Up-conversion in Rare-Earth-doped systems: past, present and future

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. Here, we want to discuss the "unconventional case" where the above principle is not respected in its simple statement. It shall be shown that RE-doped solids may deviate rather easily from above principle giving rise, under moderate to strong excitation density to unconventional emissions of the anti-Stokes types. Four mechanisms may be involved in up-conversion: i) Besides multistep excitation due to classical Excited State Absorption (ESA), there is ii) the very efficient process of up-conversion by sequential energy transfers due to the fact that in many RE-doped solids, ions may be connected energetically by energy transfer diffusion. This last phenomenon has to be distinguished from the third one iii) namely co-operative up-conversion either between two ions or between a pair of ions and a third one. Though some of their theoretical behaviours are rather analogous with the ones of up-conversion by energy transfers, their efficiencies are usually much weaker because they involve virtual levels which have to be described in a higher order of perturbation. iv) A fourth process shall also be considered: the photon avalanche effect. It is also based on sequential energy transfers but of the down-conversion type (usually called cross-relaxation), whereas the up-conversion step itself is due to ESA. The various experimental techniques, which allow distinctions between the behaviours of these various processes, are analysed and some perspectives for applications of these processes in various types of RE-doped solids such as crystals and glasses under bulk or fibre form are given. Finally the latest developments are discussed.