Abstract
A stack of Ta2O5/SiO2 layers is presently used as coating layer of mirrors in interferometric detectors for gravitational waves. The sensitivity of these detectors is limited in the 50–300Hz frequency range by the mirror thermal noise, and it was suggested that mechanical losses in the Ta2O5 are the dominant source of noise. We focus here on Spectroscopic Ellipsometry (SE) results (in the 0.75÷5eV spectral range) obtained on high quality Ta2O5 films deposited on SiO2 substrates by Double Ion Beam Sputtering at the Laboratoire des Matériaux Avancés (Lyon, France). The films are extremely flat as indicated by the 0.2nm RMS roughness determined by Atomic Force Microscopy (AFM) on (20×20)μm2 areas. The comparison of the optical properties determined by SE with literature data, corroborated by X-ray Photoelectron Spectroscopy (XPS) data, suggests that the films present a non-ideal bulk stoichiometry and/or some degree of nanoporosity. The possible influence of an interface layer is also discussed.
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