Increase of the blue upconversion emission in YAG:Tm3+ nanopowders by codoping with Yb3+ ions

The YAG nanopowders were prepared by a co-precipitation method using nitrate and ammonium hydrogen carbonate as raw materials. To obtain homogenous precipitate, reverse-strike (adding salt solutions to the precipitant solution) technique was adopted. Therefore, single (Tm³⁺) and codoped (Tm³⁺–Yb³⁺) YAG nanopowders with a size between 40–90 nm have been obtained.Blue upconversion emission at around 480 nm has been found in YAG: Tm³⁺ nanopowders under excitation to the ³H₄ level of Tm³⁺ at around 800 nm. However, this upconversion emission in nanopowders codoped with Tm³⁺–Yb³⁺ ions is increased by a factor of about 10. The analysis of the temporal evolution of the involved levels and the dependence of the upconversion intensity on the pump power at 800 nm allowed to distinguish the upconversion mechanism. In YAG: Tm³⁺ nanopowders the upconversion mechanism is due to excited state absorption processes. However, in the codoped samples, Yb³⁺ ions acts as the sensitizers; in consequence, the blue upconversion is strongly increased.