Pure red upconversion emission from CaF₂:Yb³⁺/Eu³⁺

Single phase cubic CaF₂ microcrystals co-doped with Eu³⁺ and Yb³⁺ ions were synthesized through a coprecipitation process followed by annealing at 1200 °C. Under the excitation of 980 nm laser, the intensive emission at ~614 nm (⁵D₀ → ⁷F₂) in the red region was observed at room temperature. The color coordinate of CaF₂:0.1%Yb³⁺/0.1%Eu³⁺ was calculated with the Commission Internationale de I'Eclairage (CIE) chromaticity coordinate of (x=0.572, y=0.265). The excitation of Eu³⁺ ions came from the energy transfer of Yb³⁺-dimers through a cooperation sensitization process. Furthermore, not only the emissions from the ⁵D_i (i=0, 1, 2, 3)→⁷F_j (j=0, 1, 2, 3) transitions but also the rarely reported emission (~398 nm) from the ⁵L₆ →⁷F₀ transition was recorded. The cooperation sensitization process was so effective in the Yb³⁺/Eu³⁺ co-doped system that all the cooperative luminescence (CL) from Yb³⁺-clusters were quenched and only the red upconversion luminescence of Eu³⁺ ions could be seen.