Amorphous mixed titania-tantala
coatings are key components of
Bragg reflectors in gravitational-wave detectors (GWDs). Attaining
the lowest possible values of optical absorption and mechanical losses
in coatings is of paramount importance for GWDs, and this requires
a complex optimization of the coating deposition and postdeposition
annealing. We present here a systematic investigation of the optical
properties and internal friction of amorphous mixed titania-tantala
coatings grown by ion beam sputtering. We consider coatings with six
different cation mixing ratiosdefined as Ti/(Ti + Ta)and
we study them both in the as-deposited and annealed states. All coatings
have been subject to the same annealing of 500 °C for 10 h in
air, which is the postdeposition treatment adopted so far for Bragg
reflectors in GWD applications. By exploiting spectroscopic ellipsometry
data and modeling, along with ancillary techniques, we retrieved the
dielectric function of the coatings in a wide spectral range. When
varying the mixing ratio and performing the annealing, we find monotonicin
some cases, almost lineartrends for most of the aforementioned
properties. Remakably, the postannealing Urbach energy displays a
definite minimum for a mixing ratio around 20%, very close to the
composition of the coatings showing the lowest optical absorption
for GWD applications. We suggest that the observed minimum in the
Urbach energy depends not only on the mixing ratio but also on the
annealing parameters. On the other hand, the minimum coating loss
angle was found to be weakly dependent on the considered measurement
frequency and to lie within a rather broad range of Ti content (cation
ratios of 21 and 44%), suggesting that the search for an absolute
minimum following postdeposition annealing should be rather sought
in the study of the best annealing parameters for each specific cation
ratio considered. This work constitutes a reference for the optical
properties of the amorphous mixed titania-tantala coatings and highlights
the relevance of the Urbach energy as an additional parameter to guide
the optimization process of materials for high-performing Bragg reflectors.