【作者】 龚明；

【导师】 郭光灿； 何力新；

【作者基本信息】 中国科学技术大学 ， 光学， 2010， 博士

【摘要】 自组织量子点在量子点激光器和量子信息中已经得到了广泛的应用。实验上已经实现个各种波长的激光器；在量子信息方面,已经实现了单光子源,纠缠光源以及比特的Rabi振荡、初始化、比特的读取、集成化等等。近10年来在实验和理论方面都取得了令人瞩目的进展,但是量子点离它的实际应用还有很大的距离,尤其是在量子信息方面,还有很多重要的问题没有得到解决,比如耗散问题、比特集成问题以及量子点理论计算的问题等等。这些问题是我们这篇论文的出发点。本文在第1章回顾了自组织量子点在量子点激光器和量子信息中的应用,在第2章我们介绍了自组织量子点的重要物理问题。在第3章,我们给出了处理量子点能级结构的三种方法,它们是k·p方法、经验赝势方法以及紧束缚模型。在这些模型中我们尤其关注量子点的微观对称性以及对应力的处理,因为它们对量子点的光学性质有很重要的影响。由于我们在第4-7章的计算需要用到经验赝势方法,所以我们详细介绍了经验赝势方法。在第3章最后我们讨论了量子点中的规范问题,我们证明磁场会破坏周期性系统的平移变换不变性,所以Bloch定理不再成立。我们给出了解决经验赝势中规范变换不变性的基本方法。这篇博士论文主要关心量子点的光学性质,我们得到的主要结论有,1.系统研究了InAs/InP量子点的单粒子能级、激子能量、激子寿命、量子点中的多体效应以及电子／空穴的相图,同时我们得到的结果和InAs/GaAs量子点进行了比较。在第5和第6章中我们利用经验赝势方法研究了InAs/InP和InAs/GaAs量子点的能级结构和光学性质。我们发现InAs/InP和InAs/GaAs量子点的电子(空穴)的束缚势以及应力有很大的差别,所以它的电子结构和光学性质也有很大的差别。我们理论计算的结果和实验结果非常一致。2.我们证明InAs/InP量子点适合用于实现通讯波段的纠缠光源。利用量子点中的双激子级联过程可以实现确定性的纠缠光源,但是在实际量子点中,由于量子点的真实对称性只有C2v,激子的两个明态头一个精细结构劈裂(几十μeV),所以实际上得到的只有经典关联态。在第7章,我们利用经验赝势方法证明InAs/InP量子点的精细结构劈列非常小,此外它的发光波长在1.55μm附近,所以InAs/InP量子点适合用于实现通讯波段的纠缠光源。3.我们证明(InAs)n／(GaAs)m量子阱的自旋劈裂∝|k|。在第8章我们也开发了一套基于平面波的能带计算方法,其中我们采用和经验赝势一样的计算方法,在这个程序中我们用两种方法计算了自旋轨道哈密顿。由于自旋轨道耦合是非局域的,所以处理起来会非常困难,我们的这个方法可以处理非常大尺寸的体系。我们用这个方法计算了(InAs)n/(GaAs)m量子阱的自旋劈裂,我们的结果证明,选择合适的InAs厚度,可以让自旋劈裂∝|k|。这个结果可能在自旋电子学中有很大的用处。4.我们实验证明InAs/GaAs量子点中激子-光学声子的耦合进入了强耦合区域。在第9章我们研究了InAs/GaAs量子点中的激子-光学声子强耦合问题。我们发现在s峰的左边有一个新的峰s′,随着温度升高,s和s′会出现能级交叉。这些结果是激子-光学声子强耦合导致的,它使得量子点的光学性质有很大的改变。我们的结果证明,基于弱耦合近似的Huang-Rhys模型已经不能用来解释我们的实验结果。我们提出了一个二能级模型并用它可以很好地解释我们的实验结果,但是我们没有办法确定导致强耦合的机制,所以我们期待更多理论支持。更多还原

【Abstract】 The self-assembled quantum dots (QDs) have been widely used QDs laser and quantum information science (QIS). In experiments the QDs laser with different wave-length has been achieved; In the field of quantum information science, the single photon source, entangled photon source, Rabi oscillation of qubit, initialization, qubit readout and integration et al. Huge progressions both in theory and experiments have been achieved in the past ten years, however, there are still a lot of things to do before prac-tically applications, especially in QIS, we still have a lot of questions that unsolved, such as decoherence of qubits, integration of qubits and theoretical modelling of QDs. These questions are the starting point of this dissertation.In this work, we fist review the application of self-assembled QDs in QDs laser and QIS in Chap.1 and then the basic physics in QDs in Chap.2. In Chap.3 we give three methods in the calculation of electronic structure of QDs, including k·p method, empirical pseudopotential method and tight-binding method. In these methods, we pay much attention to the symmetry and the strain effect, which is of essential importance in determination of optical properties of QDs. We give a detailed description of the empirical pseudopotential method, as it will be used in Chap.4-7. At the last of Chap.3, we discuss the gauge problem under uniform magnetic field, we show that the magnetic field will breakdown the translation symmetry of the periodic lattice, so the Bloch theorem should be modified, we give the method to fix the gauge problems.In this dissertation, we mainly focus on the optical properties of QDs, and the main results we have obtained in this dissertation are as following,1. We systematically study the optical properties of InAs/InP QDs, including the single particle levels, exciton energy, exciton lifetime, many-body effect in QDs and the phase diagrams of electron and hole; We also make a comparative study with that in InAs/GaAs QDs.In Chap.5 and Chap.6, we study the electronic structure and optical properties of InAs/InP and InAs/GaAs QDs using empirical pseudopotential method. We found that the confinement potential of electron(hole) and the strain differs greatly in these two kind of QDs, which render the huge different between the electronic structure and optical properties of QDs. Our results agree well with the available experiments datas.2. We proof that InAs/InP QDs can be used as entangled photon source in the communication regime.The biexciton cascade process in QD can be used as entangled photon source, but in actually QDs the symmetry is C2v, so the two bright state of exciton has a small splitting, know as fine structure splitting, which is about tens ofμeV, so the output two photon states have only classical correlation. In Chap.7 we proof that the fine structure splitting in InAs/InP QDs is very small using empirical potential method, ant the exciton energy is around 1.55μm, so InAs/InP QDs can be used as entangled photon source in the communication regime.3. We show that the spin splitting in (InAs)n/(GaAs)m quantum well∝|k|.In Chap.8 we develops a software to calculate the energy bands using pseudopo-tental method, where the pseudopotential is the same as that used in QDs. In our codes, two method has been used to deal with the spin-orbit interactions. One should know that the spin-orbit interaction in non-local, so the treatment of this is very complex. The method used in codes enable us to calculate the energy bands of very large systems. We use our codes to calculate the band splitting of (InAs)n/(GaAs)m quantum well, and we found that if we choose a suitable width of InAs, we can get spin splitting∝|k|. This results may have potential application in spintronics.4. We show that the coupling betwcen exciton and optical phonon in InAs/GaAs QDs has entered the strong coupling regime.In Chap.9 we study the strong exciton-optical phonon coupling in InAs/GaAs QDs. We find a new peak s’on the left side of s shell, and these two peaks can an-ticross with each other. This results is a strong evidence for strong coupling between exciton and optical phonon, hence the optical properties of QDs is greatly modified. Our results show that the Huang-Rhys model, which is based on weak-coupling, can not be used to interpret our results. We use a two-band model to explain the experimen-tal observations, and it work quite well. However, our experiments can not determine the mechanism that render the strong coupling between exciton and optical phonon, and it still ask for further theoretical investigation。更多还原