單一砷化銦量子點與微型柱狀共振腔之偏振解析耦合作用

Abstract

本論文旨在利用偏振解析之方法探討單一砷化銦量子點與微型柱狀共振腔光子的耦合作用。我們所使用的長晶方法導致砷化銦量子點形狀帶有些微不對稱性，這不對稱性使量子點中的價電帶發生混成作用以及產生非等向交換能，分別影響了量子點激子的發光偏振性以及破壞了激子的能量簡併態。利用能量高的雷射光激發在高品質共振腔內的量子點產生激子，其與共振腔內光子發生耦合作用，激子與光子的耦合作用依其耦合強度可界分為弱耦合區與強耦合區，論文將基於此分類架構下進行探討分析。 首先，在第一部份，我們於實驗上呈現了量子點激子因與共振腔光子發生弱耦合作用，使得其發光偏振性與偏振角度隨著激子與光子間能量差距而改變，針對此觀察，我們考慮了激子與光場間的夾角，提出理論來解析並擬合實驗結果，其不僅能解釋本實驗之發現，更充分吻合先前他人的實驗結果；此外，我們亦提出了利用上述理論模型來萃取與偏振相關的Purcell因子的方法，並比較此萃取Purcell因子的方法與過去已知的方法，可得出非常一致的結果。 接著在第二部分，我們觀察到線性偏振的激子與光子發生了強耦合作用，此時同樣利用偏振解析方法來分析量子點激子的精細結構與共振腔的模態分裂，可獲悉此耦合系統中具有相同偏振之激子態與光子模態間的耦合強度；過去幾年間，數篇理論文章提出利用激子與光子強耦合作用來產生偏振糾纏光子對，我們亦研究了此願景在實現上將會面臨的困難，亦即量子點在不同調光激發下所發生的退相效應對強耦合系統帶來的影響，具體而言，因為兩個激子偏振態與兩個光子偏振模態間能量差距不同，造成退相效應對於耦合程度的影響不同，進而破壞了產生偏振糾纏光子對的理想系統，文中將以一簡單理論模型詳細說明之，並突顯了不同調光激發對於「利用強耦合系統產生糾纏光子對研究」中的重要性。

This thesis aims to investigate the coupling behavior of single InAs quantum dot (QD) with photons in a micropillar cavity by the technique of polarization-resolved photoluminescence (PL). The studied InAs QDs grown in Stranski-Krastanow mode exhibit slight asymmetry in the shape that could bring about valence-band mixing and introduce the anisotropic exchange interaction, giving rise to the polarization anisotropy in the emission and lifting the degeneracy of excitonic states, respectively. The QD exciton, created by the excitation of laser with high energy, will couple to the cavity photon in a high quality microcavity. Depending on the coupling strength, the coupling behavior can be sorted into weak coupling regime and strong coupling regime. This thesis proceeds into the analysis based on these two distinct coupling regimes. In the first part of thesis, we experimentally study the polarization properties of QD emission weakly coupled with the fundamental cavity modes. The change of polarization degree accompanying with the rotation of polarization angle are observed as the detuning between QD and cavity mode is varied. To explain this, we develop an analytical model considering the polarization misalignment between exciton dipole and cavity field. This model explains the experiments well and, at the same time, corresponds with previous work. In addition, our model also provides a new approach to extract the anisotropic Purcell factors. The Purcell factor obtained from this approach is further compared to the one obtained from known approach, showing good coincidence. In the second part of thesis, we observe the strong coupling between QD exciton and cavity photons in experiment. With the assistance of polarizationresolved PL, the exciton fine structure and cavity mode splitting can be obtained, and moreover, the coupling strengths for specific polarization of detection can be determined. In the past few years, there are several theoretical works proposed for the generation of polarization-entangled photon pairs in a strongly coupled exciton-photon system. Aiming to bring the theoretical proposals into practice, we investigate the influence of incoherent pumping upon the strong coupling between QD exciton and cavity. For two exchange-split excitons, the inherent difference in the detuning with respect to the cavity modes leads to an unequal reduction of Rabi splitting for two orthogonal polarizations as the excitation power is increased, which could destroy the indistinguishable decay paths built by the coupled states. Our work prompts a careful implementation of optical pumping for the pursuit of polarization-entangled photon pairs in the strong coupling regime.