Cooperative radiation and stimulated emission at high excitation densities in poly(para-phenylene)-type ladder polymer

We have studied the radiative recombination processes in poly (para-phenylene)-type ladderpolymer films using ultrafast spectroscopy techniques at excitation densities up to approximately 5 X 10 20 photons/cm 3 per pulse. In addition to the vibronic manifold of singlet exciton emission the photoluminescence spectrum at such elevated conditions is characterized by an optical transition, which is not present at low pump fluence. This spectral new component is identified as biexcitonic radiative recombination, an assignment that is supported by the hyperlinearity of its total emission as a function of pump intensities and the kinetics revealing a thermal equilibrium state between excitonic and biexcitonic populations. The biexciton binding energy is evaluated to E b,xx equals 120 meV. Stimulated emission in the spectral blue region is investigated by femtosecond pump and probe experiments.