半導體量子點光源中激子自旋動態、精細結構匹裂與光學異向性的理論研究

Abstract

半導體量子點光源由於擁有高度量化的電子結構，而能展現出優異發光特性，此前瞻性的半導體奈米材料不僅可應用在量子光源元件上(如單光子發射器或糾纏態光子對發射器)，可應用在廣泛的光量子資訊的領域上，例如:量子遠傳、量子位元、量子密碼學等。然而，光激子在固態材料中，無法避免的量子自旋鬆弛效應、精細結構匹裂與光學偏振不對稱性，往往大幅地限制了量子點在量子資訊上的應用。在本論文中，我們將以k∙p模型為基礎，發展一套解析與數值的模擬方法，藉由大型的數值運算，計算出自組式量子點的電子結構，建構出一套激子在量子點中的自旋動態行為、精細結構匹裂與光學理論。 在自旋鬆弛時間的研究中，我們完整考慮各種自旋軌道耦合效應與核子的作用力，我們發現自組性In(Ga)As/GaAs量子點自旋鬆弛效應是由電洞Dresselhaus自旋軌道耦合效應所主導，計算出暗激子自旋翻轉為亮激子的時間快達100奈秒，與實驗估算極為接近。在光學偏振的研究中，我們研究了液滴磊晶GaAs/AlGaAs量子點的幾何形狀，探討重電洞和輕電洞能態混合對光學偏振異向性的影響。過去普遍認為，量子點形狀不對稱性是造成量子點光學偏振異相性的主因，但在實驗上與我們的研究皆顯示，增加量子點的高度更會增加光學偏振的不對稱性；最後，在精細結構匹裂的研究中，我們完整考慮了液滴磊晶量子點中輕電洞與重電洞能態混和對於電子電洞的交換能的影響，計算出如何透過外加應力來消除精細結構匹裂。本研究對量子點中激子動態行為、精細結構匹裂與發光特性提供完整的理論基礎，提供未來研究者能有效的設計或控制半導體量子點光源元件。

With highly quantized energy spectrum and excellent optical properties, quantum dots (QDs) are promising nanostructures for the implementation of optical quantum information technology such as quantum teleportation, quantum cryptography, and quantum light sources. In QD-based photonic devices, e.g., single photon sources or entangled photon pair emitters, the dynamics of excitons in the fine structures plays an essential role in the quantum operations. However, inevitable population relaxation, loss of quantum coherence, fine structure splitting (FSS), and optical anisotropy limit the usability of dot-based photonic devices. In this thesis, we use both analytical and large-scale numerical approaches based on the multi-band k∙p model to carry out a comprehensive theoretical investigation of the spin dynamics, fine structures and optical anisotropies of quantum-confined neutral exciton states confined in QDs. As a main result, the hole-Dresselhaus spin orbital couplings (SOCs) is identified as the dominant spin-admixture mechanism that lead to relaxation rates as fast as ∼10−2 ns−1. In our study of optical anisotropies, we report the substantial impact of vertical confinement on valence heavy-hole (HH) and light-hole (LH) mixings, which enhance polarization anisotropy in taller strained self-assembled InGaAs/GaAs QDs. At last, we study the effects of HH-LH mixing on the fine-structures and polarization properties of a single exciton confined in single GaAs/AlGaAs QDs under the application of anisotropic stress. We demonstrate how possible to design and prepare any desired exciton states of QD photon sources, by mechanical means, prior to photon generation. Our investigations of the spin dynamics, fine structures, and optical properties of excitons in photo-excited QDs establish useful guidelines for the optimal design of dot-based quantum light source devices.