Figure 2.

The energy transfer mechanisms of the up conversion processes in sensitizer Yb³⁺ crystals codoped activator Tm³⁺ and Er³⁺ under 980 nm diodelaser excitation. The photon excitation, energy transfer, multi-photon relaxation and emission processes are shown as dashed-dotted, dashed, dotted and full arrows respectively. Only visible and NIR emissions are shown here. (Based on ideas from Wang F, Liu X. Recent advances in the chemistry of lanthanide-doped upconversion nanocrystals. Chem Soc Rev. 2009;38:976–989. doi:10.1039/b809132n.²³