节点文献
半导体自组织量子点物理性质的外场调控
Tuning Physical Properties of Semiconductor Self-assembled Quantum Dots
【作者】 王建平;
【导师】 何力新;
【作者基本信息】 中国科学技术大学 , 光学, 2017, 博士
【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 photon sources is fundamentally important in quantum information science. Traditionally, entangled photons are generated via the parameter-down-convention process in nonlinear optical media. However, this method is proba-bilistic, that it has small probability to generate the unwanted multiply entangled photon pairs. It has been proposed that deterministic entangled photon pairs can be generated via biexcition cascade process in self-assembled quantum dots. However, the finite fine structure splitting, which is much larger than the natural broadening of the emission line,is the major obstruction to this proposal. This thesis is devoted to tune the fine structure splitting of exciton by external stresses.The major results in this work are listed below.1. Finite temperature empirical pseudopotential theory: To study the tem-perature dependent optical properties of quantum dots, we developed a temperature-dependent empirical pseudopotential theory. The temperature may dramatically affect the optical properties of quantum dots, such as the red shift of emission lines. In our ap-proach, the temperature dependence is introduced by the temperature-dependent Debye-Waller factor. We determine the corresponding parameters by fitting the effective mass,energy gap as well as the temperature dependent emission energies at the high symmet-ric points of bulk materials. We calculate the temperature dependent exciton energies in quantum dots, which agree with the experimental data very well. We find that the fine structure splitting of exciton will not be dramatically affected by the temperature effect.2. Tuning of fine structure splitting of exciton by external stress: We show that the fine structure splitting of exciton can be efficiently tuned by external stresses.We discuss the role of uniaxial stress and biaxial stress on the fine structure splitting of quantum dots. We show that the fine structure splitting can be tuned to nearly zero for general quantum dots via two independent stresses, which has been verified experimen-tally.3. Theoretical model for fine structure splitting in quantum dots: By using Bir-Pikus model, we derived the theoretical model that describe how the fine structure splitting change under external stresses, which allow us to understand the microscopic mechanism of how the stresses change the fine structure spitting and exciton polariza-tion angles.4. An wavelength tunable entangled photon source: The quantum dots entan-gled photon emissions have been demonstrated experimentally. However due to the structure difference in quantum dots, the emission wave lengths in quantum dots are generally different from dot to dot, and therefore the entanglement between photons from two quantum dots are impossible to be realized. We demonstrate that with three independent external stressed we can eliminate the fine structure splittings and tune the wave lengths simultaneously. This provides a first step to the future realization of scalable entangled photon-pair generators for quantum information applications.
【Key words】 self-assembled quantum dots; empirical pseudopotential; entangled photon source; fine structure splitting;