Study On The Third-order Nonlinear Optical Properties Of Graphene And Its Composites

Since the first ruby laser was invented in 1960,many new disciplines have been born.One of the most important subjects is nonlinear optics.Nonlinear optics and nonlinear optical materials always play an importantrole in many areas,such as laser technology,optoelectronic technology,photochemical,optical limiting,optical switch,optical computing,communication technology and other fields.Recently,the performance indexes of nonlinear optical materials have been gradually unable to meet practical requirements with the development of photoelectric technology.Therefore,exploring and improving the performance of nonlinear optical materials has become an important direction of research.Semiconductor nanomaterials have been praised for their outstanding advantages in terms of their special energy band structure and their photoelectric properties,which implies the application value of the semiconductor nanomaterials is immeasurable in functionalization of special materials,devices and other related areas.The quantum confinement effect is significant because of the size of the material is generally some nanometers,and often smaller than its own exciton Bohr radius,which makesthe tendency of the semiconductor nanocomposites to be agglomerated and poor in stability.Graphene is a two-dimensional structure containing only hexagonal carbon atoms in lattice structures and has a thickness of only one carbon atom.It attract much attention in many fields such as energy materials,biomedicine,physics and chemistry,which depends entirely on its stable and orderly structure,ultrafast electron transfer,mechanical properties and excellent thermal conductivity,high surface area,electricity and other properties.Therefore,graphene hasbecome anideal carrier of semiconductor nanomaterials.It can not only solve the problem of easy agglomeration of semiconductor nano materials,but also greatly improve theperformance of composite materialsvia the effect of synergies afterthe combination of semiconductor quantum dots and graphene in covalent connection?Graphene is easily oxidized by oxidants under acidic conditions,and the oxidation process can also be regarded as a process in which graphene is modified by oxygen-containing groups.Because there are many types and quantities of theoxygen-containing functional groups on the graphene oxide,the structure of graphene oxide is very different compared with graphene,and thus the properties of the two are also very different.Therefore,it will become meaningful to explore the effect of the structural change on the properties of graphene oxide and graphene.The main contents of this paper are as follows:Firstly,the nonlinear optical properties graphere oxide were studied.Graphene oxide with high oxygen content was prepared by chemical means,and then was reduced by using the reducing agent,and thedifferent degree of reduction of graphene oxide with different oxygen content was obtained by controlling the degree of reduction.The samples were characterized by UV-Vis spectroscopy,Raman spectroscopy,Fourier transform infrared spectroscopy,X-ray diffractometer and so on.Then,the picosecond Z-scan technique(laser wavelength 532 nm,pulse width 30 ps,frequency 10Hz)was used to test the optical nonlinear properties of the samples.The results show that the graphene oxide has good two-photon absorption,but with the increase of the degree of reduction,the two-photon absorption is gradually weakened and the saturated absorption gradually increases.This phenomenon is mainly due to conversion fromthe sp3 cluster to the sp2 domains in the graphene under the effect of the reducing agent,and then cause the change of the internal electronic structure,and the electron transition mode also changes,resulting in nonlinear absorption characteristics change gradually.In addition,under the large light intensity,graphene oxide and different degree of reduction of graphene oxide all have an anti-saturated absorption,which is related to the excited state absorption.Secondly,the third-order optical nonlinearities of CdS quantum dots and graphene Composites were studied.The nano-quantum dots of cadmium sulfide nanoparticles were covalently linked with graphene.and analyzed by X-ray diffraction,Raman spectroscopy,Fourier transform infrared spectroscopy,UV-Vis absorption spectroscopy and transmission electron microscopy.The results show that the material has a good sphalerite structure,and the quantum dot size is about 7 nm.Finally,the third-order nonlinear optical properties of the composites were measured under picosecond laser.The result show that the optical nonlinearity of G/CdS composites which connected with covalent bond was significantly enhanced and the structural defects were significantly increased.The enhancement of optical nonlinearity is due to the synergistic effect of charge transfer in the local electric field between CdS nanocrystals and graphene,whilein G/CdS hybrid structure,the main reason of the increase of nonlinear optics remaining in the same order of magnitude is the increase of nonradiative defects.In addition,we found that the synergistic polarizability of G/CdS increased rapidly in the range of 4.3GW/cm2 to 8.1GW/cm2,and increased slowly beyond this range.Thirdly,summary and outlook of this work are made.It points out the problems to be solved and the basic ideas.