Origins of fundamental limits for reflection losses at multilayer dielectric mirrors

Fundamental limits on reflection losses are set by internal material losses associated with the Urbach tail near a band gap and by thermodynamic density fluctuations in fabrication. In materials such as SiO(2) and TiO(2), these limits are of the order of parts in 10(9). The current quality of supercavity mirrors, in contrast to that of optical fibers, is still far from these limits because of purely technological limitations in surface preparation and in the reduction of impurity levels. Overcoming these would greatly benefit, for example, Fabry-Perot interferometers, ring lasers, and gravitational wave detectors.