Enhanced downconversion luminescence of NaLuF4:Yb3+,Er3+ micrometer hexagonal prismatic crystals under 980 nm excitation

NaLuF₄:Yb³⁺,Er³⁺ micrometer prismatic crystals are potential materials for the preparation of optical waveguide devices. However, structural defects and OH^– groups are generated inside and on their surfaces during growth processes. The structural defects and OH^– groups lead to a large depletion of excitation energy and severely quench the downconversion luminescence of Er³⁺. In addition, the Yb³⁺/Er³⁺ co-doping ratio significantly affects the downconversion luminescence of the microcrystals, which in turn affects the performance of the optical waveguide amplifiers based on the microcrystals. By adjusting the co-doping ratio of Yb³⁺ and Er³⁺ in the microcrystals and removing the defects in the crystals, their downconversion luminescence performance was significantly improved. The results showed that the downconversion luminescence under the excitation from a 980 nm laser reached the maximum value when the doping ratio of Yb³⁺ and Er³⁺ was 20:1.5. In addition, the OH^– groups and structural defects in the micrometer crystals were significantly eliminated by ion-exchange and high-temperature annealing treatments, and the crystalline quality was significantly improved. After ion-exchange and high-temperature annealing treatments, the downconversion luminescence enhancement of NaLuF₄:Yb³⁺,Er³⁺ microcrystals was 2.5 times that of the untreated samples.