Development of large size sapphire crystals for laser interferometer gravitational-wave observatory

Because of its high density and superior quality factor, sapphire crystal, as a candidate material for test masses, has attracted much attention in gravitation wave communities. The use of sapphire crystal, however, is limited by its size, homogeneity, and absorption. An effort has been made to overcome these difficulties at the Shanghai Institute of Optics and Fine Mechanics in collaboration with the Laser Interferometer Gravitational-Wave Observatory Laboratory. By using a directional temperature gradient technique (TGT), sapphire crystals of 11 cm in diameter and 8 cm in length were grown at the C plane (0001). The results indicate that a homogeneity of <5×10^-7 and an absorption of 35-65 ppm/cm have been achieved at wavelength of 1.06 μm. This paper presents the TGT growth of sapphire crystals, their transmittance spectra, optical homogeneity, and absorption. Future applications for gravitational wave experiments are discussed.