves of a photonic 2-D crystal made with coaxial circular cavities and made from a real metal. We show that the super-enhanced transmission of the finite structure is due to a cavity resonance of a single guided mode. An extensive characterization of this mode is presented, in terms of the spatial mode structure and effective index dispersion curve.