Preparation And Investigation On Third-order Nonlinear Optical Properties Of ZnS/MWCNTs Composite Materials

The research on the nonlinear optical material with a large third-order nonlinear refractive index and an ultra-fast response speed is of great significance in many aspects of optics such as optical communications, optical limiters, as well as all-optical switch. The research hot topics in the area of nonlinear optical materials at present are semiconductor quantum dots(QDs), carbon nanotubes(CNTs), organic material and metal material.The unique one-dimensional tubular structure and special physical and chemical properties of CNTs make them playing a vital role in physics, chemistry, biology and materials science. The unique one-dimensional p electronic structure and fast nonlinear response characteristic of the CNT makes it a hot topic for research workers. The unique quantum size effect of the semiconductor QD makes it showing different linear and nonlinear optical properties. The semiconductor QD will show different nonlinear optical effects woth different synthetic methods, different production process, and different sizes. Therefore, the nonlinear optical properties of semiconductor QDs are also in need to investigate in detail. Semiconductor materials can be dispersed in the organic polymer, and made into films. They can also be composited with other materials(e.g. CNTs and graphene) through physical or chemical methods. The composite materials not only show the nonlinear optical effect of semiconductor QDs, but also keep the film characteristics of plasticity, and are convement to process. Since both the two materials have strong third-order nonlinear optical properties, the third-order nonlinear optical properties of modified composite is worthy to inquiry.This thesis is divided into five chapters. Chapter 1 is introduction. Firstly, the current research progress of the third-order nonlinear optical is summarized mainly. Then the development of nonlinear optics and the principle of third-order nonlinear optical effects are introduced. The research progresses of the third-order nonlinear optical effects of MWCNTs and semiconductor QDs are presented respectively at last. The measuring methods of the nonlinear optical properties and their principles are shown in Chapter 2, and the Z-scan principle of single beam and the calculation method are introduced mainly. The purification experiment of MWCNTs and the third-order nonlinear optical properties of MWCNTs are shown in Chapter 3. The synthesis of Zn S QDs and their third-order nonlinear optical properties are introduced in Chapter 4. The synthesis of MWCNTs and Zn S QDs with different concentration are presented in Chapter 5. Z-scan test of the composite materials and body materials under the same concentration are performed, the variations of the third-order optical properties versus the concentration of Zn S QDs are discussed. Finally, the summary and prospect of the work are shown.The main contents of this thesis are as follows:Firstly, the MWCNTs are purified experimentally by washing with the strong mixed acid etching method. By comparing the transmission electron micrograph(TEM) and X-ray diffraction(XRD) of the MWCNTs, their surfaces are found to be smooth, and there are no other elements. Through the Z-scan test, the MWCNTs are found to be a self-focusing material and exhibit saturated absorption characteristics.Secondly, the Zn S QDs with the size of 10 nm synthesized, and their sizes are distributed uniformly. The Zn S QDs are found to be self-defocusing and two-photon absorption properties experimentally. The third-order nonlinear susceptibility of the Zn S QDs is measured to be 3.06 × 10-12 esu, and is nearly 10 times higher than that of MWCNTs.Thirdly, the MWCNTs are wrapped with the Zn S QDs by adopting the chemical adsorption method. The composite materials of MWCNTs with different concentrations of Zn S QDs are synthesized by controlling the reaction process. Z-scan test of the composite materials and the original materials are performed under the same concentration. The composite materials show the saturation absorption characteristic of the MWCNTs and the negative nonlinear refraction effect of Zn S. The third-order nonlinear susceptibility of the composite materials reach the maximum value of 2.67× 10-11 esu when both the concentration of zinc source and S source are 6.25 × 10-3 mol/L. Comparing with MWCNTs, the sign of the third-order nonlinear refractive index is changed and the third-order nonlinear absorption is reduced for the composite materials. This indicates that the composite materials have potential applications in optical information storage, all optical switch and so on.Finally, the work during the graduate student period is summarized, and the future work is prospected.