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The knowledge of the power density for which luminescence intensity reversal begins with power increase is of valuable importance for practical applications. Under 980 nm CW diode laser excitation, the maximum upconversion luminescence intensities were obtained at 4800, 5700, and 7100 W/cm 2 for 2 H 9/2 → 4 I 15/2 , 2 H 11/2 & 4 S 3/2 → 4 I 15/2 , and 4 F 9/2 → 4 I 15/2 transition luminescences, respectively, in compact powder composed by cubic-phase NaYF4:Yb 3+ ,Er 3+ nanoparticles with a most probable diameter of ∼ 46 nm. It was revealed experimentally that the reversal power density decreases as luminescent level rises. Excitation increase combined with radiative quantum efficiency decrease as power increase can reproduce this "n"-shape power dependence relationship.

Upconversion luminescence (UCL) of lanthanide-doped nanomaterials is usually a low-efficiency nonlinear process, involving multi-step, multi-channel transitions in a multi-level system. Here, we demonstrate quantum control and enhancement of multi-color (e.g., red and green) UCLs of NaYF4:Yb3+/Er3+ nanoparticles with metallic Fabry-Perot micro-cavities. Besides realization of controlled single-color UCLs, their internal quantum efficiencies are strongly enhanced, up to 3–4 orders of times. Experimental results indicate that the controlled single-color UCLs and enhancements are caused not only by modifications of the spontaneous radiation rates for the red- and green-color transitions but also by influencing the intermediate transitions to result in modified distributions of electrons in each of the multiple Er3+ levels, facilitating emission of either red- or green-color light. This work suggests a way to control photon emissions in systems with multi-channel transitions and/or multi-step excitations.