J. F. Suyver, J. Grimm, K. W. Krämer and H. U. Güdel
Highly efficient near-infrared to visible up-conversion process in NaYF₄:Er³⁺,Yb³⁺
J. Lumin. 114, 53-59 (2005)                 

Abstract: Up-conversion (UC) entails the addition of two (or more) photons to generate one of higher energy. This process is very interesting, both from a fundamental point of view as well as for (future) applications. Here, dependence of the emission spectra on excitation power and measurement temperature are reported for the very efficient near-infrared to visible photon UC material NaYF₄: 2% Er³⁺, 18% Yb³⁺. In the high-power limit for continuous-wave laser excitation, roughly 50% of all near-infrared photons absorbed by the material are up-converted and emitted in the visible spectral range. The excitation spectrum shows a 39 cm^–1 energy gap between the two lowest crystal-field components |0> and |1> of the Yb^{3+ 2}F_5/2 multiplet, which sensitizes the UC emission. There is very efficient energy transfer to the lowest energy crystal-field component of the Er^{3+ 4}I_11/2 state from ²F_5/2|1>, resulting in an activation energy for all Er³⁺ related up- and down-conversion emissions that scales with the number of excitation photons required multiplied by the 39 cm^–1 energy gap.