Interferometric gravitational-wave detectors have opened a new window into the Universe, creating exciting opportunities in the field of astrophysics. While current detectors enable successful detections, there is a need to increase their sensitivity and expand their frequency band to observe additional types of gravitational-wave sources. One of the primary limitations is thermal noise originating from the mirror coatings, which restricts the detectors' performance in their central and most sensitive frequency region. This noise source will also impact future detectors, such as the Einstein Telescope.

Thermal noise arises from thermally activated transitions between equilibrium configurations within the material’s structure and is quantified by the so-called mechanical loss angle. As a result, thermal noise is determined by the atomic organization of the material. In the context of optical measurements, Urbach tails describe a sub-gap exponential broadening of the absorption edge, which is linked to structural and thermal disorder. Consequently, understanding the structural origin of these exponential tails provides valuable insights into the atomic organization of the material.

In this seminar, we will examine how the mirrors used in gravitational-wave detectors must simultaneously meet specific mechanical and optical requirements. Moreover, although optical and mechanical properties can be measured independently, a correlation has been observed between the mechanical loss angle and Urbach tails. This finding presents new opportunities for advancing research on optical coatings for gravitational-wave detectors.