Characterization of Berreman modes in metal/dielectric Ag/SiO2 and Ag/MgF2 multilayers

The study of Berreman modes, well known in dielectric layers deposited on metallic substrates, is extended in this paper to metal/dielectric multilayers. Infra-red reflectance measurements were performed from 0.05 eV to 0.24 eV under conditions of nonnormal-incidence and polarized light incident on Ag/SiO2 and Ag/MgF2 multilayers prepared respectively by means of magnetron sputtering and thermal evaporation. Optical ellipsometric measurements were performed in the infra-red on the Ag/SiO2 multilayers. In the case of p-polarized light, strong absorptions appear, near the longitudinal optical frequencies of SiO2 and MgF2, characteristic of the Berreman modes. Other structures were observed in the reflectance and in the ellipsometric parameters at the transverse optical frequencies of SiO2 and MgF2. We compare here the results of the two types of optical measurement, and we conclude that photometry and ellipsometry clearly indicate the Berreman modes, ellipsometry giving more information than photometry. We discuss the dependence of these effects on the angle of incidence, the dielectric layer thickness, and the number of periods of the multilayers.