Ultrafast Optical Nonlinearities In Hybrid Semiconductor-Noble Metal Nanostructures

Semiconductor Quantum Dots (QDs) can be seen as the zero-dimensional structures in which quantum confinement of electron and hole in all three dimensions leads to a tunability of the electronic and optical properties by simply changing the crystal size. Semiconductor nanocrystal (NC) QDs have interesting features such as size-controlled optical properties, optical nonlinearity, and high-efficiency carrier multiplication due to their discrete electronic states and strong Coulomb interactions. They have been intensively studied due to such interesting features and their potential for optoelectronic applications such as low-threshold lasers, light-emitting diodes, solar cells and nonlinear photonic devices. Due to the interaction of the exciton in a semiconductor QDs with the enhanced local electromagnetic field of the metal surface plasmon (SP), nanostructured plasmonic metal systems can greatly enhance a variety of optical processes, including SP enhanced photocatalysis, light-harvesting and photovoltaics, SP-enhanced fluorescence, Forster resonance energy transfer and nonlinear optical response. These render semiconductor QDs-metal nanoparticle systems as promising candidates for linear and nonlinear optical applications.Via Z-scan technique, femtosecond pump-probe and microscope spectroscopy, the optical nonlinearity and ultrafast dynamics in semiconductor films, semiconductor QDs, metal nanostructures and hybrid semiconductor QDs-metal nanoparticle structures were investigated in this thesis. The main contents are as follows:Firstly, performing femtosecond pump-probe technique at different excitation intensities, the carrier transition and relaxation dynamics in thioglycolic acid-capped CdTe quantum dots nanocrystals with different diameters were systemically investigated.Secondly, single-phase cuprous oxide (CU2O) film deposited on a quartz substrate was obtained using the pulsed laser deposition technique. And its third-order optical nonlinearities at different excitation intensity were investigated by performing Z-scan method with a femtosecond laser.Thirdly, combining nanosphere lithography method and pulsed laser deposition technique, the Au periodical particle arrays (PPAs) were fabricated on quartz substrates. Annealing the Au PPA at different temperatures and for different times, the surface plasmon absorption peaks of the Au PPAs can be tuned. Dispersing the CdTe QDs onto the Au PPAs from their diluted aqueous solution via drop-casting method, the hybrid CdTe QDs-Au PPA structures were obtained. The linear and nonlinear optical properties of the hybrid structures were investigated, when the Au SP is spectrally tuned to be in resonance with the exciton transition in CdTe QDs, the highest photoluminescence efficiency and largest nonlinear refractive index can be obtained.Fourthly, under the excitation of800nm and400nm femtosecond laser, the fluorescence intensity and polarization of CdTe quantum dots controlled by single silver nanowire were investigated by the far-field microscope spectroscopy, and higher-order plasmon modes in single Ag nanowire were verified under the excitation of800nm.Fifthly, performing the finite-difference time-domain methods, the surface plasmon modes of the single Ag nanowire at the excitation with different wavelenth (800nm and400nm) and polatization direction were numerically simulated and theoretically analysed.