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 5H3–7 → 7F0–3, 5L6 → 7F0, 5D3 → 7F2,3, and 5D2 → 7F0,1,2,3 of Eu3+ ions in NaYF4 nanocrystals were observed. 5H3–7, 5L6, and 5D2,3 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.