Physical origin of the resonant mode deep inside the stop band of a metallodielectric photonic crystal

The first band of a typical layer-by-layer metallodielectric woodpile structure corresponds to the waveguide cutoff of the rod-to-rod opening. However, a nonlocalized propagating mode beyond waveguide cutoff was previously observed if the metallic gratings are separated by dielectric spacers. We show the physical origin of this mode by studying the electromagnetic fields inside the photonic crystal using the finite-difference time-domain analysis. Our results indicate that it is due to an unusual Fabry-Perot resonance between every other grating layer. Moreover, the polarization dependence of transmission spectrum with odd-number layer is a consequence of the symmetry of the grating structure.