Dependence of the random laser behavior on the concentrations of dye and scatterers

We have studied stimulated emission in a random laser based on rhodamine 6G dye and TiO2 nanoparticles. At both small and large concentrations of nanoparticles, the minimum threshold was found at ultra-high concentrations of dye, 20 g L-1. With the increase in concentration of TiO2 nanoparticles, the threshold reaches its maximum at the transition from a weak-scattering regime to a strong-scattering regime, when the transport mean free path l(t) is approximately equal to the absorption length l(a). At the same value of l(t), the random laser emission qualitatively changes its behavior. The experimental results are in good agreement with the predictions of a heuristic model.