Upconversion mechanism for two-color emission in rare-earth-ion-doped ZrO2 nanocrystals

Two-color upconversion (UC) emissions with anomalous power dependence were observed in spectrally tailored rare-earth-ion-doped ZrO2 nanocrystals. Single green and red UC radiation in the visible range was induced by 980 nm diode laser excitation in ZrO2:Er3+ and ZrO2:Er3++Yb3+ nanocrystals, yielding quadratic and linear laser power dependences, respectively. The UC spectrum design is enabled by the saturation of the I-4(13/2)(Er) state through the efficient energy back-transfer process S-4(3/2)(Er)+F-2(7/2)(Yb)-> I-4(13/2)(Er)+F-2(5/2)(Yb) induced by high Yb3+-ion concentration. The anomalous power dependence originates from the fact that the UC rate of the I-4(11/2)(Er) state at high Yb3+-ion concentrations is fundamentally much larger than its decay rate. These conclusions obtained are confirmed by theoretical investigations based on steady-state rate equations.