Insights into luminescence and energy transfer processes in Ce3+- and Tb3+ co-doped (Gd, Y)3Al2Ga3O12 garnet single crystals

Driven by the pursuit of optimizing the luminescent properties of garnet-based single crystals for X-ray imaging applications, this work reports on the successful synthesis and in-depth characterization of GYAGG crystals doped with Ce³⁺ or Tb³⁺ ions, and doubly-doped with both of them. Micro-pulling down synthesis yielded crystals with a single cubic phase confirmed by X-ray diffraction. Photoluminescence (PL) and radioluminescence (RL) measurements on Ce³⁺,Tb³⁺ co-doped GYAGG revealed bidirectional energy transfer processes. Characteristic broadband 5d1→4f emission of Ce³⁺ centres peaking at 530 nm and narrow 4f→4f emission lines of Tb³⁺ ions starting from 5D4 level within 480-630nm were observed. At higher doping levels, cross-relaxation in Tb³⁺ pairs resulted depleted 5D3 state and only the emission from 5D4 one was observed in the spectra. Analysis of PL decay characteristics corroborated the spectral observations, confirming progressively decreasing Ce³⁺ decay time (down to 40 ns at 15% Tb3+) due to enhanced Ce³⁺→Tb³⁺ energy transfer. Similarly, the Tb³⁺ decay time accelerated by more than 60% after the Ce³⁺ co-doping. Notably, co-doping with 0.5% Ce³⁺ and 10-15% Tb³⁺ doubled the luminescence intensity of RL spectra compared to 0.5% Ce³⁺-doping alone, attributed to the increased density of emitting centres.