Low thermal noise mirror coatings utilising titanium dioxide and germanium dioxide mixtures

• DOI: 10.48550/arxiv.2502.07679
• Publication Date: 2025-02-11
• Authors: Mariana Fazio; I. W. Martin; Peggy S. M. Hill; M. Ben Yaala; M. Chicoine; Caspar Clark; Nicholas Demos; M. M. Fejer; D. R. Gibson; S. Gras; J. Hough; A. S. Markosyan; G. I. McGhee; S. Rowan; J. R. Smith; F. Schiettekatte; S. C. Tait; G. Vajente; S. Reid
• Keywords: Titanium Dioxide; Silicon dioxide

Upgrades to ground-based gravitational-wave observatories will require mirror coatings with reduced thermal noise, enabling improved detector sensitivity and extended astrophysical reach. Recent studies have shown that optical utilising amorphous materials exhibit a larger fraction of corner-sharing between adjacent structural units metal-centered polyhedra are promising route for reducing mechanical dissipation thus noise at room temperature. We report on multilayer fabricated using germanium dioxide mixed titanium (TiO$_2$:GeO$_2$) the high index layers, silicon (SiO$_2$) low material. Single layers TiO$_2$:GeO$_2$ characterised optimise mixture proportion based highly reflective stacks were deposited. Exceptional absorption 1064 nm below 1 part-per-million (ppm) is observed in after heat treatment. The annealing process also induces formation blisters which leads increased scattering. However, there indication can be suppressed by decreasing water partial pressure deposition chamber. Direct measurements provide experimental verification significant 25\% reduction over mirrors currently employed, combined sub-ppm levels show potential improve observatories.