Highly efficient near-infrared to visible up-conversion process in image

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 image: 2% image, 18% image. 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 image energy gap between the two lowest crystal-field components image and image of the image multiplet, which sensitizes the UC emission. There is very efficient energy transfer to the lowest energy crystal-field component of the image state from image, resulting in an activation energy for all image related up- and down-conversion emissions that scales with the number of excitation photons required multiplied by the image energy gap.