Use of Coupled Rate Equations to Model NIR-to-Visible Upconversion Kinetics in Er3+, Yb3+: NaYF4 Nanocrystals

Upconversion phosphors are able convert incident light into light of a shorter wavelength. The (erbium) Er³⁺, (ytterbium) Yb³⁺: NaYF₄ system is the most efficient upconversion phosphor known, and yet the quantitative aspects of the mechanism responsible for upconversion, such as the values of key microscopic rate constants, have not been determined. In this work, the dynamics of the photo-physical processes leading to near-infrared (NIR) to visible upconversion in Er³⁺, Yb³⁺: NaYF₄ nanocrystals are investigated using a nonlinear rate-equation model. Following selective 950nm (NIR) pulsed excitation of Yb³⁺, the population density of the excited states of the Er³⁺ and Yb³⁺ ions are followed as a function of time. The results of these time-resolved luminescence measurements are compared with the simulated results of our rate equation model. Based upon the quality of the resulting fits we conclude that the model successfully describes the upconversion process within the nanocrystals.