Self-consistent optical constants of SiO₂ and Ta₂O₅ films

Luis V. Rodríguez-de Marcos, Juan I. Larruquert, José A. Méndez, and José A. Aznárez

Abstract

Self-consistent optical constants of SiO₂ and Ta₂O₅ films have been obtained for their relevance in optical coatings from the near ultraviolet to the near infrared spectral ranges, where they are transparent and have a high refractive index contrast. Particular attention has been paid to wavelengths close to and shorter than each material cutoff. The far and the extreme ultraviolet ranges are also covered here, where few (SiO₂) or almost no optical constant data (Ta₂O₅) were available for films of these materials. This work is aimed at filling the lack of self-consistent sets of optical constants with data in a very broad spectral range, which can be widely applied in multilayer design for the everyday use of these materials in multilayer coatings. Oxide films were deposited by reactive electron-beam evaporation onto various sorts of substrates at 573 K. Transmittance, reflectance, and ellipsometry measurements were performed for each oxide in spectral intervals jointly covering from the extreme ultraviolet to the near infrared; starting with these measurements along with extrapolations, an iterative and double Kramers-Kronig analysis procedure has been followed to obtain a self-consistent set of optical constants per material. With the final data sets, we have satisfactorily reproduced the experimental measurements. Global data self-consistency was successfully evaluated through sum rules, and local consistency at each photon energy range was also evaluated through a novel sum-rule method which involves window functions.