Competition between Two Upconversion Mechanisms for Red Emission in Er3+ and Yb3+ Codoped Rare-Earth Sesquioxides

Er3+/Yb3+ codoped rare-earth sesquioxides are widely used red upconversion (UC) phosphors under near-infrared (NIR) 980 nm excitation. Besides the old mechanism, a newly identified round-trip energy transfer (RTET) between Er3+ and Yb3+ accelerates relaxation from the green-emitting to the red-emitting state, making the red and green UC lifetimes comparable. Here, we show that the effectiveness of RTET is closely related to multiphonon relaxation (MPR) of Er3+; that is, slow MPR is conducive to RTET. The contributions of the two mechanisms are studied in Gd2O3 and Sc2O3, which have the slowest and fastest MPR, respectively, in rare-earth sesquioxides. The effect of RTET is verified based on the spectral and temporal behavior of down-shifted and upconversion luminescence. Upon NIR pulse excitation, RTET contributes 94 ± 9.3% in Gd2O3, leading to nearly identical red and green lifetimes, but drops to 70 ± 6.2% in Sc2O3, showing a lifetime of red UC emission significantly longer than that of green UC emission due to the nonnegligible effect of the old mechanism. Upon NIR CW excitation, the old mechanism and RTET make a main contribution in Sc2O3 and Gd2O3, respectively. These findings provide new insights into controlling UC lifetimes for time-gated optical applications.