Mirror coating solution for the cryogenic Einstein telescope

Craig, Kieran and Steinlechner, Jessica and Murray, Peter G. and Bell, Angus S. and Birney, Ross and Haughian, Karen and Hough, Jim and MacLaren, Ian and Penn, Steve and Reid, Stuart and Robie, Raymond and Rowan, Sheila and Martin, Iain W. (2019) Mirror coating solution for the cryogenic Einstein telescope. Physical Review Letters, 122 (23). 231102. ISSN 1079-7114 (https://doi.org/10.1103/PhysRevLett.122.231102)

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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.