Power-dependent upconversion luminescence intensity in NaYF4,Yb3+,Er3+ nanoparticles

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/cm2 for 2H9/2→4I15/2, 2H11/2&4S3/2→4I15/2, and 4F9/2→4I15/2 transition luminescences, respectively, in compact powder composed by cubic-phase NaYF4: Yb3+,Er3+ 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.