Judd-Ofelt spectroscopic study of Mg/Er-codoped near-stoichiometric LiNbO₃ crystals for integrated optics

Near-stoichiometric (NS) (Mg:)Er:LiNbO₃ crystals were grown from melts containing 0.0/0.5, 0.5/0.5, and 1.0/0.5 mol%/mol% MgO/Er ₂O₃. Crystal composition and optical properties studies show that the Li₂O contents in these crystals increase all by ∼1 mol% relative to the congruent point. The 1.0 mol% MgO-doped NS crystal is just near optical damage threshold and withstands a 488 nm light intensity >0.74 MW/cm² without optical damage. Unpolarized absorption spectra of these NS crystals were measured, and the Er³⁺ absorption cross-section spectra were determined. The Er3+ spectroscopic properties were studied by Judd-Ofelt theory. The results show that as the crystal composition approaches the stoichiometry, the Er³⁺ spectroscopic properties change definitely. The Er³⁺ ion in the NS crystal has smaller absorption cross section and hence weaker oscillator strength, lower emission rate, and longer radiative lifetime. Nevertheless, the radiative quantum efficiency is retained. In addition, the MgO codoping has less effect on the Er³⁺ spectroscopic properties.