Stoichiometry-Controlled Defect Evolution and Enhanced Up-conversion in In: Dy: LiNbO ₃ Crystals

Controlling the [Li]/[Nb] stoichiometry in In 3+ /Dy 3+ co-doped LiNbO 3 is demonstrated as a powerful defect-engineering strategy that not only enhances up-conversion luminescence but, counterintuitively, dramatically shortens the emission lifetime. Single crystals with nominal [Li]/[Nb] ratios spanning 0.946-1.38 were grown by the Czochralski method and systematically characterized. The Li-rich crystal ([Li]/[Nb]=1.38) exhibits 2.77-fold and 2.51-fold increases in yellow (583 nm) and red (654 nm) up-conversion intensity, respectively, while the 4 F 9/2 lifetime plummets from 288 μs to 42 μs. This paradoxical behavior is unraveled through a combination of defect-sensitive vibrational spectroscopy, Judd-Ofelt analysis, and a semi-quantitative kinetic rate model. As the stoichiometry shifts from Li-deficient to Li-rich, the dominant defect transforms from lithium vacancy/anti-site niobium complexes to dopant on Nb site configurations, promoting Nb-site-related, lower-symmetry local environments around Dy 3+ . This stoichiometry driven site occupation change induces a marked boost of the radiative transition probability (k r ), which outcompetes the concomitant increase in non-radiative decay rate (k nr ), thereby simultaneously enhancing emission intensity and shortening lifetime. The spectroscopic quality factor X = Ω 4 /Ω 6 rises from 1.20 to 2.25, providing solid spectroscopic verification for the enhanced radiative capability. Power-dependent measurements are consistent with a two-photon up-conversion mechanism, with contributions from excited-state absorption and energy-transfer up-conversion. These findings establish a new paradigm-radiative-rate activation via stoichiometric engineering-and highlight In: Dy: LiNbO 3 as a promising platform for compact visible lasers and integrated photonics.