来源：ACS Publications
The popular Er3+–Yb3+ upconversion (UC) system generates green and red emissions under near-infrared (NIR) 980 nm excitation. The green emission is a two-photon process, whereas for the red emission two and three photon process are both possible, depending on host. Fluoride hosts generally favor a three-photon process because fluorides have low phonon energies and multiphonon relaxation (MPR) involved in the usual two-photon process is suppressed. However, CaF2:Er3+/Yb3+ exceptionally produces an intense two-photon upconverted red emission. Here, we show that the red UC emission in CaF2 is generated mainly by an unusual two-photon UC mechanism that enables efficient relaxation from the UC pumped green emitting state to the red emitting state via round trip energy transfer (RTET) between Er3+ and Yb3+. The efficient RTET in CaF2 is attributed to the unique characteristic of CaF2 in which rare earth ions are prone to clustering. The contribution of RTET to red UC emission is evaluated for various Yb3+ and Er3+ concentrations based on analysis of spectral and temporal behavior of luminescence. The results indicate that RTET dominates red UC emission for a wide range of dopant concentrations in CaF2:Er3+/Yb3+. Furthermore, the origin of the commonly observed decrease of red-to-green intensity ratio on increasing Yb3+ concentration companied by UC luminescence color change from yellow to green in (Ca, Sr)F2 is revealed based on the RTET model.