Research On The Nonlinear Optical Properties Of Nanocomposites By Femtosecond Laser Z-scan Technology

The nanocomposites are new nonlinear optical materials with great application prospects.Because of their unique size effect and assemble easily, they have been received extensiveattention. But how to research on the nonlinear optical properties of nanocomposites deeply isstill lacks a powerful tool. To solve this problem for the Z-scan technique in optical signalanalysis extraction and high-precision, high efficiency we set up the femtosecond z-scan testsystem in this thesis. Then using the improved Adomian operator to deduce the analyticalmodels of saturable absorption and summarizes the theoretical models of two-photonabsorption, three photon absorption, multiphoton absorption. Based on these works, wedesigned software on matlab to analysis the Z-scan test results, acquired relative nonlinearoptical parameters.At last, by using the femtosecond Z-scan technology and the Z-scan data analysissoftware we have researched the nonlinear optical properties of metal nanoparticles and metalnanocomposites. Then we founded that the nonlinear optical properties of these nanoparticleswere influenced by the morphology, size, constituent and surface modification, et al. Throughquantitative analysis of the experiment results, we came to the summery about how to designmetal nanocomposites for control their nonlinear optical properties which make metalnanoparticles as a good candidate for new optical nonlinear materials.The detail work divided into the following several parts:1,For build an accurate and stable femtosecond Z-scan measurement we choose stablefemtosecond laser light source, large area silicon photoelectric detector (S1337-1010BR)with relatively high precision, and efficient collection plan with professional digitaloscilloscope which can not only enhance the precision of Z-scan measurements and ensure the stability of the system. Simultaneously, the control strategy, realize the Z-scancontrol program on the operating system of digital oscilloscope, saves the communicationtimes to transfer test data between PC and collecting instrument. This method improvedthe test efficiency by10times than using PC as controller. The Z-scan system we builtwas tested by measuring the nonlinear refraction standard sample CS2and nonlinearabsorption samples Rhodamine6B. The test results proved that the femtosecond Z-scanmeasurement works stable and correctly.2,With the research of diversity of materials, some other nonlinear absorption processes,which even appeared simultaneously, were observed form open aperture Z-scan traces.However, the existing analytical model is relatively homogeneous. Therefore we establishan improved modeling regarding Z-scan technique for charactering a nonlinear opticalthin absorber with saturable absorption. Subsequently we summarized analytical model oftwo-photon absorption, three-photon absorption, multi-photon absorption and saturableabsorption and according these theories the Z-scan data process program was implement.3,The nonlinear absorption properties of small organic molecules were characterized by thefemtosecond Z-scan system we built and analyzed by the Z-scan data process program. Inthe thesis, porphyrin derivatives, which easy assemble with metal nanoparticles, include5,10,15,20-Tetraphenyl-21H,23H-porphine,5,10,15,20-Tetrakis(4-methoxyphenyl)-21H,23H-porphine and Porphine-5,10,15,20-tetrakis were investigated. At the sameexperimental conditions, we have measured the two-photon absorption crossection ofthese three samples and chose Porphine-5,10,15,20-tetrakis, which has the largesttwo-photon absorption coefficients, as a good candidate for wrap up the metalnanoparticles in order to improve the nonlinear absorption crossection of nanocomposits.4,the nonlinear absorption properties of silver nanocrystals with different shape and Aunanorods were investigated. Not only the size effect the nonlinear optical properties ofmetal nanoparticles as previous reported, but also the morphology. The Ag nanoprismswith75nm size show700nm SPR peak and exhibit both negative and positive nonlinearabsorption. The nonlinear absorption process is considered to generated by ground-stateplasmon bleaching,free-carrier absorption, and interband transitions. The Ag nanodiskswith35nm show495nm SPR peak. However, Ag nanodisks displayed only positivenonlinear absorption process. The different nonlinear absorption process is attributed to the morphology influence by the Ag nanocrystals.5,Molecular engineering method was used to cover the same size of Au nanorods withSiO2and polymer PSS respectively. The influence of core-shell structure on the nonlinearabsorption properties of metal nanopaticles was researched. The SPR peak of SiO2structure was moved to the red direction relative to the Au nanorods, which more close tothe wavelength of laser beam and the SPR peak of PSS structure turn to the direction ofblue light. The Z-scan experimental results of the SiO2structure, Au nanorods and PSSstructure preserved that the saturable absorption process in SiO2was the most notable inthe three samples. Saturable absorption indensity of SiO2structure is two orders ofmagnitude than the PSS's. Surface modification can change the nonlinear opricalproperties of metal nanocomposits.