Optical properties of germania and titania at 1064 nm and at 1550 nm

One of the main noise sources in current gravitational-wave detectors is the thermal noise of the high-reflectivity coatings on the main interferometer optics. Coating thermal noise is dominated by the mechanical loss of the high-refractive index material within the coating stacks, Ta 2 O 5 mixed with TiO 2 . For upgrades to room-temperature detectors, a mixture of GeO 2 and TiO 2 is an interesting alternative candidate coating material. While the rather low refractive index of GeO 2 increases with increasing TiO 2 content, a higher TiO 2 content results in a lower threshold temperature before heat treatment leads to crystallisation, and potentially to a degradation of optical properties. For future cryogenic detectors, on the other hand, a higher TiO 2 content is beneficial as the TiO 2 suppresses the low-temperature mechanical loss peak of GeO 2 . In this paper, we present the optical properties of coatings—produced by plasma-assisted ion-beam evaporation—with high TiO 2 content at 1550 nm, a laser wavelength considered for cryogenic gravitational-wave detectors, as a function of heat-treatment temperature. For comparison, the absorption of pure GeO 2 was also measured. Furthermore, results at the currently-used wavelength of 1064 nm are presented.