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 39 cm(-1) energy gap between the two lowest crystal-field components 10) and 11) of the Yb3+ F-2(5/2) multiplet, which sensitizes the UC emission. There is very efficient energy transfer to the lowest energy crystal-field component of the Er3+ I-4(11/2) state from (2)F(5/2)vertical bar 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. (c) 2004 Elsevier B.V. All rights reserved.