Measurement of the mechanical loss of a cooled reflective coating for   gravitational wave detection
  We have measured the mechanical loss of a dielectric multilayer reflective coating (ion-beam sputtered SiO$_2$ and Ta$_2$O$_5$) in cooled mirrors. The loss was nearly independent of the temperature (4 K $\sim$ 300 K), frequency, optical loss, and stress caused by the coating, and the details of the manufacturing processes. The loss angle was $(4 \sim 6) \times 10^{-4}$. The temperature independence of this loss implies that the amplitude of the coating thermal noise, which is a severe limit in any precise measurement, is proportional to the square root of the temperature. Sapphire mirrors at 20 K satisfy the requirement concerning the thermal noise of even future interferometric gravitational wave detector projects on the ground, for example, LCGT. 
Kazuhiro Yamamoto, Shinji Miyoki, Takashi Uchiyama, Hideki Ishitsuka,   Masatake Ohashi, Kazuaki Kuroda, Takayuki Tomaru, No...