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 effective length of the oligomer. The corresponding three-photon absorption coefficient α3 presents at resonance high values for the longest oligomers (α3 very much greater than 10000 cm3/GW2 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 μJ for an input energy of up to 350 μJ in a F/5 optical geometry).