Two Photon NIR-to-Red Upconversion Mechanism in CaF ₂ :Er ³⁺ , Yb ³⁺

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.