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 at the Shanghai Institute of optics and Fine Mechanics in a collaboration with the Laser Interferometer Gravitational Wave Observatory Laboratory to overcome these difficulties. By using the directional temperature gradient technique, sapphire crystals of 11 cm in diameter and 8 cm in length were grown at the C plane. 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.

The flexure and compressive strengths of sapphire are dependent on crystal orientation and temperature. Most notably, the c -axis compressive strength decreases below the tensile strength at temperatures >400°C and falls to 2% of the room-temperature compressive strength at 800°C. Loss of compressive strength complicates the interpretation of flexure tests. Four-point flexure specimens with no component of c -axis compression increase in strength at temperatures >500°C; however, specimens that have c -axis compression decrease in strength. It has been observed that c -axis compression causes twinning on rhombohedral crystal planes. Intersection of twins on different rhombohedral planes causes fracture that leads to mechanical failure.


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