Effect of energy back transfer from Er3+ to Yb3+ ions on the upconversion luminescence of Er:NaYb(MoO4)2 and Yb,Er:NaBi(MoO4)2

Under the excitation of a 980 nm laser, the visible upconversion (UC) luminescence of Er³⁺ ions doped Yb³⁺ ions self-activated NaYb(MoO₄)₂ phosphor and crystal, as well as the Yb³⁺/Er³⁺ ions codoped NaBi(MoO₄)₂ crystal were investigated comprehensively. The results indicate that all three samples exhibit two significant green emission bands and a weak red emission band in the visible band corresponding to the transitions of ²H_11/2/⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 of Er³⁺ ions, respectively. Through the variable power density spectra of three different samples, the relationship between the energy back transfer (EBT) process of Yb³⁺-Er³⁺ ions and the power density point and Yb³⁺ ion concentration was investigated. The EBT process was observed in both the Er³⁺ ions doped Yb³⁺ ions self-activated NaYb(MoO₄)₂ phosphor and crystal, as confirmed by the luminescence image of the sample. At high power density, the Yb³⁺ ions self-activated sample exhibited yellow luminescence, with the crystal appearing later than the phosphor. In contrast, the NaBi(MoO₄)₂ crystal displayed bright green emission within the measured power density range. In addition, by monitoring the relative intensity change of Yb³⁺ emission in 5 at% Er³⁺:NaYb(MoO₄)₂ crystal, the generation of EBT process in self-activated samples at high power density is more directly explained. These experimental results provide a reliable basis for our comprehensive understanding of the EBT mechanism, and also provide a reliable direction for the final determination of the optimal excitation power density for optical temperature measurement.