Green and red upconversion luminescence in Er³⁺-doped and Er³⁺/Yb³⁺-codoped SrTiO₃ ultrafine powders

Er³⁺-doped and Er³⁺/Yb³⁺-codoped strontium titanate (SrTiO₃) ultrafine powders were prepared in a molten NaCl flux. The intense green and red emissions around 528 nm, 550 nm and 662 nm corresponding to the ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions, respectively, of Er³⁺ ions were observed under excitation at 980 nm. A great enhancement of the visible upconversion emissions in Er³⁺/Yb³⁺-codoped sample was observed. Excited state absorption and energy transfer process are discussed as the possible mechanisms for these emissions.