Mode interaction and light transport in bidimensional organic random lasers in the weak scattering limit

We report on the modes interaction and light transport in weakly scattering neat films of small molecular weight organic molecules, showing coherent random lasing. The lasing modes interaction exhibits peculiar properties, with spatially overlapped modes at low excitation density and modes competition at high excitation density. This results in a progressive decrease and in a saturation of the number of lasing modes with increasing the excitation density. The weak scattering results in diffusive light transport with a transport mean free path in the range 14.6–125 μm, which is much higher that the lasing wavelength λ≈620 nm. The average transport properties are correlated with the single scattering event cross section. We demonstrate that the light propagation is mainly affected by single scattering properties rather than collective effects.