激發光於氧化鋅微米共振腔之理論與實驗分析

Experimental and Theoretical study of emission properties of ZnO-based microresonators

吳寶仲(Huynh-Buu-Trong Ngo)

指導教授 ： 張亞中 ； 王本誠

國立清華大學/原子科學院/工程與系統科學系/博士(2017年)

摘要

藉由充分的耦合方式，共振波長的光可以被束縛在光共振腔內。這種獨特的束縛行為是因為光與物質的交互作用改變，使得空腔內光子的生命期延長。在各種光學共振腔中，由半導體所組成的共振腔是目前的主流，因為較能滿足高Q因以及低雷射等條件。因此，研究氧化鋅（ZnO）共振腔不只在基礎研究，更在各種應用領域有很重要的影響力。
這份論文中系統性地研究了以水熱合成法製作的氧化鋅微米球共振腔(MSRs)中的迴廊模態(WGM)，包含多孔ZnO和 Au@ZnO的MSRs。我們研發了低溫下單步驟合成高品質微米球的製程標準。接著用微光激發(micro-PL)光譜儀探索這些共振腔中的WGM。這篇論文也提出了改良版介電常數(MRI) 以及等效介質理論(EMT)等模型，以用來分析從ZnO MSRs 量到的PL激發光譜。這些模型不只能夠精確指出共振模數和模態類型，也能處理共振腔元件必要的物理參數。最後，可以用格林函數理論，建構出任一MSR的激發光譜。和PL實驗相比較，我們可以更進一步了解MSRs的光學性質像是洩溢模態，激發態，以及相關係數等等這些實驗無法直接量測的特性。
這篇論文的科學進展涵蓋了：合成MSRs的製程標準、從PL光譜分析WGM的數學模型以及能計算MSRs的激發光譜的理論，並將這些整合成ZnO MSRs內光交互作用之下所產生WGM的自洽物理圖像。基於這些發現，我們提出一個解決方案去克服微米球尺寸和WGM品質間的兩難困境：修改共振腔內部結構減少孔隙率或增加激發源數量（例如電漿奈米粒子或量子點）。藉由這個方法我們可以縮小共振腔尺寸達奈米尺度，同時保存了有高品質因子的WGM。因此這篇論文對於未來奈米尺度WGM的研究提供了重要資產，兼且對主模態數的基礎研究，也對奈米雷射或奈米共振腔生物元件的前瞻應用而言是不可或缺的。

關鍵字

氧化鋅 ； 回音壁微腔

並列摘要

It is known that light at resonance wavelengths would be confined to optical resonators by a sufficient coupling scheme. Such confinement behaviors are unique and alter the interaction of light with matter due to the lifetime extension of confined photons. Among various states of an arbitrary optical resonator, semiconductor resonators have been considerably attractive for their potential to achieve high Q-factor and low lasing threshold. Hence, the investigation of ZnO-based resonators has a very important impact not only on fundamental research but also on numerous applications in different fields.
This thesis presented systematically studies on whispering gallery mode (WGM) in ZnO based-micro spherical resonators (MSRs) (including porous ZnO and Au@ZnO MSRs) grown by hydrothermal synthesis. The protocol of synthesized high-quality spherical resonators by single-pot, low-temperature technique is developed. WGM in these resonators is then explored by micro photoluminescence (μ-PL) spectrometer. The thesis also proposes Modified Refractive Index (MRI) and Effective Medium Theory implemented (EMT) schemes to analyze the PL spectra measured from those ZnO MSRs. These schemes are shown that they are useful not only for precisely assigning resonance mode number and type but also for addressing the unneglectable role of resonator’s components. Finally, the emission luminescence spectra of an arbitrary MSR are reconstructed based on Green functional theory. By comparison to experimental PL, we can get a further understanding of optical properties of MSRs such as leaky modes, stimulated modes and their correlation properties which are not able to be clarified by observation of experimental spectrum only.
The scientific advancements covered within this thesis, including the build-up of protocol of synthesis MSRs, models to analyzed the WGM from PL spectra and theory to calculating emission spectra of MSRs, are integrated together to perform a consistent picture of how light interacts inside ZnO MSRs and exhibit WGM behaviors. We show that not only geometry but also components of an MSR affect their WGM. Based on this finding, we propose a solution to overcome the obstacles between size and WGM quality, that is to modify the inner structure by reducing the air fraction or adding more emitting sources (such as plasmonic nanoparticles or quantum dots). By utilizing that approach we can reduce the resonator size to the nanoscale, and yet maintain high-quality WGM. The thesis thus plays a vital asset for the future studies of WGM in nanoscale, which is essential for both fundamental studies of principles mode number and advanced studies on potential applications such as nano-lasing or nano-resonator based bio-devices.

並列關鍵字

ZnO ； Whispering Gallery Mode

參考文獻

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