Enabling cryogenic gravitational wave detectors: growth of sapphire crystals with record low absorption in the near infrared

15 ultra-pure sapphire single crystals of diameter 32 mm and 100 mm long were grown under stationary stable regime using Czochralski (Cz) technique. Despite varying several growth parameters, the obtained crystals were transparent without visible macroscopic defects such as cracks, inclusions and grains boundaries. The optical absorption coefficients () at the 1064 nm wavelength of the grown sapphire crystals were mapped by the Photothermal Deflection Spectroscopy (PDS). C-axis crystals grown using low pulling rates of 1.5 mm/h and lower present very low 1064nm optical absorption coefficients of α=11 ppm/cm. At such low levels the origin of the optical absorption has never been explained. However, in this paper we point toward the role of Fe 2+ -Fe 3+ and Ti 3+ -Ti 3+ pairs. These results open the route to upscaling the growth process to achieve ultra-large sapphire crystals with outstanding optical performances, which are a key for the next generation of gravitational wave detectors.