Optical Nonlinearity Of II-VI Semiconductor Coupled/hybrid Low Dimensional Structure

Recent years, The II-VI coupled/hybrid semiconductor quantum dots have attracted much attention in both Science research and industry application, and their optical nonlinearity is one of the hottest spots. The quantum dots shows quantum size effect and small size effect because of the carrier confinement. Their optical nonlinearity is stronger than traditional bulk material, and can be controlled by changing the manufacture process. Compared with single quantum dots, the coupled/hybrid quantum dots have more complex structure and show various properties because of the interaction among the components. Besides, the coupled/hybrid quantum dots have great potential in application due to their extendibility and the integration. In this paper, the optical nonlinearity of the coupled core-shell structured quantum dots composed of CdSe and ZnS arid the hybrid complex composed of CdTe quantum dots and Au nanoparticles is theoretically investigated. The main work is included as follows:1. Size-dependent optical nonlinearity of ZnS/CdSe cylindrical quantum shellFor the quantum shell with the core of wider-band ZnS and the shell of narrower-band CdS, the conduction band structure is calculated under the effective mass approximation, and the size-dependent 3rd-order nonlinear susceptibility of quadratic electro-optic effect is calculated using perturbation method. The parameter "barrier" is introduced due to the differences of the band gap between the core and the shell. When the core radius is fixed, the energy will decrease smoothly as the the shell width increases if the energy is higher than the "barier", and speed of energy decreasing as the shell width increases will fluctuate if the energy is lower than the "barrier"; when the shell width is fixed, as the core radius increases, the energy levels above the "barrier" decrease and the energy levels below the "barrier" hardly change. The 3rd order nonlinear optical susceptibility induced by the interband transition is calculated using the result mentiond above, and the result reveals that when the core radius is fixed and the shell width increases, the positions of the 3rd-order susceptibility resonant peaks have a red shift and the intensities of the peaks become stronger.2. Polaron effects on the third-order susceptibility of a CdSe/ZnS quantum dotquantum well For the quantum dot quantum well with the core of narrower-band CdSe and the shell of the wider-band ZnS, the polaron effects on the third-order susceptibility associated with intersubband transition in the conduction band The results reveal that the polaron effects are quite important especially around the peak value of the third-order susceptibility.3. the optical nonlinearity of semiconductor quantum dots in the Au-CdTe hybridcomplexFor the hybrid complex composed of Au nanoparticles and CdTe quantum dot, the band structure of the CdTe quantum dot are calculated using six-band and eight-band theory. The result shows that the energy levels are no longer in direct ration to the inverse square of the radius, and the splitting can be observed for the small sized quantum dots. As the radius of the CdTe quantum dots increases, the 3rd-order susceptibility resonant peaks have a red shift and become stronger..In the Au-CdTe hybrid complex, the local field effect and the dipole-dipole interaction between metal and semiconductors are taken into consideration. The local field enhancement will increase the optical nonlinearity while the dipole-dipole interaction will increase the speed of damping and thus decrease the optical nonlinearity. The local field contribution can be significant if the metal-semiconductor distance is not close enough, and hence The nonlinear susceptibility of semiconductor quantum dots can be enhancedBesides, the 3-level-model is used to describe the CdTe quantum dot in CdTe-Au hybrid complex to calculate the energy absorption rate and the population dynamics. Due to the metal-semiconductor interaction, the Stark splitting can be got using relatively weaker polarization, and the time-resolved population changes as the light intensity and the geometrical parameters of the material.