Optical Response And Interaction Of Semiconductor Quantum Dots-Metal Nanostructures

Nanomaterials have novel properties comparing to their bulk counterparts. For instance, semiconductor quantum dots have much stronger exciton confinements and their optical behaviors can be tuned by adjusting sizes and dopants; metallic nanostructures exhibit strong local field enhancement owing to surface plasmon resonance, which can be tuned by controlling sizes and shapes. Both semiconductor quantum dots and metal nanoparticles also have largely enhanced nonlinear responses. Moreover, efficient excitation energy transfer at the nanometer scales has critical significance to improve optical properties of the hybrid nanomaterials. In this thesis, we investigate linear and nonlinear optical responses of Ag doped CdTe semiconductor quantum dots and metal nanoparticles in the following five aspects.Firstly, we report the synthesis of water-soluble Ag-doped CdTe semiconductor quantum dots (SQDs) via a facile aqueous approach for the first time. Experimental characterizations demonstrate the efficient doping of the Ag impurities into the CdTe SQDs. The Stokes shift is decreased by 120 meV, the fluorescence intensity is enhanced more than 3 times, the radiative rate is enhanced 4.2 times, and the nonradiative rate is efficiently suppressed by doping 0.3% Ag. The physical mechanism of the fluorescence enhancement in Ag-doped CdTe SQDs is attributed to the minimization of surface defects, filling of the trap states, and the enhancement of the radiative rate by the silver dopants. Our results suggest that the silver doping is an efficient method for tuning the optical properties of the CdTe SQDs.Secondly, we investigate optical third-order nonlinearity and figure of merits (FOMs) of the CdTe quantum dots heavily doped with Ag. After doping Ag, the excitonic resonant absorption is enhanced and the 1S peak is red-shifted and broadened. Interestingly, the nonlinear refraction near the band edge is enhanced more than 35 times, while the nonlinear absorption keeps very small at the crossover of the one-photon saturation absorption and two-photon excitation near the band edge, leading to the desired one- and two-photon FOMs for the demands of all-optical waveguide switching. Our observations offer a strategy to prepare doped semiconductor quantum dots with large third-order susceptibility and good nonlinear FOMs and thus show prospective applications in optical information processing, switching, and modulating.Thirdly, we demonstrate tunable nonlinear absorption of a nanocomplex of plasmonic and molecular-like gold nanocrystals. The SA→RSA process is efficiently suppressed, and the stepwise SA→SA process is fulfilled owing to energy transfer in the nanocomplex. Our observations offer a strategy for preparation of the saturable absorber complex and have prospective applications in liquid lasers as well as one-photon nonlinear nanodevices.Fourthly, we investigate tunable plasmon resonance and enhanced second harmonic generation (SHG) and upconverted fluorescence (UCF) of the hemispheric silver core/shell islands. The Ag, Ag/Ag2O, and Ag/Ag2O/Ag island films are prepared by using sputtering technique. The SHG and UCF of the Ag/Ag2O/Ag core/shell islands near the percolating regime is enhanced 2.34 and 3.94 times. The ratio of SHG intensity induced by p-and s-polarization is 0.86 for the initial Ag islands and increase to 1.61 for the Ag/Ag2O/Ag core/shell samples. Our observations provide a new approach to fabricate plasmon enhanced optical nonlinear nanodevices with tunable SHG and UCF.Finally, we comparatively investigate fluorescence quenching effect of the CdSe semiconductor quantum dots by using plasmonic and molecule-like gold nanocrystals (AuNCs) in aqueous suspensions and spin-coated films. In the aqueous suspensions, the plasmonic nanoparticles have larger quenching effect than the molecule-like ones. In the films, the plasmonic and molecule-like AuNCs have comparable quenching factor. Through studying the fluorescence emission dynamics characteristics, we find that the plasmonic AuNCs enhance both radiative and nonradiative rates but the molecule-like ones only enhance nonradiative rate. These observations help us to understand the quantum properties of plasmon.