Upconversion luminescence of Sm²⁺ ions

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