The optical transmittance of a slab of a two-dimensional photonic lattice composed of a triangular array of circular rods was calculated by means of the plane-wave expansion. This method naturally gives the specular and Bragg reflectivity as well. The comparison between the band structure of the relevant infinite lattice and the calculated transmittance showed a clear correspondence concerned with band gaps. However, it also showed a null coupling between the external radiation field and an internal electromagnetic mode in addition to a considerable amount of the energy transfer to the Bragg waves. This fact implies that the simple analysis of the spectra based on the state density is insufficient.

• Received 26 October 1994

DOI:https://doi.org/10.1103/PhysRevB.51.4672