Upconversion Luminescence of Sm²⁺ Ions Based on Cooperative Energy Transfer

We report the observation of upconversion luminescence from Sm²⁺ ions, which demonstrate that changeable valence lanthanides can served as the ions for optical frequency transformation. Upon the excitation of 980 nm diode laser, the doped Sm²⁺ ions in the hybrid material of BaFCl_0.5Br_0.5:1%Sm²⁺-CaF₂:1%Yb³⁺ emit red upconversion fluorescence peaked at 631, 644, 665, 689, 704, 729 nm from the ⁵D_0,1→⁷F_0,1,2 transitions, respectively. By transient dynamic analysis, we attribute the excitation of Sm²⁺ ions to the cooperation energy transfer process: two excited Yb³⁺ ions simultaneously transfer their energy to one Sm²⁺ ion.