Light scattering in porous anodic aluminium oxide films II. Polarization effects

Abstract

The apparent optical anisotropy of porous anodic oxide films grown on polycrystalline aluminium, which manifests itself as a “grain contrast” effect when a detached oxide layer is observed by transmitted light between crossed polarizers, is explained by consideration of the forward scattering of light at the pores present in such oxide layers. An essential factor is the relative tilting of pores which have grown over different substrate grains. It is shown that the pores do not grow preferentially normal to a particular crystallographic plane in the Al grain, but normal to the local oxide-metal interface, which itself becomes tilted at the beginning of anodizing through orientation-dependent electrochemical attack.