Line narrowing in the dye solution with scattering centers

Abstract

As was shown by Markushev et al. [1], laser-like emission without cavity can be obtained from a scattering gain medium, such as powder of Nd doped laser crystals. In the recent work by Lawandy et al. [2] a close phenomenon was observed in a concentrated rhodamine 640 dye solution in the presence of scatterers. The work in Ref. [2] is the first where gain and scattering are separate. In this case lasing evidently is not initiated within the particles, in contrast to the Markushev et al. case, where the gain is in the particles, which supports morphology-dependent resonances. The work by Sha et al. [3] showed similar results in concentrated dye, but it also showed that introduction of a low density of scatterers raises the threshold required for the laser-like effects in the low-concentrated dye. We carried out our own experiments in rhodamine 6G laser dye and observed the laser-like effects both in dilute and concentrated dye with and without scatterers. In our experiment the wavelength of stimulated emission was strongly dependent on the scatterers' concentration. Our observations and some results of Refs. [2,3] are explained with a simple ASE (amplified spontaneous emission) model.