Studies On The Third-order Optical Nonlinearities Of Poly (4,4'-diazobenzene-2, 5-thiophene)

In recent years,considerable attention has been focused on nonlinear optical materials for their promising applications, including image processing, all-optical switching and optical storage memory, so nonlinear optical materials and devices have stimulated much interest. Research of new materials that have great potential for use in nonlinear optical (NLO) devices is one of the current subjects of great interest. Polymers have come under critical study regarding their potential as NLO materials. A new typeπ-Conjugated copolymer[4 ,4′-diazobenzene-2,5-thiophene](PDABTh)were obtained by the polymerization of 2,5-dibromothiophene Grignard reagents and 4,4'-Diiodoazobenzene through nickel(II) complexes catalyzed cross-coupling reactions. Through the experiment we have observed that PDABTh has good heat stability, it also possesses large third-order NLO properties, which is expected to be used as polymer materials with electro-optical properties.In this article, the third-order NLO properties of the Dimethylformamide(DMF) solution of PDABTh whith concentration 6.8×10-5 mol/L was studied using Z-scan technique with 20 ps laser pulse at 532 nm. The polymer exhibits moderately strong third-order NLO absorption coefficient effectβ=8.1×10-13m/w and self-focusing performance. The third-order NLO refractive coefficient (n2) of the polymer is 5.6×10-19m2/w, while the hyperpolarizabilitiesγto be 2.5×10-30 esu. Based on the NLO experimental results, These relatively large NLO properties of PDABTh may have much to do with its symmetry of skeleton and rigidity of structures.From the obtained experimental results, the new polymer has relatively large third-order NLO properties at 532 nm. If physical mechanism is concerned, we think strong nonlinear absorption of the material is attributed to two-photo absorption of the polymer molecule, and polarization ofπ-bonding in polymer molecule, which belongs to photorefractive phenomena originated from the change of electron cloudy distribution, results in large nonlinear refractive index of the material.