Mechanical loss of a multilayer tantala/silica coating on a sapphire disk at cryogenic temperatures: Toward the KAGRA gravitational wave detector

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We report the results of a new experimental setup to measure the mechanical loss of coating layers on a thin sapphire disk at cryogenic temperatures. Some of the authors previously reported that there was no temperature dependence of the mechanical loss from a multilayer tantala/silica coating on a sapphire disk, both before and after heat treatment, although some reports indicate that ${\mathrm{Ta}}_{2}{\mathrm{O}}_{5}$ and ${\mathrm{SiO}}_{2}$ layers annealed at $600\text{ }\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ have loss peaks near 20 K. Since KAGRA---the Japanese gravitational-wave detector, currently under construction---will be operated at 20 K and have coated sapphire mirrors, it is very important to clarify the mechanical loss behavior of tantala/silica coatings around this temperature. We carefully investigate a tantala/silica-coated sapphire disk with the new setup, anneal the disk, and then investigate the annealed disk. We find that there is no distinct loss peak both before and after annealing under particular conditions. The mechanical loss for the unannealed disk at 20 K is about $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$, as previously reported, while that for the annealed disk is approximately $6.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$.

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