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

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⁺.