coatings grown by biased target ion beam deposition for gravitational wave detectors*

Advanced LIGO employs as end masses high reflectance mirrors of alternating layers of SiO₂ and Ta₂O₅ doped with 25% of TiO₂. Doping Ta₂O₅ with 25% TiO₂ decreased the mechanical loss which increased the sensitivity of the detector, but the physical reasons of this are still unknown. In this work we studied thin films of Ta₂O₅ doped with different TiO₂ concentrations grown by biased target ion beam deposition to evaluate the effect of doping in the mechanical loss and to search for high index materials with improved performance for the upcoming upgrade of LIGO and future detectors.
The deposition system consists of a low energy ion source and metallic targets individually pulsed biased. Control of each target bias allows for mixing Ta₂O₅ and TiO₂, with estimates of the doping obtained by x-ray photoelectron spectroscopy. Extensive characterization shows the films are nearly stoichiometric and dense, with an absorption loss at 1064 nm lower than 20 ppm. Mechanical loss was measured in as deposited coatings and after annealing. For all TiO₂ concentrations the mechanical loss is reduced after annealing, reaching a minimum for around 20% TiO₂ after annealing at 600 C comparable to state-of-the-art TiO₂ doped Ta₂O₅.