Far-field emission and feedback origin of random lasing in oligothiophene dioxide neat films

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Abstract
We report on random lasing in substituted quinquethienyl S,S-dioxide neat films. Despite the absence of highly efficient scatterers in the film, a fine structure with laser-like peaks as narrow as 5 Å is observed in the emission spectra. The far-field emission pattern is studied through angle-resolved emission measurements, demonstrating that random lasing emission is directional, with a 8° divergence but different individual emission patterns. The origin of the scattering centers providing the feedback for lasing has been analyzed through atomic force microscopy measurements of the film surface. We demonstrate that the random lasing is induced by sequential scattering from 50 nm diameter holes in the film with an average distance of 500 nm, while thickness fluctuations are not relevant.