Low cryogenic mechanical loss composite silica thin film for low thermal noise dielectric mirror coatings.

Thermal noise in dielectric mirror-coatings is the limiting factor for ultra-high-precision metrologies employing optical cavities and being operated at cryogenic temperatures. Silica film is an indispensable low-refractive-index material for mirror coatings but suffers from high cryogenic mechanical loss and hence contributes to high thermal noise. We partitioned a thick silica film into thin layers by introducing blocking layers of titania. The cryogenic mechanical loss of silica was significantly suppressed; the effect was more profound for thinner partitions. Elimination of the transitions of the long-range two-level systems with scales that exceed the thickness of the silica partition is hypothesized. Dielectric mirror coatings with blocking layers are proposed to reduce the cryogenic thermal noise of the coatings. The calculated reflectance spectrum is consistent with that of the conventional quarter-wave (QW) stack around 1550 nm, and the calculated absorptance increases 2.2 times over that of a conventional titania/silica QW stack where the titania is absorptive.