Spectra Properties And Optical Nonlinearities Of A Series Of CdSe_xS_1-x And CdSe_xS_1-x/ZnS Quantum Dots

The quantum dots(QDs) possess the large nonlinear absorption coefficient and nonlinear refractive index, which enable them to be the candidates for the all-optical switching and optical limiter. Therefore, it is very significant that their optical nonlinearity, their physical mechanism and their potential application are investigated. In this dissertation, we chose a series of CdSe_xS_1-xand CdSe_xS_1-x/ZnS QDs as the research object, and discus the effects of the composition and shell of QDs on the spectra. Their nonlinear optical properties have been investigated by Z-scan technique. At last, the potential application in optical limiting also has been attempted for these QDs. The main results of the research work are as follows:1.A detailed analysis of the series of CdSe_xS_1-xand CdSe_xS_1-x/ZnS QDs was presented by means of absorption and fluorescence spectra. First, based on the absorption spectra and effective mass approximation, the composition of these QDs was determined, which is more accurate than feed ratio; it was also found that the spectra properties can be tuned by the composition. Second, the intrinsic radiative lifetime of these QDs was determined on the basis of the Gaussian fitting for the absorption spectra; it was found that the intrinsic radiative lifetime is decreased and the oscillator strength is enhanced by the surface modification of shell structure. Comparing the fluorescence spectra of core-shell QDs with that of their counterpart, it was found that the fluorescence intensity is enhanced by surface modification. These results indicate that surface modification can reduce their defect concentration, which results in the decrease of non-radiative channel and enhancement of fluorescent intensity.2.We investigated the optical nonlinearity of the different composition of the non-core/shell and core/shell QDs with 1064nm ps pulse by Z-scan technique. The Z-scan results show that two-photon absorption(TPA) occurred for core/shell CdSe_0.5S_0.5/ZnS QDs, and no TPA occurred for the others, including CdSe_0.3S_0.7,CdSe_0.5S_0.5,CdSe_0.8S_0.2,CdSe_0.8S_0.2,/ZnS and CdSe_0.3S_0.7/ZnS QDs. First, we established the energy level model of the optical physical process and interpreted the change of the nonlinear refractive indices with the increase of input intensity; the third-order nonlinear indices is determined to be 1.50×10^-17m²/W(SI)(5.1×10^-11esu), the refraction cross section is 6.74×10^-24cm³, their average TPA cross section is up to the order of 1.3×10⁴GM. Second, the nonlinear refractive indices of CdSe_0.3S_0.7,CdSe_0.5S_0.5,CdSe_0.8S_0.2,CdSe_0.8S_0.2/ZnS and CdSe_0.3S_0.7/ZnS QDs is obtained by close-aperture Z-scan, it was found that their nonlinear refractive indices hardly vary with input intensity. Base on the experimental results, the optical nonlinearity in the nonresonant region is ascribed to the bound electronic response, which results from the distortion of electron clound and indicate that the input intensity have less influence on them. It was also found that the nonlinear refractive indices increase with the decrease of bandgap, which is caused by Se content in these QDs. Then we give a qualitative explanation for this phenomenon. This result shows that not only the peak wavelength of these QDs but their nonlinear optical properties can be altered by composition of these QDs. Moreover, the nonlinear refractive indices of core/shell QDs is larger than the counterpart with the same composition, which indicates that surface modification by coating shell structure on the QDs can improve their nonlinear optical properties.3. We investigated the optical nonlinearity of a series of CdSe_xS_1-x and CdSe_xS_1-x/ZnS QDs with 532nm ps pulse by Z-scan technique. The Z-scan results show that the nonlinear absorption occurred for all the QDs. Among them, the nonlinear absorption of CdSe_0.3S_0.7 QDs originates from TPA process and the TPA-induced excited state absorption(TPESA) process, that of the other QDs originates from excited state absorption(ESA). First, based on the energy leve model of two-photon-excitation dynamics, we interpreted the phenomena that their TPA coefficients linearly increase with increase of input intensity and their nonlinear refractive indices linearly decrease with input intensity. This result shows that the nonlinear refractive index is ascribed to the contribution of bond electronics and TPA-generated free carriers. Both the TPESA and the nonlinear response of TPA-generated free carriers are fifth-order nonlinear process. The TPA coefficient of CdSe_0.3S_0.7 QDs is 1.2cm/GW by open-aperture Z-scan, their TPA cross section is 1.5×10⁴GM, the TPA-induced ESA cross section is 0.63×10^-19cm², the third-order nonlinear index is 2.6×10^-14cm²/W(8.7×10^-12esu) and the refraction cross section of free carrier is -2.9×10^-23cm³. Second, in order to interpret the nonlinear absorption of CdSe_0.5S_0.5,CdSe_0.8S_0.2,CdSe_0.3S_0.7/ZnS,CdSe_0.5S_0.5/ZnS and CdSe_0.8S_0.2/ZnS QDs in resonant region, the energy level model of ESA dynamics is established. It is confirmed that the nonlinear absorption originates from ESA. The ESA cross sections and the nonlinear refractive of these QDs are determined by Z-scan. It was found that the nonlinear refractive indices for these QDs increase with the increase of Se content while their ESA cross sections decrease with it, for which a qualitative explanation is presented by us. Moreover, we also found that the QDs with ZnS shell exhibit larger ESA cross section and nonlinear refractive indices than their counterpart, which confirms that surface modification can improve the nonlinear optical properties.4. Through the measure of nonlinear transmittance of these QDs, it was found that the nonlinear transmittance decreases fast with input intensty. The result shows that these QDs exhibit excellent optical limiting properties, which enable them to be a promising candidate material for optical limiters.