Mirror Coating Solution for the Cryogenic Einstein Telescope

This is the authors' abstract. We don't add key points for this paper.

Planned cryogenic gravitational-wave detectors will require improved coatings with a strain thermal noise reduced by a factor of 25 compared to Advanced LIGO. We present investigations of HfO₂ doped with SiO₂ as a new coating material for future detectors. Our measurements show an extinction coefficient of k=6×10⁻⁶ and a mechanical loss of ϕ=3.8×10⁻⁴ at 10 K, which is a factor of 2 below that of SiO₂, the currently used low refractive-index coating material. These properties make HfO₂ doped with SiO₂ ideally suited as a low-index partner material for use with a-Si in the lower part of a multimaterial coating. Based on these results, we present a multimaterial coating design which, for the first time, can simultaneously meet the strict requirements on optical absorption and thermal noise of the cryogenic Einstein Telescope.