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Nonlinear absorption and refraction measurements of Fluorene-based molecules via Picosecond Z-scans

Fluorene-based molecules were studied in detail as candidates for two-photon absorption (2PA) applications. Previous studies were carried out over the wavelengths range 560 - 900 nm. In this work we have extended our studies to shorter wavelengths in order to determine behavior closer to the 2PA resonance peak. We used picosecond pulses at 532 nm for performing open and closed aperture Z-scan measurements, in order to obtain both the real part of the third order nonlinearity (χr(3)), corresponding to nonlinear refraction (n2) and the imaginary part of the third order nonlinearity (χi(3)), corresponding to nonlinear absorption (β) due to 2PA. We studied several fluorene-based molecules, which differ by length of conjugation system and substitution moieties. From open aperture Z-scan measurements we obtained the imaginary part of the third order nonlinearity (χi(3)). We measured 2PA cross-sections, δ, varying from 1,000 up to 10,000 GM at 532 nm. The influence of the molecular structure on δ was examined. For some of the dyes we also measured n2 via closed aperture Z-scan measurements. By subtracting the contribution of the solvent from the observed value for the solution, we obtain the contribution to n2 due to the solute.