Far-infrared spectroscopy study of an uncoupled mode in a two-dimensional photonic lattice

We fabricated a two-dimensional photonic lattice composed of a triangular array of circular air rods with the lattice constant of 170 {μ}m. The comparison of the observed transmission spectra in the far-infrared region with the calculated transmittance and band structure gave clear evidence for the existence of an uncoupled mode which cannot be excited by an external plane wave. The wave function of the uncoupled mode is antisymmetric under a relevant mirror reflection defined on the triangular lattice, which implies that the above conclusion is consistent with the earlier observation on the other bands in two-dimensional lattices by Robertson et al.