Optical properties of Au and Al blacks: annealing effects

Optical properties of Au and Al blacks have been studied in the spectral range from 0.35 to 1.0 mu m. The films, formed by thermal evaporation in a He atmosphere of 1 torr, were submitted to heat treatments at 250 degrees C and changes in their optical properties were recorded and analysed. It appears then that the metal particles forming the Al blacks would be of very fine dimensions with properties differing considerably from the bulk properties of Al. Annealing in air of the Al blacks would promote further oxidation of the metal particles, making the films more transparent. In the case of Au blacks annealed in air for 200 h, the optical properties are reasonably well described by the Maxwell-Garnett theory, assuming bulk properties for the metal particles forming the films