Gain dynamics in conjugated polymers at room temperature

We report on stimulated emission and optical gain in BEH-PPV polymer films at room temperature. Performing femtosecond pump-probe experiments we find very fast (≤100 fs) relaxation of excitons within a ladder of vibronic states. From spectra taken at very early times after photoexcitation we identify several vibronic transitions with a linewidth of about 8 nm. Strong coupling of excitons to these vibrations leads to stimulated emission and large optical gain of up to 10⁴ cm^-1. For a pump-probe delay of 200 fs the different tiansitions are no longer distinguishable due to broadening through spectral diffusion and exciton migration. The lifetime of the optical gain corresponds to the exciton lifetime. Population depletion by stimulated emission leads to a density dependent decay within a few hundred fs up to about 2 ps.