Studies On Third-order Nonlinear Optical Properties Of Reduced Graphene Oxide And Graphene/Titanium Dioxide Composites

The birth of the first ruby laser has revolutionized optical technology and has had a profound impact on optical theory.Its application has spread to all areas of production and life.One of the important branch areas is nonlinear optics.Nonlinear optical materials play an important role in laser technology,spectroscopy,optical communication,integrated optics,optical switches,etc.Graphene is a honeycomb two-dimensional nanomaterial composed of carbon atoms and sp2 hybrid orbitals.It is considered to be the basic unit for constructing fullerenes,carbon nanotubes and graphite.Due to the stable planar structure of graphene and rapid electron mobility,it has attracted much attention in electronic products,optical sensors,nanoelectronic devices and solar cells.There are five main methods for preparing graphene.They are micromechanical stripping method,liquid phase stripping method,chemical vapor deposition method,epitaxial growth method,and graphite oxide reduction method.This paper uses the graphite oxide reduction method.Because the introduction of oxygen atoms in the graphene oxide changes the orbital hybridization type of carbon atoms,the sp3 carbon hybrid orbitals appear inside the graphene,and this change affects the optical properties of graphene.Therefore,it is necessary to explore the third-order nonlinear optical properties of graphene and graphene oxide.Finding materials with large third-order nonlinear coefficients and fast nonlinear response has become the mainstream target of nonlinear optics.The third-order nonlinear optical materials are mainly focused on semiconductor materials and metal materials.However,the light absorption near the plasma peak of the metal surface is greatly enhanced.Oxide has become a research hotspot because of its excellent optical,electrical and magnetic properties.Titanium dioxide is the most classic semiconductor material and also a metal oxide.With high transmittance,high refractive index and large third-order nonlinear coefficient,it has become the top priority of oxide materials research.Compared with pure titanium dioxide,titanium dioxide nanocomposites have attracted more and more people's research due to their potential applications in optoelectronics and optoelectronic devices.Some studies have shown that titanium dioxide composites can inhibit electron/hole pairs and introduce new energy levels in the band gap to accelerate the transfer of photogenerated electrons,thereby improving the third-order nonlinear properties of the material.The main work of this thesis is:First,the third-order nonlinear optical properties of graphene oxide with different degrees of reduction were studied.A higher oxygen content graphene oxide was prepared by the modified Hummers method,and then graphene oxides having different degrees of reduction were prepared by gradually increasing the dose of sodium borohydride.Finally,the samples were characterized by X-ray diffractometry,Fourier transform infrared spectroscopy,Raman spectroscopy,transmission electron microscopy,etc.We used the picosecond laser Z-scan method to measure and analyze the nonlinear optical properties of the samples.The test showed that the graphitized graphene showed a peak shape in the Z-scan opening pattern at 10 ?J,indicating that the graphene oxide has saturated absorption characteristics.The closed-cell Z-scan of graphene oxide exhibits the shape of the peak after the valley,indicating that graphene oxide has a positive nonlinear refractive index and has self-focusing properties.We analyze the reason because the electronic structure of graphene is a massless Dirac fermion.The graphene conduction band and the valence band intersect at the Dirac point,and the graphene has a band gap of zero,and it is easy to absorb all photons of any wavelength.Therefore,graphene has saturated absorption characteristics and can be used for laser Q-switching technology.In graphene oxide with different degree of reduction,the electron energy structure of graphene oxide changes due to the conversion of graphene sp2 hybrid structure and graphene oxide sp3 hybrid structure,which leads to the change of its absorption characteristics.Third-order nonlinear study of second titanium dioxide and graphene oxide composites.The structure and size of the composites were characterized by X-ray diffractometry,UV-Vis absorption spectroscopy and transmission electron microscopy.The results show that the composites of titanium dioxide and graphene exhibit saturated absorption characteristics and self-focusing properties.The third-order nonlinear characteristics are best at 10 h.We speculate that this is because the complex inhibits the recombination of electron-hole pairs during the reaction,resulting in a change in the energy level and affecting the nonlinear optical properties of the material.The third summarizes the main work of this paper and looks forward to the future work.And pointed out the problems and basic ideas that need to be further studied and solved in the future.