Li‐Deficient, Off‐Congruent MgO:LiNbO3 Crystals Prepared by Postgrown Li‐Poor Vapor Transport Equilibration for Integrated Optics

Abstract

Li‐deficient, off‐congruent Z‐cut MgO:LiNbO3 (MgO:LN) crystals for integrated optics were prepared by carrying out postgrown Li‐poor vapor transport equilibration (VTE) treatments on congruently grown MgO (5 mol% in melt):LiNbO3 plates at 1100°C for durations ranging from 40 to 395 h. Secondary ion mass spectrometry analysis, surface ordinary refractive index measurement, and neutron activation analysis were carried out on the VTE crystals to verify that the Mg and Nb ions did not diffuse out of the crystal during the VTE procedure and their distributions over the whole plate retain its homogeneity. The VTE duration dependence of the Li2O content reduction was determined using gravimetric method, and the crystalline phase was by powder X‐ray diffraction. The results show that the Li2O content decreases with a prolonged VTE and the Li2O content reduction in the saturation regime is about 2.9 mol%. All of the VTE crystals still retain the LN phase, and Li‐vacancy and NbLi are the major defects in the VTE crystal. OH absorption study reveals that the doped MgO concentration is below the photorefractive threshold for all VTE crystals. The optical absorption edge (OAE) of the VTE‐treated MgO:LN was also measured as a function of the VTE duration. On the basis of the known Li2O content and measured OAE, the photon‐energy fit reported previously, valid for the evaluation of Li2O content in a pure LN, is corrected for the Li‐poor VTE‐treated MgO:LNs. Finally, the applicability of the Li‐deficient off‐congruent MgO:LN crystals prepared by the Li‐poor VTE method is demonstrated by characterizing the optical damage and Er diffusion properties of a single‐mode Ti:MgO:Er:LiNbO3 strip waveguide fabricated on an Li‐poor VTE‐prepared MgO:LN crystal.