Excitation density and photoluminescence studies of polyfluorene excited by two-photon absorption

Highly efficient photoluminescent organic materials are excellent alternatives to inorganic crystals for non-linear optics applications. One candidate, polyfluorene with an absorption onset of about 3 eV, was studied in this paper by means of transmission and photoluminescence measurements with an ultra-fast laser source at 1.54 eV with 250 fs pulses. We studied the two-photon absorption and emission properties of polyfluorene as a function of the concentration in toluene solvent at 300 K. It is demonstrated that both the two-photon absorption and the associated emission exhibit the same saturation intensity, which does not depend on the concentration. It is shown that although we achieve excitation rates of 1 % to 10% of all repetition units, the excited units do not interfere before luminescent recombination occurs.