Lasing in Active Sub Mean Free Path Sized Systems with Dense Random Weak Scatterers

We report enhanced emission and gain narrowing in Rhodamine-590 perchlorate dye in an aqueous suspension of polystyrene microspheres. A systematic study of the threshold condition for and the gain narrowing of the stimulated emission over a wide range of dye concentration and the scatterer number density showed several interesting features even though the transport meanfree path far exceeded the system size. We proposed a new probabilistic approach for the present case of dense random week scatterers involving the otherwise rare and ignorable sub-meanfree path scatterings, now made effective by the high gain in the medium, which is consistent with experimentally observed features.