Dynamical study on the stimulated absorption and emission in a conjugated polymer

By applying a femtosecond electric pump pulse to a poly(p-phenylene vinylene) (PPV) molecule, we theoretically investigate the dynamical processes for its stimulated absorption and emission. The simulations are performed within the framework of an extended version of one-dimensional Su-Schrieffer-Heeger tight-binding model combined with a nonadiabatic evolution method. Firstly, we set the molecule initially lying in the ground state, by which we give the relation between different stimulated transition modes and the photoexciting pulse. Analysis of the final states shows that we can only obtain some electron-hole binding states by an external optical excitation for the molecule, which includes exciton, biexciton, and high-energy exciton. We have calculated their yields and find that they are determined by the optical excitation energy. In addition, based on the experimental observations, we separately investigate the effect of the optical excitation intensity on the yields of biexciton and high-energy exciton. The calculated results are consistent with the corresponding experimental speculations. Finally, by setting the molecule lying in an exciton or a biexciton, we focus on the stimulated emission process between their generated intragap states. Effects of the optical excitation energy and intensity on them are separately analyzed. These results might be of great importance for further improving the optical applications of polymers, especially for optimizing the polymer photovoltaic and laser properties.