Study On The Optical Transient Coherence Effects In Quantum Confined Semiconductor Structures

In the past several decades, more and more attention is focused on the novel photoelectric properties of quantum confined semiconductor structures, and they were widely studied for their potential applications in many fields, such as photoelectrics and biomedicine fields. The CdSe/ZnS core-shell quantum dot quantum well (QDQW) is made of II-VI semiconductors, its special nonlinear optical properties and the optical transient coherence effects were favored because of the excellent fluorescence quantum yield and stability in room temperature. In this dissertation, the optical transition dynamics process of charge carriers will be studied, and the corresponding optical transient coherence effects in the CdSe/ZnS core-shell QDQW will be investigated particularly. We got the optical nutation signal and the two-pulse photon-echo signal (2PPE signal) in the CdSe/ZnS core-shell QDQW, which driven by femtosecond laser pulses, through numerical simulations. It was found that the optical nutation effect and the photon echo effect can be controlled effectively by the variation of the size and the structure of the multiple QDQW. Furthermore, a particular explanation will be given in terms of the theory of quantum size confined effect (QSCE). The details are the following:The stationary Schrodinger equation of charge carriers in the CdSe/ZnS core-shell QDQW was solved numerically based on the?·p Method and Effective Mass Approximation (EMA) after a reasonable theoretical model was made. And the confined energy eigenvalues and eigenfunctions of charge carriers were obtained. Furthermore, the dynamical processes of 1s-2s optical transitions both inside the well and outside the well were analyzed. Especially, the optical transition process of electrons'barrier penetration was investigated completely, and it's meaningful to the specific applications of CdSe/ZnS core-shell QDQW. On the other hand, we had the core-shell QDQW's size and structure changed in our numerical calculations, and studied the influence on the quantum confined energy and the energy space between energy levels. And the variation of electrical transition dipole moments (ETDM) was also analyzed. The results indicated that optical transition properties are size and structure dependent. A comprehensive explanation was made in terms of QSCE theory after a careful investigation of the electron's distribution probability in the core-shell QDQW. More details will be introduced in Chapter 4.The Chapter 5 is about the numerical simulation of optical nutation signals. Based on the optical Bloch equations and Density Matrix Approach (DMA), and supposed the CdSe/ZnS core-shell QDQW was driven with a 150fs laser pulse in low temperature (about10K), the optical nutation signals for the optical transitions in the QDQW were simulated successfully.Moreover, let the core-shell QDQW's size and structure vary in our numerical calculations, the influence on the optical nutation signals was investigated. It's found that the intensity and the Rabi frequency of the optical nutation signals are sensitive to the QDQW's size and structure variation. And there is an optimal structure and size for the optical nutation phenomenon in this core-shell QDQW. We found the optimized structure for the core-shell QDQW in different cases, and some explanations were also given based on the analysis we already made above about the optical transition process.In Chapter 6, the photon echo effects for excitons'optical transition were introduced. In the case of the Coulomb interaction between electron and hole was taken into account, the optical transition processes of 1se1sh excitons were investigated, and the band gap of the CdSe/ZnS core-shell QDQW was calculated. The numerical results told us the band gap of this multiple QDQW is wider a lot than the one in bulk CdSe materials.We changed the value of the core-shell QDQW's size and the structure in our numerical calculation to investigate the variations of excitons' optical transition processes between the ground state and the (1se, 1sh) state. The numerical calculation results illustrate that the ETDM is sensitive to the core's size variation, but the thickness variation of the shell has little influence on ETDM. These convince that the ZnS shell mainly provides an efficient passivation of the surface trap states, and the thickness variation, in a reasonable region, influences the nonlinear optical properties of the core-shell QDQW very little.And then, the 2PPE phenomenon in this QDQW, which driven by a 100fs Data Pulse and a 50fs Brief Pulse one after another, was studied based on the optical Bloch equations. The motion of Bloch vector M and the transient nonlinear broadening dynamical process were investigated numerically. And the 2PPE signals for 1se 1sh excitons'optical transitions were simulated when the size and structure of the core-shell QDQW varied. It was found that the intensity of 2PPE signal can be controlled efficiently by the variation of size and structure. This is meaningful to the core-shell QDQWs'specific applications, such as binary optical calculating and saving in quantum calculation field; labeling, tracking and probing in biomedicine field.At the end of this dissertation, a short introduction about the study of electron spin relaxation in GaAs quantum well under a terahertz laser field will be written in Chapter 7. It is a tentative topic relevant to my formal works, and this research was made with the financial help of Chinese Scholarship Counsel during the period of I studied in the School of Electrical and Computer Engineering of Georgia Institute of Technology as a Joint-training PhD. student. Based on the Floquet theory, the Schrodinger equation of electron in GaAs quantum well, which driven by a periodic terahertz laser field, was numerically solved in matrix method. The quasienergies and corresponding eigenfunctions of spin (spin-up and spin-down) were obtained already. The next specific research topic will focus on the electron spin relaxation controlled via the Rashba spin-orbit-coupling with an external terahertz laser field.