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.

Thickness-dependent crystallization on thermal anneal for titania/silica nm-layer composites deposited by ion beam sputter method.

Crystallization following thermal annealing of thin film stacks consisting of alternating nm-thick titania/silica layers was investigated. Several prototypes were designed, featuring a different number of titania/silica layer pairs, and different thicknesses (in the range from 4 to 40 nm, for the titania layers), but the same nominal refractive index (2.09) and optical thickness (a quarter of wavelength at 1064 nm).