Proximal selenium co-doping in Er:CaF₂ crystal for efficient 2.75 µm lasers

Non-radiative relaxation hinders the performance of erbium-based mid-infrared lasers. Here, we demonstrate that anion-engineering in fluorite crystal provides a solution to this challenge, facilitating efficient laser operation. The Er,Se-co-doped CaF₂ crystal exhibits extended lifetimes at characteristic emission bands compared to Er:CaF₂. First-principles calculations reveal the spatial proximity of Se^2- to Er³⁺ clusters within CaF₂ lattice, alongside the introduction of low-frequency phonon modes, underpinning the observed phenomena. Efficient 2.75 µm continuous-wave laser operation was demonstrated on this crystal, exhibiting high slope efficiencies of 34.4% and 12.2% under 976 nm resonant and 1532 nm upconversion pumping, respectively. Our results highlight the role of anion-engineering in designing advanced laser materials.