The fabrication and enhanced nonlinear optical properties of electrostatic self-assembled film containing water-soluble chiral polymers

An ultra-thin film containing a water-soluble chiral PPV derivative and oligo-thiophene derivative was fabricated through the electrostatic self-assembly technique. The PPV and thiophene derivatives are poly{(2,5-bis(3-bromotrimethylammoniopropoxy)-phenylene-1,4-divinylene)-alt-1,4-(2,5-bis((3-hydroxy-2-(S)-methyl)propoxy)phenylenevinylene) (BHP-PPV) and 4′,3″-dipentyl-5,2′:5′,2″:5″,2″′-quaterthiophene-2,5″′-dicarboxylic acid (QTDA), respectively. The circular dichroism (CD) spectrum of BHP-PPV cast film on quartz substrate proved the chirality of BHP-PPV. The UV–vis spectra showed a continuous deposition process of BHP-PPV and QTDA. The film structure was characterized by small angle X-ray diffraction (XRD) measurement and atomic force microscopy (AFM) images. The nonlinear optical (NLO) properties of BHP-PPV/QTDA ultra-thin film with different number of bilayers were investigated by the Z-scan technique with 8 ns laser pulse at 532 nm. The Z-scan experimental data were analyzed with the double-sided film Z-scan theory. The BHP-PPV/QTDA film exhibits enhanced reverse saturable absorption (RSA) and self-defocusing effects, which may be attributed to the conjugated strength, chirality and well-ordered film structure. The chirality may lead to the RSA of BHP-PPV/QTDA film contrary to the SA of the other electrostatic self-assembled films without chiral units. The self-defocusing effect should be due to the thermal effect.