Rare earth coordination compounds: from monodentate ligands via crown ether complexes to a variety of bidentate ligands

Though the common laser sources usually make use of optically non-scattering materials, the basic processes involved in the generation of coherent light by either ASE (amplification of spontaneous emission by stimulated one) or by synchronised spontaneous emission, namely super-radiance and superfluorescence, do not require transparent materials. In this paper, results from the literature and from our own work on ASE and on superfluorescence processes obtained on rare-earth-doped powdered materials are presented with emphasis on our recent new results on super-radiance in powder samples. A critical discussion of these results is presented with reference to existing theories of gain in diffusing media. It is shown that when stimulated amplification is taking place, several grains with centimeter long path are involved, whereas when super-radiance is observed only one grain is sufficient for the process, provided that conditions on T2 are respected.