Three-photon upconversion luminescence phenomenon for the green levels in Er3+/Yb3+ codoped cubic nanocrystalline yttria

In this paper, we observed that in Er3+/Yb3+ codoped nanocrystals (NC), with the decreasing particle size and the increasing Yb3+ concentration, the upconversion luminescence (UCL) of the red emissions of F 9 / 2 4 − I 15 / 2 4 gradually increased, while the green emissions of S 3 / 2 4 / H 11 / 2 2 gradually diminished under 980 nm diode laser excitation. In NC with lower Yb3+ concentration, both the red and green emissions result from a two-photon excitation. In NC with higher Yb3+ concentration, the red emissions result from a two-photon excitation, while the green emissions dominantly result from a three-photon excitation. A model was provided for explaining the above UCL phenomena.