Study On Ultrafast Nonlinear Optical Properties Of Composite Films Based On π Conjugated Molecules

Thin films have unique structural characteristics such as small volume,uniform film thickness,easy integration and excellent nonlinear optical properties.Developing organic thin films with ultrafast response is the only way for the final device of nonlinear optical materials.Composite films have broad application prospects in the fields of optical limiting,two-photon fluorescence imaging,all-optical switches and optical information storage.The third-order nonlinear optical properties of organic composite films depend not only on the chemical structure of the component monomers,but also on their molecular morphology and arrangement.The dense and orderly arrangement of the molecules in the composite film makes the π electrons of the conjugated structure tend to delocalize and prone to polarization,which has a large third-order nonlinear optical effect.The study of nonlinear optical properties of organic composite films,on the one hand,helps us to deeply understand the influence of composite film structure and organic molecular aggregation form on the third-order nonlinear optical properties,and establish the correlation between film micro structure and macro nonlinearity.On the other hand,it provides reference for the design,optimization and synthesis of new nonlinear optical films.In this dissertation,N-(benzothiazol-2-yl)ferrocenylhydrazone metal organic complexes with Donor-Acceptor(D-A)structure,water-soluble lithium indium phthalocyanine chloride,water-soluble fullerene derivatives were explored and synthesized,and one-dimensional,two-dimensional and three-dimensional composite films with delocalized π electrons were prepared.We systematically studied the nonlinear optical properties of the composite films by Z-scan and 4f phase coherent imaging technique.This dissertation mainly carried out the following tasks:Firstly,C18H15N3SFe/PMMA composite films were prepared by doping C18H15N3SFe molecules into PMMA polymer matrix.The linear and ultrafast nonlinear properties of samples were investigated via Ultraviolet-near infrared absorption spectroscopy and Z-scan measurements.The experimental results under 190 fs at 532 nm show that the nonlinear absorption response of C18H15N3SFe/PMMA film is derived from the excited state absorption effect induced by two photons.C18H15N3SFe/PMMA films show strong reverse saturation absorption(RSA).Secondly,an ultrathin multilayer film with well-defined structure containing anionic Poly[[4,4-bis(4-sulfobutyl)-4H-cyclopenta[2,1-b:3,4-b’]dithiophene-2,6-diyl]-1,4-phenylene sodium sa(PCPDTPhSO3Na)and cationic poly diallydimethyl ammonium chloride(PDDA)were prepared using layer-by-layer assembly technique.The ultrafast nonlinear properties of samples were investigated via the 4f coherent imaging system with phase object(NIT-PO).The combination of experimental data and theoretical simulation reveals the influence of the aggregates on the nonlinear optical properties.The results of NIT-PO demonstrate that PCPDTPhSO3Na/PDDA film exhibited excellent self-focusing effect and RSA characteristics under 190 fs at 400 nm,and considerable self-defocusing effect and saturation absorption(RA)characteristics under 190 fs at 532 nm.The RSA of the film mainly comes from the excited state absorption of PCPDTPhSO3Na molecule,and RA may be caused by the ground state bleaching effect.Thirdly,an ultrathin composite films with alternated layers containing anionic lithium tetracarboxyl indium phthalocyanine chloride(ClIn-Pc(COOLi)4)and PDDA were prepared using layer-by-layer assembly technique.The resulting films were characterized by UV-Vis and Fourier transform infrared spectroscopy,showing regularly consecutive adsorption.The ultrafast nonlinear properties of samples were investigated via NIT-PO with 190 fs laser pulses at 532 nm.The UV-Vis absorption spectra showed that the ClIn-Pc(COOLi)4 molecules in the composite films were arranged in an orderly manner.The π-π interaction of ClIn-Pc(COOLi)4 molecules in the film is strengthened,making it easier for electron clouds to overlap.The overlapping microstructure of intermolecular orbitals will promote charge transfer.The result of NIT-PO show that the film shows excellent self-focusing effect and RSA,which indicates that the high third-order nonlinear effect of the film is not only due to the large conjugated delocalization system of organic molecules,but also the collective effect produced by π aggregates.Our results indicate that the ClIn-Pc(COOLi)4/PDDA film is a promising candidate for future nonlinear optical devices.Fourthly,a set of ultrathin multilayer films with alternated layers containing PCPDTPhSO3Na as donor and amine group modified fullerene derivative as acceptor were prepared using layer-by-layer assembly technique on a quartz substrate.The growth of the film was monitored by UV-Vis absorption spectra,showing regularly consecutive deposition process.The ultrafast nonlinear properties of samples were investigated via NIT-PO.Experimental results under 190 fs at 532 nm and 400 nm demonstrate that the ultrathin films exhibit excellent self-focusing effect and RSA.The charge transfer between donor and acceptor molecules is helpful to obtain a larger nonlinear optical response of the composite films.The ultrafast nonlinear properties of PCPDTPhSO3Na/fullerene derivative composite films are affected by the charge transfer effect from PCPDTPhSO3Na donor to fullerene derivative acceptor in the ordered donor acceptor structure.