THE EXPERIMENTAL DIELECTRIC FUNCTION OF POROUS ANODIC ALUMINA IN THE INFRARED REGION - A COMPARISON WITH THE MAXWELL-GARNETT MODEL

The infrared reflectance from thin alumina films on metal substrates h as a deep minimum for p-polarized light at oblique incidence. This ori ginates from absorption when light couples with a longitudinal optical (LO) phonon mode with k-vector zero. The absorption band is wide for amorphous alumina and is shifted to longer wavelengths for porous oxid es compared to non-porous ones. Anodic alumina, prepared in phosphoric acid, with a pore volume fraction of 0.3, has been investigated. The s- and p-polarized reflectance has been measured for selected angles o f incidence between 15 degrees and 75 degrees, and the dielectric func tion has been determined from these measurements. The effective dielec tric function has been calculated using Maxwell-Garnett effective-medi um theory for a two-component anisotropic medium consisting of air-fil led cylindrical pores perpendicular to the surface in an alumina matri x with optical constants of nonporous evaporated alumina. The theoreti cal and experimental results are in good agreement, which shows that t he redshift of the LO mode absorption for p-polarized light can be exp lained by the presence of pores.