Figure 7

(a) Power-dependent UC external quantum efficiency at 980 nm light excitation in Er³⁺/Yb³⁺ doped Na₅Lu₉F₃₂ single crystals, the inset shows the external quantum efficiency on Yb³⁺ concentrations; (b) Comparison of UC external quantum efficiency of Er³⁺/Yb³⁺ co-doped Na₅Lu₉F₃₂/LiYF₄ single crystals under 980 nm excitation.