半导体自组织量子点物理性质的外场调控

发布时间：2018-05-15 03:41

本文选题：自组织量子点 + 经验赝势　；参考：《中国科学技术大学》2017年博士论文

【摘要】：确定性纠缠光源在量子信息技术中有重要的应用。在传统的实验方案中,它们一般是通过非线性参量下转换过程实现的。但是这种纠缠光源不是确定性的,有很小的概率产生冗余的多光子纠缠,并且在每个脉冲中光子对的有和无完全是随机的。为了克服这一困难,人们提出了基于量子点双激子级联跃迁产生确定性的纠缠光子对。这种纠缠光源的优势在于它每次可以确定性地产生一对纠缠光子对。但实现这种量子点纠缠光源的最大困难是,在级联过程中单激子的两个偏振态的能量不是简并的,存在一个能量差,被称为精细结构劈裂。精细结构劈裂的存在会破坏光子的纠缠性,所以在实验上很难实现量子点纠缠光源。在过去的十多年中,对量子点的精细结构的调控研究受到了广泛关注。本论文主要讨论外应力对量子点精细结构劈裂的调控。本文包含的主要工作如下:1.发展了有限温度经验赝势方法研究温度对量子点光学性质的影响:温度对量子点的光学性质影响很大,比如它会使得量子点的发光位置红移。在我们的方法中温度对经验赝势的影响主要是通过Debye-Waller因子引入的。通过拟合体材料高对称点的有效质量,能隙以及它们的能带随着温度的变化关系,我们确定了有限温度经验赝势理论中的相关参数。利用该方法我们计算了量子点发光能量和温度的关系,与实验结果吻合很好。我们发现量子点的精细结构劈裂不随温度显著变化。2.应力对量子点精细结构的调控:量子点的精细结构劈裂可以通过外力调控。我们讨论了单轴应力和组合应力对量子点精细结构劈裂的影响。我们证明利用组合的两个单轴应力,量子点的精细结构劈裂可以被调节到接近零,从而可以在实现纠缠光源。该方案已经被很多实验验证。3.应力调控量子点物理性质的模型理论:我们利用Bir-Pikus方法讨论了应力对量子点精细结构劈裂的影响,并且解析推导了精细结构,激子偏振角等在外应力下的行为。该方法得到的结果和经验赝势方法得到的结果在数量级上是一致的。利用该方法,我们可以深入理解应力对量子点精细结构影响的微观机制。4.波长可调的纠缠光源:尽管量子点可以作为理想的纠缠光源,而且也已经在实验上获得了验证,但是量子点的差异很大,不同的量子点的发光能量完全不同,无法实现不同量子点之间的纠缠。我们证明了利用三个独立应力,不仅可以完全消除其精细结构劈裂,而且可以调节其波长,为实现可扩展的量子点纠缠光源打下了基础。
[Abstract]:Deterministic entangled light sources have important applications in quantum information technology. In the traditional experimental scheme, they are usually realized by nonlinear parametric downconversion process. However, the entangled light source is not deterministic and has a small probability of generating redundant multiphoton entanglement, and the existence and absence of photon pairs in each pulse are completely random. In order to overcome this difficulty, a deterministic entangled photon pair based on quantum dot double excitons cascade transition is proposed. The advantage of this entangled source is that it can produce a pair of entangled photons at a time. However, the biggest difficulty in realizing this kind of quantum dot entangled light source is that the energy of two polarization states of single excitons is not degenerate in the cascade process, and there is an energy difference, which is called fine structure splitting. The existence of fine structure splitting will destroy the entanglement of photons, so it is difficult to realize quantum dot entangled light source experimentally. In the past decade, the regulation of the fine structure of quantum dots has attracted much attention. In this paper, we mainly discuss the regulation of external stress on the fine structure splitting of quantum dots. The main work of this paper is as follows: 1. The empirical pseudopotential method of finite temperature is developed to study the effect of temperature on the optical properties of quantum dots: temperature has a great influence on the optical properties of quantum dots, for example, it can make the luminous position of quantum dots red shift. In our method, the effect of temperature on empirical pseudopotential is mainly introduced by Debye-Waller factor. Based on the effective mass, energy gap and the relation of energy band with temperature, we determine the relevant parameters in the empirical pseudopotential theory of finite temperature. By using this method, we have calculated the relationship between the luminescence energy and the temperature of quantum dots, which is in good agreement with the experimental results. We find that the fine structure cleavage of quantum dots does not change significantly with temperature. Stress regulation of fine structure of quantum dots: fine structure splitting of quantum dots can be controlled by external force. The effects of uniaxial stress and combined stress on the splitting of fine structure of quantum dots are discussed. We prove that the fine structure splitting of quantum dots can be adjusted to close to zero by using two uniaxial stresses of the combination so that the entangled light source can be realized. This scheme has been verified by many experiments. Model theory of physical properties of quantum dots regulated by stress: we use the Bir-Pikus method to discuss the effect of stress on the splitting of fine structure of quantum dots, and analyze the behavior of fine structure, polarization angle of exciton and other external stresses. The results obtained by this method are consistent with those obtained by the empirical pseudopotential method. By using this method, we can deeply understand the microscopic mechanism of the effect of stress on the fine structure of quantum dots. Wavelength adjustable entangled light source: although quantum dots can be used as ideal entangled light sources and have been verified experimentally, the quantum dots differ greatly and the luminous energy of different quantum dots is completely different. Entanglement between different quantum dots cannot be realized. We prove that using three independent stresses can not only completely eliminate the fine structure splitting, but also adjust its wavelength, which lays the foundation for the realization of an extensible quantum dot entangled light source.
【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：O471.1

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