Mirror Coating Solution for the Cryogenic Einstein Telescope.

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 SiO(2 )as a new coating material for future detectors. Our measurements show an extinction coefficient of k = 6 x 10(-6) and a mechanical loss of phi = 3.8 x 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.