Perfect two-dimensional porous alumina photonic crystals with duplex oxide layers

A perfect two-dimensional porous alumina photonic crystal with 500 nm interpore distance was fabricated on an area of $\text{4\hspace{0.5em}}{\mathrm{cm}}^2$ via imprint methods and subsequent electrochemical anodization. By comparing measured reflectivity with theory, the refractive indices in the oxide layers were determined. The results indicate that the porous alumina structure is composed of a duplex oxide layer: an inner oxide layer consisting of pure alumina oxide of 50 nm in thickness, and an outer oxide layer of a nonuniform refractive index. We suggest that the nonuniform refractive index of the outer oxide arises from an inhomogeneous distribution of anion species concentrated in the intermediate part of the outer oxide.