Random lasers based on organic epitaxial nanofibers

We present a review on random lasing in organic nanofibers made of oligophenyl nanocrystals grown by molecular epitaxy on polar substrates. The nanofibers have sub-wavelength cross-sectional dimensions and can extend in length up to the millimeter scale. We report on random lasing properties of nanofibers, under subpicosecond photopumping, both in the coherent and incoherent regimes. With the aid of both optical and morphological studies on individual fibers, we get insight into one-dimensional coherent feedback taking place along the nanofibers' axes. Model calculations of light propagation in disordered media allow us to give a semiquantitative description of one-dimensional coherent random lasing near the lasing threshold. We also report on amplified simulated emission in individual nanofibers and demonstrate that nanoscale linear optical amplifiers can be obtained by molecular self-assembly at surfaces. Photophysical studies of nanofibers resorting to subpicosecond luminescence and pump-probe spectroscopy give us valuable information on temperature-dependent, excited-state nonlinear processes, such as exciton-exciton annihilation and photoinduced absorption. Excited-state effects strongly influence lasing thresholds under quasi-continuous-wave photoexcitation conditions, as demonstrated in photoexcitation experiments performed with nanosecond pulses. Last, we briefly discuss the potential of organic epitaxial nanofibers featuring low-threshold random lasing for photonic sensing applications.