Highly efficient near-infrared to visible up-conversion process in NaYF4:Er3+,Yb3+

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 39cm-1 energy gap between the two lowest crystal-field components |0〉 and |1〉 of the Yb3+F5/22 multiplet, which sensitizes the UC emission. There is very efficient energy transfer to the lowest energy crystal-field component of the Er3+I11/24 state from F5/22|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 39cm-1 energy gap.