Optical gain in Förster energy transfer based organic guest-host-systems

We study the optical gain for various doping concentrations in a dye doped polymer (poly-[9,9-dioctylfluorene] with 6,6-[2,2-octyloxy-1, 1-binaphthyl] spacer groups (BN-PFO) doped by the stilbene dye 1,4-bis[2-[4-[N,N-di[p-tolyl]amino]phenyl]vinyl-benzene] (DPAVB)). In such a guest-host-system (GHS) the occupation of the upper laser level (dopant site) is due to Förster energy transfer (FET), which strongly depends on the donor acceptor distance and hence on the concentration of the laser dye. Therefore, the doping concentration is varied over a wide range and the gain coefficients are measured at various excitation densities to analyze the stimulated emission cross section. For the investigated GHS maximum gain coefficients up to 340 cm-1 were found at absorbed pump energy densities of around 50 J/cm2. It will be shown that the stimulated emission cross section ( = 1.8 × 10-16 cm2) is concentration independent which is quite different to a recently investigated small m olecule based GHS. These effects will be discussed considering the rate and exciton diffusion constants.