Low Mechanical Loss TiO_{2}:GeO_{2} Coatings for Reduced Thermal Noise in Gravitational Wave Interferometers.

Abstract

The sensitivity of current and planned gravitational wave interferometric detectors is limited, in the mostcritical frequency region around 100Hz, by a combination of quantum noise and thermal noise. Thelatter is dominated by Brownian noise: thermal motion originating from the elastic energy dissipation in the dielectric coatings used in the interferometer mirrors. The energy dissipation is a material property characterized by the mechanical loss angle. We have identified mixtures of titanium dioxide (TiO_{2}) andgermanium dioxide (GeO_{2}) that show internal dissipations at a level of 1×10^{-4}, low enough to provide improvement of almost a factor of 2 on the level of Brownian noise with respect to the state-of-the-art materials. We show that by using a mixture of 44% TiO_{2} and 56% GeO_{2} in the high refractive indexlayers of the interferometer mirrors, it would be possible to achieve a thermal noise level in line withthe design requirements. These results are a crucial step forward to produce the mirrors needed to meetthe thermal noise requirements for the planned upgrades of the Advanced LIGO (Laser Interferometer Gravitational-Wave Observatory) and Virgo detectors.