量子點激子能態檢測與操控

Abstract

本論文旨在研究對半導體量子點中激子能態的量測與操控。本實驗主要以砷化銦量子點為主，利用外加磁場以研究量子點的磁光特性。實驗中可觀察到不同的激子態，對外加磁場所反應的反磁特性不盡相同。此結果可歸因於載子(電子及電洞)間庫倫作用力的影響。並藉由激子的反磁特性，可分析載子的波函數分布情況。其中負電荷激子的反磁特性不同於理論中與磁場平方正比的結果，可導因於激子復合發出螢光前後，受庫倫作用力影響下載子波函數的劇烈變化所致。 此外，雙激子與激子的連續復合過程被認為是發出糾纏光子對的重要機制之一。此系統對量子資訊的實踐提供了可能性。但量子點系統中，由於激子存在著精細結構分裂，使得光子對的糾纏特性遭到破壞。精細結構分裂往往伴隨著量子點系統的不對稱性而出現。不論是量子點中束縛位能、量子點形狀及組成的不對稱，或是量子點所受的應力不對稱，都會造成精細結構分裂的出現。在本研究中，所提出之精細結構分裂與應力不對稱所致重電洞輕電洞耦合的關係，可順利解釋量子點系統出現的精細結構分裂與偏振不對稱間的關係。藉由外加水平磁場，可進一步釐清水平磁場中黎曼效應與應力不對稱性間的關連。 由於精細結構分裂的出現受量子點所受的應力不對稱的影響，我們使用一壓電陶瓷鈮鎂酸鉛-鈦酸鉛 (PMN-PT)來對量子點提供單軸或雙軸向之應變。旨在減少量子點中應力的不對性。藉由實驗的觀察，量子點中精細結構分裂受到單軸向壓電陶瓷的影響，可自由的調整大小。儘管實驗中我們將精細結構分裂降至5 eV，但仍不足以產生量子糾纏的特性。當藉由外加水平磁場調控精細結構分裂接近零的附近時，我們發現有反交差的現象出現。另外同時外加單軸向應變，反交差的現象並沒有特別的變化。藉由實驗與理論的歸納，我們提出當量子點處在非理想的C2V對稱時，不對稱的位能會提供些微精細結構分裂。這個些微的精細結構分裂，就是造成不論是用單軸向應變或水平磁場，往往無法順利使精細結構分裂降至零的原因。

The main target of this paper is the characterizations and manipulations of exciton states in single quantum dots (QDs). First, we focus on the diamagnetic responses of different exciton complexes in single InAs/GaAs self-assembled QDs. Magneto-photoluminescence (PL) is a useful tool for measuring the carrier wave function confined in QDs. The result shows a systematic trend of different diamagnetic shifts of each exciton complexes caused by the imbalanced magnetic responses of inter-particle Coulomb interactions, which could be observed only when the confined electron and hole wave functions exhibit a large difference in their lateral extents. Furthermore, the difference of the diamagnetic shift between biexciton and exciton was found to scale as the cube of its single-particle wave function extent and therefore can be a sensitive probe to the electron hole wave function asymmetry. On the other hand, the magnetic response of negatively charged exciton in QDs also shows obvious -change in the electron wave function extent after photon emission due to the strong Coulomb attraction induced by the hole in its initial state. All the results give the information that the inter-particle Coulomb interactions are strongly correlated to the wave function distribution in weak confined QDs. Second, we focus on the generation of entangled photon pairs from the biexciton-exciton cascade process in semiconductor QDs. However, practical applications are limited by the fine structure splitting (FSS) in the bright exciton state, which arises from symmetry reduction in QDs, including the intrinsic atomistic asymmetry of the underlying lattice and the extrinsic dot shape asymmetry due to a preferential elongation developed during QD growth. Both of the effects are closely related to the size, shape, and composition profile of QDs. Here, our experiments point out an anticorrelation between the FSS and the polarization eigenaxes. Such relationship includes the exchange interactions and the strain induced valence-band mixing. Moreover, the strain induced valence-band mixing also couples with the Zeeman interaction by an in-plane magnetic field. Because of the FSS is correlated to the strain anisotropy, we used a piezoelectric material [Pb(Mg1/3Nb2/3)O3]0.72-[PbTiO3]0.28 (PMN-PT) to apply either a biaxial or an uniaxial stress on the QDs. We observed that the FSS will increase or decrease by applying an uniaxial stress. However, the FSS did not reduce enough to achieve quantum entanglement with Hanbury-Brown and Twiss (HBT) interferometer. Moreover, when we manipulated the FSS by applying an in-plane magnetic field, it shown the anticrossing as the FSS approaching to zero. When we applied the in-plane magnetic field and the uniaxial stress at the same time, the anticrossing did not have obvious changes. Here we figured out the QD asymmetry will offer a minimum splitting s0. It caused the anticrossing irrelevant applying the in-plane magnetic field or the uniaxial stress.