Blue upconversion of Tm3+ using Yb3+ as energy transfer bridge under 1532 nm excitation in Er3+, Yb3+, Tm3+ tri-doped CaMoO4

Abstract

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 upconversion process, the upconversion spectra in these samples were investigated, and the possible mechanism was proposed based on experimental 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 upconversion of other rare-earth ions under the excitation of 1532 nm was possible with the presence of Er3+ and Yb3+.