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Journal of the Physical Society of Japan

Parameter-Dependent Stimulated Photon Echo in CdSe/ZnS Quantum-Dot Quantum Well

Rongjun Zhang, Duanzheng Yao, Hongliang Jiang, Hua Xiao

Published May 11, 2009• 2 citations
Publisher

Abstract

The energies of 1s states inside and outside the quantum well of a CdSe/ZnS quantum-dot quantum well (QDQW) and the transition dipole moment between these two states have been calculated by effective mass approximation. By using the calculation results, the longitudinal and transversal relaxation times of CdSe/ZnS have been approximated. We obtain a three-pulse photon echo (TPE) signal on the basis of the two states using the optical Bloch equations. We find that there should be four echoes in TPE from our theoretical calculation. In particular, the effects of the variation in core radius and shell thickness have been studied. It has been shown from the numerical calculation that the TPE signal is strongly dependent on the size and structure of the QDQW, and is dominated by relaxation time, which is also dependent on the size and structure.

DOI
10.1143/jpsj.78.054402

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The energies of 1s states inside and outside the quantum well of a CdSe/ZnS quantum-dot quantum well (QDQW) and the transition dipole moment between these two states have been calculated by effective mass approximation. By using the calculation results, the longitudinal and transversal relaxation times of CdSe/ZnS have been approximated. We obtain a three-pulse photon echo (TPE) signal on the basis of the two states using the optical Bloch equations. We find that there should be four echoes in TPE from our theoretical calculation. In particular, the effects of the variation in core radius and shell thickness have been studied. It has been shown from the numerical calculation that the TPE signal is strongly dependent on the size and structure of the QDQW, and is dominated by relaxation time, which is also dependent on the size and structure.
The optical free induction decay (OFID) induced by the transition between 1s and 1p state in a small isolated spherical CdSe/ZnS quantum dot quantum well (QDQW) has been studied numerically under the framework of effective-mass approximation. The size and relaxation time-dependent properties of the OFID have been obtained and analyzed. It shows that the OFID-change mechanisms dependent on shell thickness and core size are different. Moreover, the OFID signal decays sharply in amplitude and life as the transverse relaxation time being reduced while the change is slight to the longitudinal relaxation time. By comparing these two factors, we infer that the size mainly influences the amplitude of the decay signal and the delay time is basically determined by the relaxation time
The energy eigenvalues and eigenfunctions of a core–shell CdSe/ZnS quantum dot structure have been obtained under an effective-mass approximation. The electric transition dipole moment induced by the 1s (inside the well)–1s (outside the well) transition has been calculated. The optical nutation signal induced by the transition has been calculated numerically based on the optical Bloch equations. In particular, the influence of the size variation of the core's radius and the shell's thickness has been studied. It has been shown from the results that the optical nutation signal is sensitive to the size and structure, and that there is an optimal structure and size for the optical nutation phenomenon. The quantum size dependent Rabi frequency and intensity of the optical nutation are also discussed.
在建模和理论分析的基础上, 对三脉冲飞秒激光作用下核壳结构CdSe/ZnS量子点量子阱中1s e 1s h 激子光跃迁引起的受激光子回波效应进行了深入研究.运用有效质量近似方法求解了载流子的静态薛定谔方程,得到能量本征值和对应波函数.基于光学Bloch方程,分析了受激光子回波的参量相关性.结果显示受激光子回波信号可以通过量子点量子阱结构和尺寸的改变进行有效调节.同时,在量子尺寸限制理论的基础上讨论了结构和尺寸的变化对受激光子回波信号的具体影响.
Abstract The optical properties of the CdSe/ZnS spherical core/shell quantum dot (CSQD), related to the laser-driven transitions 1 s → 1 p − 1 and 1 s → 2 p − 1 , are the subject of this study. We investigate in detail how transition energies, oscillator strengths, imaginary and real parts of the total, linear and third-order susceptibilities change with the variation of the model parameters, such as CSQD radius, core radius, relaxation rates and laser intensity. The energy levels and wave functions, needed for the calculations of the model parameters, were obtained by solving the radial Schrödinger equation in the effective mass approximation with the corresponding boundary conditions. The interaction of CSQD with laser light was studied in a density matrix approach using a two-level model and, for the first time, a five-level model. Optical Bloch equations were derived under dipole and rotating wave approximations and solved in the steady-state regime to obtain an analytic expression for susceptibility. Special attention was paid to the cases in which nonlinear susceptibility prevailed over linear susceptibility, leading to the appearance of the window in the absorption peak. It was found that susceptibility is more sensitive to the change in the core radius than to the change in the CSQD radius and that a greater laser intensity is needed to change the nature of the absorption and dispersion curves for 1 s → 2 p − 1 then for 1 s → 1 p − 1 transition.