Strong Influences of Melting Time and Tm ³⁺ Concentration on Blue Up‐Conversion Photoluminescence for Tm ³⁺ /Yb ³⁺ Co‐Doped TeO ₂ –TlO _0.5 –ZnO Glass

In this study, by a conventional melt quenching method, we synthesized novel up‐conversion phosphors of 60TeO ₂ –30TlO _0.5 –(9− x )ZnO– x Tm ₂ O ₃ –1Yb ₂ O ₃ ( x = 0.1–0.5) glasses, whose system was recently developed in our collaborative group, and their blue up‐conversion photoluminescence (UCPL) of Tm ³⁺ ions via three‐step energy transfer from near‐infrared (NIR) sensitizer of Yb ³⁺ ions was observed. In particular, the substantial rate of the energy transfer <γ _d5 > in the third step from Yb ³⁺ to Tm ³⁺ under excitation at 975 nm, which determined the final blue UCPL intensity, was estimated as a function of the rare‐earth concentration. With an aid of analytical methods of PL lifetime and Judd–Ofelt theory, it was revealed that the highest energy transfer rate <γ _d5 > was achieved to be 2.07 × 10 ⁻¹⁷ cm ³ /s for x = 0.2, and further increasing Tm ₂ O ₃ content x in the fixed Yb ₂ O ₃ resulted in the decrease in the energy transfer rate <γ _d5 >. One of the plausible causes was concentration quenching of Yb ³⁺ ions. The other was back‐transfer from Tm ³⁺ to Yb ³⁺ ions. The influence of the condition of glass synthesis and the melting time on <γ _d5 > was also discussed.