Low-threshold blue lasing in epitaxially grown para-sexiphenyl nanofibers

Random laser action and amplified spontaneous emission are observed near 425 nm in self-assembled para-sexiphenyl nanofibers following subpicosecond optical pumping. The threshold excitation fluence (photoexcited density) is as low as 0.5 μJ/cm2 (6×1016 cm−3). The high degree of material crystallinity results in a very large singlet-singlet annihilation rate of ≈10−7 cm3/s. In stationary conditions, assuming a standard singlet-to-triplet density ratio of 0.3 and bimolecular recombination as the only density-dependent loss mechanism, the equivalent current density necessary for lasing threshold is estimated to be as low as 3 kA/cm2. The experimental findings suggest that such highly ordered molecular nanoaggregates have great potential as blue-emitting devices for integrated photonic applications.