Pascal Gerner and Hans U. Güdel
Absorption and upconversion light emission properties of Er³⁺ and Yb³⁺/Er³⁺ codoped NaYS₂
Chem. Phys. Lett. 413, 105-109 (2005)                 

Abstract: The upconversion properties of Er³⁺ doped and Yb³⁺/Er³⁺ codoped NaYS₂ crystals are presented. Under near-IR excitation around 10 000 cm⁻¹ intense upconversion emission from the Er^{3+ 4}F_7/2, ⁴S_3/2, ⁴F_9/2 and ⁴I_9/2 excited states is observed at 12 K. A favorable green ⁴S_3/2 to red ⁴F_9/2 photon ratio of 8 is observed in Er³⁺ doped NaYS₂. Low lying S²⁻ to Yb³⁺ ligand to metal charge transfer transitions interact with Er³⁺ emitting states in Yb³⁺/Er³⁺ codoped NaYS₂ and essentially quench the green emission. In contrast to many oxide and halide materials, Yb³⁺ is not a suitable sensitizer for Er³⁺ UC emission in this sulfide host lattice.