The role of $TiO_{2}$ in the optical coatings for gravitational wave detectors.

Since the expected gravitational wave signals are extremely weak, reducing the predominant noise, $i.e.$, thermal noise due to the highly reflective optical coatings of the cavity mirrors, is the biggest challenge for today's detectors. The current mirrors in the operating interferometers are based on the alternation of two materials with high ($TiO_{2}$ doped $Ta_{2}O_{5}$) and low ($SiO_{2}$) refractive index, selected to achieve the lowest optical and mechanical losses, and hence the lowest noise. In particular, the high-index material suffers considerably from mechanical losses, which can be reduced by a nanolayering strategy in which this homogeneous layer is replaced with layered nano-composites of dielectric oxides. In this scenario, $TiO_{2}$ material represents a good candidate for its low mechanical losses and its higher crystallization temperature. We present an experimental study of the morphological and structural properties of $TiO_{2}$ in the form of thin films and in nanolayered structure with other metal oxides, in order to address the extent of the structural and morphological reliability for high-refractive index candidates in the new-generation Bragg-like reflectors.