The OA cell is comprised of two polished quartz plates of 6 mm in thickness and 25 mm in diameter and a quartz cylinder of 5 mm in thickness and 20 mm in hole diameter in between the quartz plates. The outer diameter of the cylinder was the same with those for the quartz plates. All the surfaces of the cell components were gently polished and made plane parallel with the thickness variation of about 2 μm to avoid any possible distortions of the OA signal PROFILE due to a non-parallelism of OA source and detector surfaces.