Defect Structure and Upconversion Luminescence Properties of LiNbO₃ Highly Doped Congruent In:Yb:Ho:LiNbO₃ Crystals

Congruent In:Yb:Ho:LiNbO₃ crystals with different In³⁺ concentrations have been grown by Czochralski method. Red and green upconversion emissions were observed under 980 nm excitation at room temperature. The effect of In³⁺ on the upconversion luminescence was studied in association with the defect structure caused by In³⁺ doping. Infrared transmission spectra indicate a clear shift of the OH⁻ absorption peak along with an increase of doped In³⁺ concentrations, owing to the occupation of In²⁺ Li blow the threshold concentration (3%) that suppresses the Nb⁴⁺ Li defects, and the occupation of In²⁻ Nb and In2+Li above the threshold. Moreover, In³⁺ induced upconversion luminescence intensity changes were observed to be consistent with the measured lifetime and the OH group content changes, manifesting the highest upconversion luminescence at In³⁺ concentration of 1 mol%. Mechanistic investigations indicated that both the green (at 548 nm) and the red (666 nm) emissions involve two-photon processes, being ascribed to two sequential energy transfers from to the excited Yb³⁺ ions to the Ho³⁺ ions. These results suggests that In:Yb:Ho:LiNbO₃ crystals provide opportunities for strong short wavelength up conversion lasers and related laser devices.