Off-Congruent LiNbO3 Crystals Prepared by Li-Poor Vapor Transport Equilibration for Integrated Optics

Abstract

X- and Z-cut off-congruent LiNbO3 (LN) crystals for use for integrated optics were prepared by carrying out Li-poor vapor transport equilibration (VTE) treatment on 1-mm-thick congruent plates at 1100 °C for durations ranging from 65 to 170 h. The Li2O contents and crystalline phase in these Li-poor VTE-treated crystals were characterized by using Raman scattering, optical absorption, and powder X-ray diffraction spectroscopy. The studies of optical absorption in the ultraviolet region show that the Li-poor VTE treatments result in different extents of red-shifts of the optical absorption edges (OAEs), and the amount of the red-shift increases with the prolonged VTE duration. The mean Li2O contents in all VTE samples were evaluated from the measured OAEs. The results show that the VTE-induced reduction of the mean Li2O content can reach as much as ∼1 mol% for both cases of X-cut and Z-cut, and the mean Li2O content and the VTE duration follows the linear relations: cLi = (48.62 ± 0.06) − (0.0052 ± 0.0004)t for the X-cut crystals and cLi = (48.53 ± 0.08) − (0.0063 ± 0.0005)t for the Z-cut crystals. The out-diffusion of Li shows a slight anisotropy. The diffusivity in a Z-cut crystal is slightly larger. The results of visible absorption, Raman scattering, and powder X-ray diffraction show that all the VTE crystals still retain the LiNbO3 phase, consistent with the result predicted from the LiNbO3 phase diagram. The critical VTE duration, longer than which the VTE will induce LiNb3O8 phase in the crystal, is roughly evaluated to be about 577 and 476 h for an X-cut and a Z-cut plate, respectively.