Investigation On Ultrafast Optical Properties Of CdSeTe Nanoparticles

Compared with the bulk,nanomaterials have special properties.So they have been paid more and more attention by researchers in physics,chemistry,biology,medicine,and materials field.The process and mechanism of carrier generation and transport,energy transfer and conversion affect directly the photoelectric properties.Therefore,the investigation on the photoelectric properties of semiconductor materials in the nano-scale and ultrafast time scales has become a hot spot in the field of physics and chemistry.In this dissertation,the ultrafast optical properties of CdSeTe nanoparticles（NPs）are studied using Z-scan,pump-probe and time-resolved fluorescence technique,specific work is as follows:1.Nonlinear absorption of CdSeTe NPsThe nonlinear absorption properties of CdSeTe NPs were investigated by usingZ-scan technique with 800nm femtosecond laser.At low and high repetition rate,the nonlinear absorption properties of CdSeTe NPs in toluene solution were measured.It was found,that CdSe Te NPstoluene solution showed reverse saturable absorption characteristics.Based on the theoretical analysis,at a repetition rate of about 83 MHz,the nonlinear absorption coefficient is obtained to be1.7×10^-2cm/GW.While at a repetition rate of about 1000Hz,the nonlinear absorption coefficient is 3.1×10^-2cm/GW.The nonlinear absorption properties of three kinds of CdSeTe-doped glass filters（RG780,RG830 and RG850）were investigated by using open-aperture Z-scan at a repetition rate of 1000 Hz.It was found that RG780 showed the reverse saturable absorption characteristics,while RG830 and RG850 showed saturable absorptionunder weak intensity.With the increase of excitation intensity,the nonlinear absorption in RG830 and RG850 changes from saturable absorption to reverse saturable absorption.2.Carrier dynamics of CdSeTe NPsThe carrier dynamics of CdSeTe NPs（MB652）in toluene is studied by use of femtosecond transient absorption.The results indicated that the change of transient absorption is at 440⁴70 nm region.The maxmium of transient absorption is at 510nm,which is due to the band-edge absorption of MB652.The excited-state lifetime were obtained by analyzing experimental results.The ultrafast decays of CdSe_XTe_1-X（x=0.32）quantum dots in the glassmatrix are studied by using one-colorpump probe experiments.Obvious coherent phonons of CdSe_XTe_1-X quantum dots were found.The oscillation frequencies of coherent phonon are obtained to be 6.04 and 4.83THz,and the former results from CdSe phonon mode,the latter from CdTe phonon mode.The dynamics of photoinduced exciton in three kinds of samples were studied under the excitation of 400nm laser pulses at 1000 Hz repetition rate.The results show that the kinetic process is composed of three different decay processes.Among them,the fast decay component is attributed to the relaxation of carrier recombination,the intermediate component is related to the exciton decay,and the slow process is mainly attributed to the recombination process3.Lumisecent dynamics of CdSeTe NPsFluorescence characteristics of CdSeTe nanoparticles in toluene solution were studied by time-resolved fluorescence spectroscopy.It is found that the CdSeTe nanoparticles with different Se/Te doping ratios have obvious fluorescence in 650nm-810nm under the excitation of femtosecond laser pulses of 400 nm.With the increase of Te ratio,the fluorescence peak is red shifted and the fluorescence lifetime increases from 14.46 ns to40.60 ns.The theoretical analysis shows that the fluorescence of CdSeTe nanoparticles mainly originates from the excitonic state and the surface state of the electron emission.With the increase of Te ratio,surface state and defectstates components increase,causing the corresponding fluorescence peak to redshift,fluorescence lifetime to increase.