Bismuth(III)-Doped NaYbF₄:Tm³⁺ Fluorides with Highly Efficient Upconversion Emission under Low Irradiance

Concentration quenching of upconversion (UC) luminescence (UCL) is a common phenomenon in rare-earth-doped materials that seriously restricts the concentration of the activator and sensitizer and withholds their UC emissions and quantum yields. In particular, it remains a tremendous challenge to develop one novel strategy based on the introduction of trivalent bismuth (Bi³⁺) ions to exceed the typical thulium (Tm³⁺) ion concentration and reach high-efficiency UC under low illumination. In this work, the Tm³⁺ accommodation capacity can be increased from 2.0 to 8.0 mol % in NaYbF₄:Tm³⁺ materials with the assistance of Bi³⁺ ions, which maintains strong UC emissions with large absolute UC quantum yields under low illumination. Specifically, the total upconversion quantum yield (UCQY) of the as-obtained Na(Tm_0.08Yb_0.60Bi_0.32)F₄ (8Tm60Yb32Bi) sample can reach as high as 1.45% upon continuous-wave (CW) laser excitation at 40 W cm^-2. Strikingly, the total UCQY still remains at a high level (0.41%) even though the CW power density decreases to 1.5 W cm^-2. Moreover, the intrinsic mechanism of the breakthrough in the threshold of concentration quenching of UCL by Bi³⁺ ions was also fully explored. These advances in enhancing UC emissions and UCQYs under a low pump power density offer exciting opportunities for important photonic applications.