Low Mechanical loss coatings for reduced thermal noise in gravitational wave detectors

The sensitivity of current gravitational wave interferometric detectors is limited by Brownian noise: thermal motion originating from the elastic energy dissipation in the dielectric coatings used in the interferometer mirrors. We have identified mixtures of titanium dioxide (TiO2) and germanium dioxide (GeO2) that allows an improvement of almost a factor of 2 on the level of Brownian noise with respect to the state-of-the-art materials, the largest reduction in almost two decades of research. We show that by using a mixture of 44% TiO2 and 56% GeO2 in the high refractive index layers of the interferometer mirrors, it would be possible to achieve a thermal noise level in line with the design requirements for future upgrades. These results are a crucial step forward to produce the mirrors needed to meet the thermal noise requirements for the planned upgrades of the Advanced LIGO (Laser Interferometer Gravitational-Wave Observatory) and Virgo detectors.