Far-infrared transmittance and band-structure correspondence in two-dimensional air-rod photonic crystals

Two-dimensional photonic crystals of triangular and square lattices both composed of parallel air rods with a lattice constant of 170 {μ}m were fabricated and their transmittances in the far-infrared region were measured to examine to what extent they quantitatively correspond to the calculated photonic band structures. In addition to an opaque region due to the band gap, an uncoupled mode, which cannot be excited by an external electromagnetic wave, has been clearly observed in both lattices. The calculated results agree very closely with the observed features. Furthermore, other regions with very low transmittance have been unexpectedly found to exist for both lattices. Tentatively, their manifestation is explained only qualitatively as arising from a poor coupling due to wave-vector mismatch, between the relevant band and that of incident wave. The present result implies that information on transmittance alone does not suffice for properly understanding the photonic band structure.