Fig. 15. ETU mechanism for NIR of VIS upconversion using a Yb³⁺/Er³⁺ doped UCNP with a β-phase NaYF₄ host lattice. Pumping the Yb³⁺ sensitizer with 980 nm light gives an ES that undergoes energy transfers to the Er³⁺ activator. Continued excitation of Yb³⁺ and energy transfer to Er³⁺ gives higher energy states from which several visible range emissions occur. Figure adapted from ref. 131.