Upconversion emission spectra of ZnEr1%Yb1%S QDs under 980 nm excitation ( 4 I 15/2 – 4 I 11/2 ) with excitation power varying from 1 to 10 mW. The inset shows the dependence of the upconversion emission intensity on the excitation power.
Ngan et al. (2026) studied this question.