Unusual radiative transitions of Eu³⁺ ions in Yb/Er/Eu tri-doped NaYF₄ nanocrystals under infrared excitation

By using Yb³⁺ and Er³⁺ as double sensitizers and 980 nm laser as pump light, unusual radiative transitions ⁵H_3-7 → ⁷F_0-3, ⁵L₆ → ⁷F₀, ⁵D₃ → ⁷F_2,3, and ⁵D₂ → ⁷F_0,1,2,3 of Eu³⁺ ions in NaYF₄ nanocrystals were observed. ⁵H_3-7, ⁵L₆, and ⁵D_2,3 levels of Eu³⁺ have been populated by two-step energy transfer process of Yb³⁺ → Er³⁺ → Eu³⁺ and become efficient radiative levels in the upconversion scheme. Experimental data indicate that when the excited Er³⁺ ions serve as donors, it is easier to transfer energy to acceptors after they are excited to the levels with longer lifetime.