Highly efficient up-conversion luminescence in Er³⁺/Yb³⁺ co-doped Na₅Lu₉F₃₂ single crystals by vertical Bridgman...

Er³⁺/Yb³⁺ co-doped Na₅Lu₉F₃₂ single crystals with different concentrations of Yb³⁺ ions were prepared to investigate their phase structure, up-conversion (UC) properties and mechanism of UC luminescence by Bridgman method. Under 980 nm near-infrared (NIR) excitation, three sharp UC emission bands topping at green ~525 nm, ~548 nm and red ~669 nm were obtained in Er³⁺/Yb³⁺ doped Na₅Lu₉F₃₂ single crystals which are attributing to the transitions of ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2, respectively. The quadratic dependence of pump power on UC emission indicated that two-photon process is in charge of the transition from excited state of Yb³⁺ ions to lower state of Er³⁺ ion in Na₅Lu₉F₃₂ single crystals. The long-accepted mechanism for the production of red and green emissions through up-conversion (UC) under 980 nm excitation in Er³⁺/Yb³⁺ co-doped materials apply in the Na₅Lu₉F₃₂ host was displayed. The enhancement of the red emission was observed due to a cross-relaxation (CR) process of the form: ⁴F_7/2 + ⁴I_11/2 → ⁴F_9/2 + ⁴F_9/2. Furthermore, an ideal yellowish green light performance could be achieved with 1.0 mol% Er³⁺ doped certain Yb³⁺ concentrations samples, and its external quantum efficiency approached to 6.80% under 5.5 Wcm^-2 980 nm excitation which can be applied in developing UC displays for electro-optical devices.