Fig. 2: Switchable NIR upconversion mechanism through non-steady-state excitation.

a Time-dependent upconversion emission profiles of Er³⁺ at 660 nm and Tm³⁺ at 800 nm for the UCNPs under PW 980 nm excitation with a duration time of 8 ms. b Time-dependent upconversion emission profiles of Er³⁺ at 660 nm and Tm³⁺ at 800 nm for the UCNPs doping with only Er³⁺ and Tm³⁺, respectively, under PW 980 nm excitation with a duration time of 8 ms. c NIR-to-red emission intensity ratios under different excitation frequencies for the UCNPs. d Schematic dynamic control of upconversion in Yb³⁺/Er³⁺/Tm³⁺ codoped system for 660 nm red and switchable 800 nm NIR emissions under 980 nm excitation with CW or PW laser. e Comparison of the time-dependent upconversion emission profiles of Er³⁺ at 660 nm for the UCNPs with and without codoping of Tm³⁺ under 980 nm excitation with a duration time of 8 ms.