Optical Properties of Magnesium Fluoride Films in the Ultraviolet

Abstract

The dispersion of films of magnesium fluoride evaporated on quartz substrates was determined interferometrically in the ultraviolet from the wavelengths of the maxima and minima of reflectance and the thickness of the film. The refractive index of freshly deposited films was 1.375 at 4200 A and increased to 1.42 at 2100 A. The index increased approximately 1% as the films aged. The index of refraction of magnesium fluoride at the air-film and film-substrate interfaces, and the extinction coefficient, were determined from the reflectance and transmittance at normal incidence. The index at the film-substrate interface was consistent with the interferometrically determined index for the new and aged films. However, the apparent refractive index at the air-film interface depended upon the deposition rate and the thickness of the film and coincided with the interferometrically measured index only for the thinnest films investigated. The equations for the transmittance and reflectance of absorbing inhomogeneous films are derived. The density of the films was 2.96 g/cm³ and together with thickness was invariant with age.