Up-conversion in rare-earth-doped systems: past, present and future

Abstract

As for other fluorescence light emitters, rare-earth (RE)- doped solids usually follow the well known principle of the Stokes law which simply states that excitation photons are at higher energy than emitted ones or in other words that out-put photon energy is weaker than input photon energy. Such principle is valid of course only when one ion system is considered.