Random laser tailored by directional stimulated emission

The spectrum of a random laser (RL) may appear either as a set of sharp resonances or as a smooth line superimposed on the fluorescence. Recently, Leonetti, Conti, and Lopez [Nat. Photonics 5, 615 (2011)] accounted for this duality with the onset of a mode-locked regime that is triggered by the increase of both the number of activated modes and of the intermode interaction and is accompanied by a pulse shortening. Here we report an extensive review of the experimental approach used by Leonetti et al., including the sample preparation and the particulars of the setup. Here we describe also the way in which our approach allows us either to set the degree of interaction between modes or to have a certain degree of control over the effective set of resonances brought to lasing. Moreover we report an investigation on the spatial properties of the RL which brings further confirmation of the synchronization picture, whose physical origin is deepened.