Graded index profile of anodic alumina films that is induced by conical pores

The size and the shape of micropores in anodic alumina films are studied by electron microscope observation and mercury intrusion porosimetry. Pores have conical shapes with ~ 10-nm diameters and are widened at the rate of ~ 0.05 nm/min by immersion in a sulfuric acid solution. Such conical pores give rise to the refractive-index distribution and anisotropy in the alumina films. By analyzing interference fringes, we find that a graded-index profile, as well as birefringence, exists in the anodic alumina films. The experimental results are in good agreement with theoretical predictions.