Visible green upconversion luminescence of Er3+/Yb3+/Li+ co-doped CaWO4 particles

The upconversion (UC) luminescence of Li+/Er3+/Yb3+ co-doped CaWO4 phosphors is investigated in detail. Single crystallized CaWO4:Li+/Er3+/Yb3+ phosphor can be obtained, co-doped up to 25.0/5.0/20.0 mol% (Li+/Er3+/Yb3+) by solid-state reaction. Under 980 nm excitation, CaWO4:Li+/Er3+/Yb3+ phosphor exhibited strong green UC emissions visible to the naked eye at 530 and 550 nm induced by the intra-4f transitions of Er3+ (2H11/2,4S3/2→4I15/2). The optimum doping concentrations of Yb3+/Li+ for the highest UC luminescence were verified to be 10/15 mol%, and a possible UC mechanism that depends on the pumping power is discussed in detail.