Control upconversion decay dynamics from the perspective of collective response

Very recently, upconversion luminescence (UCL) lifetime, as a powerful optical dimension, has attracted tremendous research interest due to its advantages of high information capacity and high photophysical stability. With the implementation and emergence of endlessly fascinating UCL features, it is particularly meaningful to understand the photophysical mechanisms inside UCL materials to enable rational subdivision-level structure design, which is however currently far from sufficient. In this work, we take an ordinary upconversion nanoparticle as an example to prove that the UCL decay curves and corresponding lifetimes are indeed a collective response of the entire UCL system to excitations, that exhibits correlated, yet quite different properties from individual ions. A specially developed theoretical random walk model combined with an experimental lifetime control for Yb3+/Er3+ UCL demonstrates that energy diffusion principally alters the decay rate. Moreover, the different extent of the influence of energy diffusion on the emissions of 2H11/2/4S3/2 (green) and 4F9/2 (red) leads to an extremely uncommon cross-over comparison of decay rates. This work provides new ideas for understanding decay dynamics and practical UCL lifetime manipulation methods.