Absorption and upconversion light emission properties of Er3+ and Yb3+/Er3+ codoped NaYS2

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 Er3+4F7/2, 4S3/2, 4F9/2 and 4I9/2 excited states is observed at 12 K. A favorable green 4S3/2 to red 4F9/2 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.