Femtosecond time-resolved laser action in poly(p-phenylene vinylene) films: stimulated emission in an inhomogeneously broadened exciton distribution

Femtosecond time-resolved studies of photoluminescence (PL) and stimulated emission (SE) have been conducted in neat thin films of poly(p-phenylene vinylene) derivatives. For high excitation intensity I, we found that the SE spectral narrowing is accompanied by a dramatic lifetime shortening from ∼250 to ∼3 ps, which enhances energy migration within the inhomogeneously broadened exciton distribution. In addition, the SE is delayed with respect to the PL onset by a few picoseconds, depending on I and the illumination stripe length. In some cases, a second delayed SE is formed in the film due to the continuing energy migration into exciton states with superior radiative coupling.