Cooperative energy acceptor of three Yb3+ ions

180a. Downconversion emission spectrum (black line) and upconversion emission spectrum (red line) of CaF2:Yb3+, Ce3+, Na+ excited by 306 nm and 980 nm, respectively. b. Schematic energy level diagram of Yb3+ and Ce3+, as well as the proposed QC mechanisms under 306 nm excitation.