Mechanical loss of the reflective coating and fluorite at low temperature

In some future interferometric gravitational wave detector projects, LCGT and EGO, the mirrors will be cooled so as to reduce the thermal noise. In order to evaluate the thermal noise of such cryogenic interferometers, the mechanical loss of the reflective coating of the mirrors was measured at low temperature. The measured loss angles were about 4 × 10^-4 and constant between 4 K and 300 K. This result implies that the thermal noise of the coating is proportional to the square root of the temperature. The cooling mirror suppresses the thermal noise of the coating loss effectively, which satisfies the goal sensitivity of the future projects (h ∼ 3 × 10 ^-24 Hz^-1/2 at 100 Hz). The Q-values of fluorite, which is a candidate for the mirror substrate of the cryogenic interferometer, were also measured at low temperature. The measurement showed that the Q-value below 12 K is at least 10⁷. The fluorite mirrors must be cooled down to less than 5 K if they are to be used in future projects.