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

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 NaYF 4 : 2% Er 3 + , 18% Yb 3 + . 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 + F 5 / 2 2 multiplet, which sensitizes the UC emission. There is very efficient energy transfer to the lowest energy crystal-field component of the Er 3 + I 11 / 2 4 state from F 5 / 2 2 | 1 〉 , resulting in an activation energy for all Er 3 + related up- and down-conversion emissions that scales with the number of excitation photons required multiplied by the 39 cm - 1 energy gap.