The Important Round-Trip Energy Transfer for Excitation of the Red Emission in the Er³⁺/Yb³⁺ System

Round-trip energy transfer (RTET) in the popular Er³⁺/Yb³⁺ upconversion (UC) system is a newly discovered mechanism for the red emission of Er³⁺ through Yb³⁺ as an intermediate ion. However, the importance of the RTET still remains a question. Here, we show in cubic Y₂O₃ that the new mechanism defeats conventional ones and dominates the red emission in both UC and down-shifting (DS) luminescence for a wide concentration range of Yb³⁺. The RTET enables the red UC emission to exhibit a similar time evolution as the green UC emission for Yb³⁺ concentration ≤10%, and its contribution to the red DS emission exceeds 90% for Yb³⁺ concentration ≥10%. As a unique feature, the temporal profile of the entire RTET process is presented for the first time. Our findings provide new physical insight into the regulation of color and temporal behavior of UC luminescence in the Yb³⁺/Er³⁺ system.