The optical constants of bulk and thin-film aluminum were determined at 6328 Å by use of ellipsometry. The samples were measured immersed in a liquid whose refractive index was within ±0.02 of that of oxide films. In addition, the effect of the oxide film on the optical constants of evaporated aluminum was eliminated by making measurements at the metal-glass interface of a strain-free glass prism of known refractive index, and by making measurements in situ at pressures of 2×10-10 torr. Electropolishing of bulk polycrystalline and single-crystal aluminum yielded n’s ranging from 1.63 to 1.72 and k’s ranging from 7.56 to 7.77. The Drude model for 2.6 free electrons yields n = 1.55, k = 7.60; the Kramers-Kronig relationship yields n = 1.40, k = 7.50. In order to determine the effect of evaporation pressure on n and k, aluminum was evaporated at rates of 25 to 30 Å/s onto optically polished fused silica at pressures of l×10-5, 5×l0-8, and 5×10-9 torr; the results were n = 1.70, k = 6.60; n = 1.62, k = 7.29, and n = 1.57, k = 7.52, respectively. The average values for aluminum evaporated at 1×10-9 torr of n and k that agree most closely with both the Drude and Kramers-Kronig relationship were 1.60 and 7.53, respectively. These values were used successfully to monitor the uniformity of samples used in all anodization experiments.
A. C. Nyce and L. P. Skolnick, "Optical constants of bulk and thin-film aluminum at 6328 Å," J. Opt. Soc. Am. 65, 792-796 (1975)