Two-photon absorption induced nonlinear absorption in fluorene oligomers

We report here the nonlinear absorption and optical power limiting properties of polyfluorenes induced by two-photon absorption (TPA). Measurements were performed in chloroform between 450 and 650 nm for nanosecond time duration pulses. The nonlinear absorption is attributed to a three-photon absorption process involving a first TPA step followed by an excited state absorption (ESA) process. The TPA cross-sections of small oligomers increases with the square of their lengths. This trend is rationalized in terms of excitonic coupling between monomers within the range of the C effective length of the oligomer. The corresponding three-photon absorption coefficient alpha(3) presents at resonance high values for the longest oligomers (alpha(3) much greater than 10000 cm(3) /GW(2) for N = 60 at a concentration of 200 g/L), leading to efficient optical power limiting in the whole visible range (the maximum transmitted energy is lower than 10 muJ for an input energy of up to 350 muJ in a F/5 optical geometry).