| More and more attentions have been focused on semiconductor quantum dots since they were first systematically researched and synthesized in the Eighties. There are more and more applications of semiconductor quantum dots including photoelectron, biomedicine, and quantum information and so on, while the optical nonlinearity is one of the hottest topics. CdSe/ZnS colloidal core-shell quantum dots is one of the Ⅱ-Ⅵ coupled/hybrid semiconductor quantum dots which have been intensively focused because of their high optical stability and fluorescence quantum yield and optical nonlinearity. While GaN quantum dots as representatives of the self-assemble quantum dots have also become one of the most importance issues because of their large direct energy gap, strong atomic bond, high thermal coefficient and chemical stability. The optical transient coherence effects are one of the most efficient methods to study the transition dynamical processes of current carriers. And new properties, phenomena and applications of optical transient coherence effects have come out as the development of laser technology and the realization of ultra-short laser pulse. The main works are showed as follows:1. First, the three-pulse stimulated photon echo (TSPE) based on Is-Is states transition inside the well and the shell of CdSe/ZnS core-shell quantum dots was studied. The energies and wave functions of these two levels were numerically calculated using k·p Method and Effective Mass Approximation based on reasonable theoretical model. The electrical transition dipole moment and longitudinal and transversal relaxation time were then calculated. Especially, the dependences of state energies and gap on the size and structure of core-shell quantum dots were highlighted. With the calculation of the electron’s distribution probability inside the well, the dependences of electrical transition dipole moment and relaxation time on the size and structure of core-shell quantum dots were analyzed in terms of quantum size confined theory. Further detailed studies of TSPE were carried out based on the previous results and the quantum size and structure dependences were highlighted. The results indicated that the size and structure of quantum dots showed a significant influence on the state energy, electrical transition dipole moment and relaxation time, and finally determined the intensity of the stimulated. It was found that the stimulated photon echo was more intense than other echo when the outer radius was larger than7.0nm with inner radius kept being4.3nm.2. Secondly, Using finite-difference time-domain (FDTD) method, the Maxwell-Bloch equation based on free-space plane-wave approximation was solved which was implemented to simulate the Self-induced Transparency (SIT) with ultra-short pulse propagating in near-resonant two-level atom medium (TLA). It was found that the SIT phenomenon disappeared when the pulse area equaled4π. Fast oscillated component appeared in the fast2π pulse which split from the incident4π pulse, and also the wave reshaping while the slow one showed SIT effect. This is called carrier-wave Rabi flopping (CWRF). On the other hand, the population difference (PD) between these two levels was studied. The results showed that the PD possessed a periodic variation with the increase of distance and the maximum population difference (MPD) almost achieved1which means total reverse. The reason was revealed to be the frequent spectra broadening of this2π pulse induced by CWRF which showed periodic variation. Meanwhile, the Carrier-Envelop Phase (CEP) of the faster2π pulse was studied. It was showed that the frequent spectra of electric field possessed more high frequent spectra component when the CEP equaled1π. All these phenomena could help to study the laser-matter interaction. The SITs of2π and6π pulse propagating in the near-resonant TLA medium were also studied. The results indicated the6π pulse split into three2π pulses with the fastest pulse had highest intensity and smallest pulse width which subsequently showed intense CWRF. The population different induced by this2π pulse was larger the PD induced by the4π pulse which almost achieved complete reverse.3. Moreover, we have tried to solve the states energy and wave function of GaN assemble quantum dots which posses irregular shape by use of First Principle method based on density function theory (DFT). Although the energy gap results were smaller than experiment, these results were believed to be show correct trend of energy gap according to other worker’s experience. |
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