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 H-2(11/2)/S-4(3/2) -> I-4(15/2) and F-4(9/2) -> I-4(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.