Two-photon absorption and optical power limiting of bifluorene molecule

We have studied the nonlinear absorption spectra and optical limiting prope rties of 2,2'-(9,9-dihexyl) bifluorene. Measurements were done in chlorofor m solution, in the visible range (450-650 nm), and for nanosecond time-dura tion pulses. The two-photon absorption (TPA) spectrum, obtained by the up-c onversion fluorescence method, shows a resonance at lambda = 534 nm with a cross-section sigma (TPA)=60 10(-50) cm(4).s/photon-molecule. Semiempirical quantum chemistry calculation on fluorene and bifluorene suggests an enhan cement of the bifluorene TPA due to coupling effects between monomers. In n onlinear transmission measurements, two-photon absorption is reinforced by the excited-state absorption that occurs during the pulse duration. At reso nance, the three-photon absorption coefficient is alpha (3) = 14 000 cm(3)/ GW(2) for a bifluorene concentration of 600 g/L. This strong nonlinear abso rption leads to an efficient optical power limiting in the green and blue p arts of the spectrum. The maximum transmitted energy is lower than 10 muJ f or an input energy of up to 200 muJ in a F/5 optical geometry. (C) 2001 Ame rican Institute of Physics.