Coherent light sources with powder: stimulated amplification versus super-radiance

Though the common laser sources usually make use of optically non-scatterin g materials, the basic processes involved in the generation of coherent lig ht by either ASE (amplification of spontaneous emission by stimulated one) or by synchronised spontaneous emission, namely super-radiance and superflu orescence, do not require transparent materials. In this paper, results fro m the literature and from our own work on ASE and on superfluorescence proc esses obtained on rare-earth-doped powdered materials are presented with em phasis on our recent new results on super-radiance in powder samples. A cri tical discussion of these results is presented with reference to existing t heories of gain in diffusing media. It is shown that when stimulated amplif ication is taking place, several grains with centimeter long path are invol ved, whereas when super-radiance is observed only one grain is sufficient f or the process, provided that conditions on T-2 are respected. (C) 2000 Els evier Science S.A. All rights reserved.