| dc.description |
We report on the design, synthesis, structural characterization and quadratic as well as cubic nonlinear optical (NLO) properties of a series of organic chromophores (6,6′-distyryl-3,3′-bipyridine derivatives disubstituted with donor (D) and/or acceptor (A) end-groups). The compounds are either dipolar (push–pull molecules) or apolar (symmetric D–A–A–D molecules). By lateral substitutions of the π-conjugated bridge and by varying the nature of the donor/acceptor (D/A) pair, we were able to tune both the mesogenic and NLO properties of the chromophores. Their second-order NLO properties were evaluated by the electric-field-induced second-harmonic technique. All the compounds are transparent at the second-harmonic wavelength of typical laser sources (1.32 and 1.55 μm). For the mesogenic chromophores, first hyperpolarizabilities extrapolated to infinite wavelength, β<sub>0</sub>, between 16 and 85 × 10<sup>−30</sup> esu were measured. These compounds exhibit enhanced hyperpolarizabilities compared to typical NLO liquid crystal chromophores reported in the literature. Both the nonlinear refractive index n<sub>2</sub> and the two-photon absorption (TPA) coefficient α<sub>2</sub> were simultaneously measured at λ 1064 nm, using a single-shot (50 ps, 10 Hz) multi-channel Z-scan method. For all molecular compounds, Z-scan measurements in dichloromethane solutions show a large nonlinear refractive index, close to that of pure carbon disulfide, without detectable TPA in most cases. These measurements lead to extrapolated nonresonant n<sub>2</sub> values between 1 × 10<sup>−16</sup> and 4 × 10<sup>−16</sup> m<sup>2</sup> W<sup>−1</sup>, whatever the electronic structure of the compounds. Hence, apolar symmetric D–A–A–D molecules appear to be an interesting novel family of NLO dyes. |
