Abstract
The upconversion (UC) luminescence of <TEX>$Li^+/Er^{3+}/Yb^{3+}$</TEX> co-doped <TEX>$CaWO_4$</TEX> phosphors and effects of <TEX>$Yb^{3+}$</TEX> concentration are investigated in detail. Single crystallized <TEX>$CaWO_4$</TEX> : <TEX>$Li^+/Er^{3+}/Yb^{3+}$</TEX> phosphor can be obtained, co-doped up to 35.0/5.0/30.0 mol% (<TEX>$Li^+/Er^{3+}/Yb^{3+}$</TEX>) by solid-state reaction. Under 980 nm excitation, <TEX>$CaWO_4$</TEX> : <TEX>$Li^+/Er^{3+}/Yb^{3+}$</TEX> phosphor exhibited strong green UC emissions visible to the naked eye at 530 and 550 nm induced by the intra 4f transitions of <TEX>$Er^{3+}$</TEX> (<TEX>$^4H_{11/2}$</TEX>, <TEX>$^4S_{3/2}{\rightarrow}^4I_{15/2}$</TEX>). The optimum doping concentrations of <TEX>$Yb^{3+}$</TEX> 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.
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