Revisiting the NIR-to-Visible Upconversion Mechanism in β-NaYF₄:Yb³⁺,Er³⁺

Here, we show that the long-accepted mechanism for the production of red and blue emission through upconversion (UC) of 1 μm excitation in Yb(3+)/Er(3+)-doped materials does not apply in the popular β-NaYF4 host. We propose a new mechanism involving Yb(3+)-to-Er(3+) energy-transfer UC out of the green-emitting (2)H11/2,(4)S3/2 states that quantitatively accounts for all of the observed optical behavior. Rate constants for the relevant radiative and nonradiative processes are reported along with a prediction of the power dependence of the pulsed and continuous-wave UC quantum efficiency.