自組式量子點中激子動態行為的理論研究

Abstract

由於擁有高度量化的電子結構，自組式量子點展現出優異發光特性。此前瞻性的奈米材料可以應用在各種光量子資訊的應用，例如量子遠傳、量子密碼學和量子光源發射器。在自組式量子點的量子光學應用中(如單光子、糾纏態光子對發射器)，光激子的動態行為在光學元件的操作上扮演決定性的角色。然而，在固態材料中無法避免的量子鬆弛(relaxation)效應與量子同調性(coherence)的破壞，往往大幅地限制了量子點在光量子資訊上的應用。 在本計畫中，我們將藉由解析方法與大型的數值計算程式，建構出一套激子在量子侷限中的動態行為理論。在解析方法中我們已經完成了激子在三維簡諧振盪模型中自旋鬆弛時間的理論，其中完整考慮了電子電洞的交換能、自旋軌道耦合與核子自旋的作用力。在這個理論架構下，我們將發展一個有效的數值模擬方法－8能帶kp模型－來計算應力自組式量子點帶有自旋混成的電子結構，其中涵蓋了所有可能的自旋軌道耦合及應力效應。以kp模型所計算的單一電子及電洞能階為基礎，進而利用組態相互作用法來解決多激子的問題，並考慮所有可能的粒子間交互作用，尤其是自旋相關的電子電洞庫倫交互作用。量子點中激子鬆弛時間的研究，可藉由求解電子與電洞對自發性復合的速率方程(rate equation)與黃金費米定則(Fermi’s golden rule)來了解。量子點中亮激子發去相(de-phasing time)時間的計算則可從求解自旋的運動方程得到，並將同調性破壞的原因：包括聲子或核子自旋交互作用，作有系統的研究。此理論將比對並解釋時間解析光譜的實驗結果，以及變溫光譜的分析。本研究可以對量子點中激子動態行為提供理論的基礎進而有效的設計或控制量子點光源元件。

With highly quantized energy spectrum and excellent optical properties, self-assembled quantum dots are promising nanostructures for optical implementation of quantum information technology, e.g. quantum teleportation, quantum cryptography, and quantum light sources. In photo-excited self-assembled quantum dots (QDs), the dynamical properties of photo-generated exciton play an essential role in the quantum operations of dot-based photonic devices, e.g. on-demand single photon sources, or entangled photon pair emitters. However, inevitable population relaxation and losing of quantum coherence of photo-excited carriers in a quantum dot due to both of intrinsic or extrinsic mechanisms limit the usability of the do-based photonic devices. In this project, we shall carry out a comprehensive theoretical investigation of dynamical properties of quantum-confined excitons in strained self-assembled quantum dots by using both analytical and large-scale numerical approaches. A theory for the spin relaxation times of excitons in a quantum dot in a 3D parabolic model is being developed with the full consideration of e-h exchange interactions, spin-orbital and hyperfine interactions. In the theoretical framework, we shall develop an efficient numerical simulator based on 8-band kp model for the calculation of the spin-mixed electronic structures of strained self-assembled quantum dots, taking into account all possible spin-orbital couplings and strain. Based on the kp -calculated single particle levels, the configuration interaction (CI) method will be employed to solve interacting multi-exciton problems with the full consideration of all possible inter-particle interactions, remarkably including spin-relevant e-h exchang interactions. The theoretical results will be compared with and used to account for measured time-resolved photoluminescence (PL) and analysis of the temperature-dependence of single-dot PL line widths. These studies of dynamical properties of excitons in a photo-excited quantum dot shall provide useful guidelines for optimal design of the dot-based quantum light source devices.