Photonic band gap in naturally occurring ordered anodic porous alumina

The two-dimensional (2D) photonic band gap was observed in naturally formed anodic porous alumina. Anodizing of Al under appropriate conditions yielded the anodic porous alumina which had an ordered air-hole array with a multidomain triangular lattice. The transmission spectra of the multidomain triangular air-hole lattice of the anodic porous alumina showed a clear dip in transmission that corresponded to the 2D photonic band gap. The band gap properties of the polycrystalline-type air-hole lattice could be interpreted as being the sum of those for the single-crystalline-type air-hole arrays of anodic porous alumina prepared by an artificial process.