LASING IN ACTIVE, SUB-MEAN-FREE PATH-SIZED SYSTEMS WITH DENSE, RANDOM, WEAK SCATTERERS

We report enhanced emission and gain narrowing in Rhodamine 590 perchl orate dye in an aqueous suspension of polystyrene microspheres. A syst ematic experimental study of the threshold condition for and the gain narrowing of the stimulated emission over a wide range of dye concentr ations and scatterer number densities showed several interesting featu res, even though the transport mean free path far exceeded the system size. The conventional diffusive-reactive approximation to radiative t ransfer in an inhomogeneously illuminated random amplifying medium, wh ich is valid for a transport mean-free path much smaller than the syst em size, is clearly inapplicable here. We propose a new probabilistic approach for the present case of dense, random, weak scatterers involv ing the otherwise rare and ignorable sub-mean-free-path scatterings, n ow made effective by the high gain in the medium, which is consistent: with experimentally observed features. (C) 1997 Optical Society of Am erica.