Studies On Third-order Nonlinear Optical Properties Of QDs/MWCNTs Composites

Nonlinear optics is a branch of nonlinear physics,it mainly studies the nonlinear phenomena and its application produced by the interaction between laser and matter.With the development of nonlinear optical materials,many excellent advantages and outstanding applications of nonlinear optics have been found,which makes it play an important role in many fields of physic,such as all optical switches,optical limiters,optical frequency conversion,optical pulse compression,ultra fine material structure analysis and so on.Therefore,in recent years,nonlinear optical materials have increasingly become the research direction of the researchers,including semiconductor quantum dots,carbon nanotubes and their various composite materials.The unique one-dimensional tubular structure and excellent physical and chemical properties make carbon nanotubes play an important role in the fields of physics,biology,chemistry and material science.The main hybrid mode of carbon nanotubes is the SP2 hybrid of carbon atoms,therefore,carbon nanotubes have strong nonlinear response and fast response speed which have laid a good foundation for the third-order nonlinear exploration of carbon nanotubes,and more and more scholars have been devoted to the inquiry of the third-order nonlinear characteristics.Semiconductor quantum dots have prominent optical quality and the third-order nonlinear optical properties have attracted more attention from researchers.Among them,the study of IV-VI and II-VI semiconductor quantum dots is the most extensive,these semiconductors have unique third-order nonlinear characteristics and strong nonlinear response.Consequently,it has potential application value in many fields,such as optical storage,optical communication,all-optical switching and optical limiting.The composites of carbon nanotubes and quantum dots can also exhibit some quite remarkable nonlinear properties and be capable of improving the limitations of the single carbon nanotubes.What's more,the third-order nonlinear of carbon nanotubes and quantum dots composite materials is also a subject worthy to probe further into.The structure of this paper includes the following aspects: firstly,not only a simple description of the nonlinear optics and Z-scanning theory is made,but also analyzes the research situation of semiconductor quantum dots and carbon nanotubes;secondly,introducing the preparation methods of Pb S quantum dots and the third-order nonlinear properties is presented.The purification method of carbon nanotubes was introduced,the composites of Pb S quantum dots with different concentrations and carbon nanotubes were prepared,and the influence of the quantum dots concentration on the third-order nonlinear properties were described;then,the composite of Cd Se quantum dots and carbon nanotubes was synthesized,and the third-order nonlinear properties were investigated;and finally,the summary and prospect of the full text content were given.The main works of this paper are as follows:First,as a direct band gap semiconductor material,Pb S has a smaller band gap of 0.41 ev and a larger exciton radius of 18 nm,which makes it has strong electron hole pair limiting effect and large nonlinear response.In this paper,Pb S quantum dots were synthesized by simple ultrasonic chemistry method,and the prepared quantum dots were cubic crystal structure,the size was about 20 nm and all of them were homogeneous.The Z-scanning results indicate that the Pb S quantum dots have two-photon absorption property and self-defocusing property.Second,the surface modification of carbon nanotubes was carried out,the EDTA group was grafted,and using water bath ultrasonic method under atmospheric environment by controlling the content of lead source and sulfur source synthesis the composite of different Pb S quantum dots concentrations and carbon nanotubes.The test results showed that with the increase of Pb source and S source,with the number of Pb S QDs attached to the surface of MWCNTs increased,the density enlarged and the thickness increase gradually,so the diameter of the composite will increase gradually.The measurement of third-order nonlinear showed that the composite has saturable absorption characteristics and self-focusing property,with increasing the content of Pb S,the third-order nonlinear susceptibility increases at first and then decreases.When the concentration of the Pb S is 1.25�10-2 mol/L,the maximum third-order nonlinear susceptibility ?(3)=3.81�10-12 esu can be obtained.Third,in the atmosphere,a composite of cadmium selenide quantum dots and carbon nanotubes was prepared by a safe and low cost "green" synthesis process.The test results revealed that the Cd Se quantum dots has a cubic sphalerite structure and good crystallinity.Not noly the ID/IG value of the composite increases significantly compared with the carbon nanotubes.But also the composite increases a absorption peak which was caused by the Cd Se quantum dots compared with the pure carbon tubes.The third-order nonlinear test of composite material showed that Cd Se/MWCNTs composite has saturable absorption characteristics and self-focusing property.Fourth,summarizing the main work of this paper,what's more,prospect of the main research work and direction in the future were done,too.