Line-focus-beam acoustic microscopy characterization of optical-grade LiTaO3 single crystals

Abstract

Basic data for evaluating homogeneities of LiTaO3 single crystals for optical use are investigated by line-focus-beam acoustic microscopy. First, the relationships among leaky surface acoustic-wave (LSAW) velocities, chemical compositions, Curie temperatures, densities, and lattice constants are experimentally investigated as the calibration lines for crystal evaluation using X-, Y-, and Z-cut substrates prepared from three LiTaO3 single crystals grown along the crystallographic Y axis with different Li2O contents ranging from 48 to 49 mol %. It is shown that as the Li2O content increases around the congruent composition, the LSAW velocities linearly increase for all specimen surfaces and all propagation directions, and the increase rate is maximum for Z-cut, Y-axis propagating (ZY) LiTaO3. Next, homogeneities of the above three crystals and a commercially available optical-grade crystal are evaluated using ZY-LiTaO3 specimens and the obtained calibration lines. The chemical composition variations along both the pulling direction and the diameter direction are successfully detected as LSAW velocity variations, due to the changes of crystal-growth conditions. In the commercial crystal, the LSAW velocity variations for the whole examined region (80 mm×60 mm) exhibit a maximum difference of 0.82 m/s, corresponding to the composition change of 0.026 Li2O mol %. It is demonstrated that this ultrasonic method has the unique and useful capabilities of detecting changes of the growth conditions and of evaluating local densities and the crystal-melt interface shape.