A study of the optical properties of nickel-pigmented anodic alumina in the infrared region

The reflectance of nickel-pigmented anodized thin alumina films on aluminium has been measured in the infrared range between 5 and 20 mu m. The strong absorption of plain porous alumina, found at around 10 mu m for p-polarized light at an oblique angle of incidence and originating from the longitudinal optical phonons interacting with light, is reduced for pigmented alumina. Electrochemical deposition of nickel in the pores of anodic alumina starts from the bottom of the pores, forming a nickel-pigmented bottom layer as the time of pigmentation increases. It is shown from p-polarized reflectance measurements at an oblique angle of incidence that with increasing thickness of the nickel-filled bottom layer the LO mode absorption is decreasing. This is consistent with effective-medium calculations which have been performed using the Maxwell Garnett model. It is found to be an effect of the geometrical shape and orientation of the nickel particles, which for these samples appear as rods with their long axes perpendicular to the sample surface.