Ion-beam sputtered amorphous silicon films for cryogenic precision measurement systems

Thermal noise resulting from the mechanical loss of multilayer dielectric coatings is expected to impose a limit to the sensitivities of precision measurement systems used in fundamental and applied science. In the case of gravitational wave astronomy, future interferometric gravitational wave detectors are likely to operate at cryogenic temperatures to reduce such thermal noise and ameliorate thermal loading effects, with the desirable thermomechanical properties of silicon making it an attractive mirror substrate choice for this purpose. For use in such a precision instrument, appropriate coatings of low thermal noise are essential. Amorphous silicon (a−Si) deposited by e-beam and other techniques has been shown to have low mechanical loss. However, to date, the levels of mechanical and optical loss for a−Si when deposited by ion-beam sputtering (the technique required to produce amorphous mirrors of the specification for gravitational wave detector mirrors) are unknown. In this paper results from measurements of the mechanical loss of a series of IBS a−Si films are presented which show that reductions are possible in coating thermal noise of a factor of 1.5 at 120 K and 2.1 at 20 K over the current best IBS coatings (alternating stacks of silica and titania-doped tantala), with further reductions feasible under appropriate heat treatments