Threshold excitation statistics and imaging of random lasers in ?-conjugated polymer films; evidence for random resonators

Random lasing is laser action that consists of a sequence of narrow, coherent spectral lines observed when stimulated emission in a disordered gain medium is excited above threshold excitation intensity, Ith. We studied three Ith distribution functions, F(n)(Ith) in ?-conjugated polymer films, where n (= 1-3) is the number of lasing random resonators in the excited film area; including their evolution with respect to the excitation area size, and polymer mixture with various concentrations of TiO2 nanoparticles. Optical images of the excited film area support the Ith measurements; since the sharp laser lines in the emission spectrum are accompanied by the appearance of lasing random resonators in the picture. At Ith, where a single dominant random lasing resonator appears in the accompanying picture, we found good agreement between the cavity diameter extracted from the emission power Fourier transform analysis, and the cavity size in the picture. This was directly confirmed by a novel technique of space/spectrum cross-correlation of the laser emission lines with 10x10 ?m2 spatial resolution; where we show that the laser modes are indeed localized within the respective lasing resonator. We found that F(1)(Ith) has both a marked asymmetry with respect to the mean threshold, (Ith)ave, and decreases with the area size. We also found that upon increasing the TiO2 nanoparticles density, ? in the polymer/TiO2 mixture the light mean free path decreases, F(1)(Ith) broadens, and (Ith)ave linearly increases with ?, indicating that the addition of extrinsic scatterers to the polymer matrix is not beneficial to random lasing. Our results provide strong evidence for the model of random resonators in the gain medium for explaining random lasing in ?-conjugated polymer films.