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.

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). The prototypes were deposited by ion beam sputtering on silicon substrates. All prototypes were found to be amorphous as-deposited. Thermal annealing in air at progressive temperatures was subsequently performed. It was found that the titania layers eventually crystallized forming the anatase phase, while the silica layers remained always amorphous. However, progressively thinner layers exhibited progressively higher threshold temperatures for crystallization onset. Accordingly it can be expected that composites with thinner layers will be able to sustain higher annealing temperatures without crystallizing, and likely yielding better optical and mechanical properties for advanced coatings application. These results open the way to the use of materials like titania and hafnia, that crystallize easily under thermal anneal, but ARE otherwise promising candidate materials for HR coatings necessary for cryogenic 3rd generation laser interferometric gravitational wave detectors.