Nano-sized CaMoO4 phosphors tri-doped with Er3+, Yb3+and Tm3+ions were successfully synthesized by sol-gel method. Intense blue emission from Tm3+ions was observed upon excitation of 1532 nm infrared light in Er-Yb-Tm system, while this blue upconversion could not be achieved with the absence of Yb3+ions in Er-Tm co-doped sample. In order to understand this upconver-sion process, the upconversion spectra in these samples were investigated, and the possible mechanism was proposed based on ex-perimental results. It showed that two different energy transfer from Er3+to Tm3+existed simultaneously in Er-Yb-Tm system, the one-step direct energy transfer from Er3+to Tm3+, and the two-step Er3+→Yb3+→Tm3+energy transfer. In particular, the 1G4 state of Tm3+could only be populated from the 3H4 state by cross-relaxation with an excited Yb3+ion, producing blue emission of Tm3+. In this upconversion process, Yb3+ions acted as an energy transfer bridge between Er3+and Tm3+, which also meant that the upconver-sion of other rare-earth ions under the excitation of 1532 nm was possible with the presence of Er3+and Yb3+.
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