Spectroscopic site determinations in erbium-doped lithium niobate

Rare-earth (e.g., Er³⁺, Nd³⁺) based, guided wave optical amplification in lithium niobate (LiNbO₃) integrated optic systems is a new and important addition to the field of integrated optics. The application of total site selective spectroscopy to rare-earth-doped LiNbO₃ provides the most complete spectroscopic characterization of this class of materials to date. In a previous publication we identified six spectroscopic sites in Er:LiNbO₃ using total site selective spectroscopy, two of which are cluster sites which upconvert light using nonradiative energy transfer between Er ions within a given site. In this paper Er³⁺ site identifications are made based on a consideration of solid solution defect equations in conjunction with an experimental study of the site distribution as a function of dopant concentration (0.4--2.0 mol % Er:LiNbO₃) and the Li/Nb ratio in the crystal. The Li/Nb ratio was altered using a vapor phase equilibration technique. Our results indicate that increasing the Li₂O content of Er:LiNbO₃ not only reduces the cluster site concentration by {~}30% but also increases the amount of light absorbed in the crystal by {~}15%. This observation is, to the best of our knowledge, the first report of post growth materials processing in rare-earth-doped LiNbO₃ to effect a change in absorption or cluster site concentration. In addition, increasing the dopant concentration increases Li₂O deficiency in Er:LiNbO₃ crystals. Simple solid solution defect model calculations agree with these experimental results. {} 1996 The American Physical Society.