Angular distribution of random laser emission.

We present an experimental analysis of the angular distribution of the emission of a random laser (RL) operating in the diffusive regime (∼1% volume fraction of scatterers). The RL ensemble was made of silica particles suspended in a 1:1 methanol:water matrix with Rhodamine 6G dye as the active medium. We found that, for specific pumping-energy-dependent scattering strength, the RL spectrum reached stable features that were angularly preserved. From the analysis, we propose that this defines a novel parametric condition that may well be the equivalent of the RL critical volume, as proposed by Letokhov.