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

15 ultra-pure sapphire single crystals of 32 mm diameter and 100 mm long were grown under a stationary stable regime using the Czochralski (Cz) technique. Despite varying several growth parameters, the obtained crystals were transparent without visible macroscopic defects such as cracks, inclusions and grain boundaries. The optical absorption coefficients (α) at the 1064 nm wavelength of the grown sapphire crystals were mapped by Photothermal Deflection Spectroscopy (PDS). The crystals grown along c-axis using low pulling rates lower than 1.5 mm h⁻¹, at 1064 nm present an very low optical absorption coefficients (α = 11 ppm cm⁻¹). At such low levels, the origin of the optical absorption has never been explained. However, in this paper we point toward the role of Fe²⁺–Fe³⁺ and Ti³⁺–Ti³⁺ 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.