Enhanced 3H4-3F4 nonradiative relaxation of Tm3+ through energy transfer to Yb3+ and efficient back transfer in lowly Tm3+ doped Lu1.6Sc0.4O3:Tm3+, Yb3+
Tm3+-Yb3+-Tm3+ efficient forward-backward energy transfer is proved. BET is benefit from the preferential excitation in the step of FET. Be propitious to Tm3+ blue upconversion with red light excitation. A new method to calculate radiative rate of Tm3+:3H4 state is demonstrated.