X-ray induced absorption of high-purity sapphire and investigation of the origin of the residual absorption at 1064 nm

We report a set of optical absorption measurements on x-irradiated high-purity sapphire. The results of the photothermal deflection absorption measurements at 1064 nm show an increase of the absorption at the x-irradiated areas. The x-ray damage recovery is achieved through annealing, where the absorption level at 1064 nm is brought from an average level of ~80 ppm cm -1 to ~24 ppm cm -1 . UV-Vis spectroscopy results suggest that the residual absorption at 1064 nm is due to complex clusters of Ti and Fe ions and oxygen vacancies. We suggest that a further reduction of Ti and Fe in sapphire (<0.1 ppm) as well as oxygen vacancies (through post growth oxidizing annealing) would further reduce the absorption. Moreover, the absorption level of 20 ppm cm -1 remains within the requirements of the second generation laser interferometric gravitational-wave detectors.