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.