Birefringence in anodic aluminum oxide: an optical method for measuring porosity

Abstract

Porous anodic aluminum oxide (AAO) is a unique platform for many high-tech applications in nanotechnology, nanophotonics, and membrane science. Although scanning electron microscopy is the technique most widely used to measure the pore diameters of an AAO structure, this method accesses only a limited area of the structure and provides data with limited statistics. The AAO characteristics, averaged over the entire thickness of a porous film, can be measured by optical spectroscopy. In this paper, we demonstrate that the angular dependence of the transmittance of AAO porous films has several features caused by birefringence, which is used to measure the porosity of AAO membranes. This method of measuring porosity is accurate, precise, cheap, and does not require specific sample preparation procedures.