Visible green upconversion luminescence of Li+/Er3+/Yb3+ co-doped CaWO4 phosphor and effects of Yb3+ concentration

The upconversion (UC) luminescence of Li+/Er3+/Yb3+ co-doped CaWO4 phosphors and effects of Yb3+ concentration are investigated in detail. Single crystallized CaWO4 : Li+/Er3+/Yb3+ phosphor can be obtained, co-doped up to 35.0/5.0/30.0 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+ (H-4(11/2), S-4(3/2)-> I-4(15/2)). The optimum doping concentrations of Yb3+ that would result in the highest UC luminescence were determined, and a possible UC mechanism that depends on the pumping power is discussed in detail.