title: Lasing in active, sub-mean-free path-sized systems with dense, random, weak scatterers.
abstract: We report enhanced emission and gain narrowing in Rhodamine 590 perchlorate dye in an aqueous suspension of polystyrene microspheres. A systematic experimental study of the threshold condition for and the gain narrowing of the stimulated emission over a wide range of dye concentrations and scatterer number densities showed several interesting features, even though the transport mean free path far exceeded the system size. The conventional diffusive-reactive approximation to radiative transfer in an inhomogeneously illuminated random amplifying medium, which is valid for a transport mean-free path much smaller than the system size, is clearly inapplicable here. We propose a new probabilistic approach for the present case of dense, random, weak scatterers involving the otherwise rare and ignorable sub-mean-free-path scatterings, now made effective by the high gain in the medium, which is consistent with experimentally observed features.
authors.forename: B R
authors.initials: BR
authors.lastname: Prasad
authors.forename: H
authors.initials: H
authors.lastname: Ramachandran
authors.forename: A K
authors.initials: AK
authors.lastname: Sood
authors.forename: C K
authors.initials: CK
authors.lastname: Subramanian
authors.forename: N
authors.initials: N
authors.lastname: Kumar