Excess mechanical loss associated with dielectric mirror coatings on test masses in interferometric gravitational wave detectors

Interferometric gravitational wave detectors use mirrors whose substrates are formed from materials of low intrinsic mechanical dissipation. The two most likely choices for the test masses in future advanced detectors are fused silica or sapphire (Rowan S et al 2000 Phys. Lett. A 265 5). These test masses must be coated to form mirrors, highly reflecting at 1064 nm. We have measured the excess mechanical losses associated with adding dielectric coatings to substrates of fused silica and calculated the effect of the excess loss on the thermal noise in an advanced interferometer.