Fig. 1. Schematic illustration of the MPA mechanism with different emitters. For the nonlinear process, nonlinear order n is defined by Iem∝Iexn, where Iem is the emission intensity and Iex is the excitation intensity. The photon avalanche firstly occurs in Yb3+/Pr3+ co-doped core and emissions from Yb3+ and Pr3+ all exhibit a nonlinear order of n=26. The avalanching energy can migrate to the third ion (Tm3+, Ho3+, etc.) via Yb3+ sublattice, triggering multiplied nonlinear upconversion luminescence. Credit: Qiuqiang Zhan, South China Normal University.