  A detailed study of the pore-widening rate of nanoporous anodic alumina
layers as a function of the anodization voltage was carried out. The study
focuses on samples produced under the same electrolyte and concentration but
different anodization voltages within the self-ordering regime. By means of
ellipsometry-based optical characterization, it is shown that in the
porewidening process, the porosity increases at a faster rate for lower
anodization voltages. This opens the possibility of obtaining three-dimensional
nanostructured nanoporous anodic alumina with controlled thickness and
refractive index of each layer, and with a refractive index difference of up to
0.24 between layers, for samples produced with oxalic acid electrolytes.
