Up-conversions in RE-doped Solids

Abstract

As for other fiuorescence light emitters, rare-earth-doped solids usually follow the well known principle of the Stokes law which simply states that excitation photons are ar higher energy than emitted ones, or in other words that out put photon energy is weak, less than input photon energy. Such a principle is valid of course only when one ion system is considered. In this chapter, we want to discuss the “unconventional case” where the above principle is not respected in its simple statement. Ir will be shown that RE-doped solids may deviate rather easily from the 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: Besides (1)multistep excitation due to classical excited state absorption (ESA), there is (2) 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 (3)a third process, namely cooperative up-conversion either between two ions or between a pair of ions anda third one. Though some of their theoretical behaviors are rather analogous to those of up-conversion by energy transfers, their efficiencies are usually much lower because they involve virtual levels which have to be described in a higher order of perturbation. (4)A fourth process, the photon avalanche effect, will also be considered. 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. Various experimental techniques that allow distinctions to be made between the behaviors of these various processes will be analyzed. Finally, perspectives for applications of these processes in various types of RE-doped solids such as crystals and glasses in bulk or fiber form are given, and recent advances are encompassed and described. At any rate this chapter is not intended to be a review; its purpose is to be a didactic description of the various processes that may be involved together with selected typical examples that may help in understanding further developments.