摘要
The use of amorphous, SiO 2 -TiO 2 nanolayered coatings has been proposed recently for the mirrors of 3rd-generation interferometric detectors of gravitational waves, to be operated at low temperature. Coatings with a high number of low-high index sub-units pairs with nanoscale thickness were found to preserve the amorphous structure for high annealing temperatures, a key factor to improve the mechanical quality of the mirrors. The optimization of mirror designs based on such coatings requires a detailed knowledge of the optical properties of sub-units at the nm-thick scale. To this aim we have performed a Spectroscopic Ellipsometry (SE) study of amorphous SiO 2 -TiO 2 nanolayered films deposited on Si wafers by Ion Beam Sputtering (IBS). We have analyzed films that are composed of 5 and 19 nanolayers (NL 5 and NL 19 samples) and have total optical thickness nominally equivalent to a quarter of wavelength at 1064 nm. A set of reference optical properties for the constituent materials was obtained by the analysis of thicker SiO 2 and TiO 2 homogeneous films (∼ 120 nm) deposited by the same IBS facility. By flanking SE with ancillary techniques, such as TEM and AFM, we built optical models that allowed us to retrieve the broad-band (250–1700 nm) optical properties of the nanolayers in the NL 5 and NL 19 composite films. In the models which provided the best agreement between simulation and data, the thickness of each sub-unit was fitted within rather narrow bounds determined by the analysis of TEM measurements on witness samples. Regarding the NL 5 sample, with thickness of 19.9 nm and 27.1 nm for SiO 2 and TiO 2 sub-units, respectively, the optical properties presented limited variations with respect to the thin film counterparts. For the NL 19 sample, which is composed of ultrathin sub-units (4.4 nm and 8.4 nm for SiO 2 and TiO 2 , respectively) we observed a significant decrease of the IR refraction index for both types of sub-units; this points to a lesser mass density with respect to the thin film reference. The results are discussed in the light of the existing literature on nanofilms of amorphous oxides.