砷化銦鎵量子點與二維光子晶體共振腔之耦合效應研究

Investigation of coupling effects of InGaAs quantum dots in two-dimensional photonic crystal nanocavities

陳文彥(Wen-yen Chen)

指導教授 ： 徐子民

國立中央大學/理學院/物理學系/博士(2007年)

摘要

本論文旨在研究砷化銦鎵量子點與二維光子晶體共振腔之耦合效應。主要的研究內容分為以下三部分：首先，在論文的第一部份，我們探討光子晶體共振腔之共振腔效應，包括共振腔對光源之自發輻射效率、光引出效率與光極化特性的改變。接著，我們利用電子束微影以及高密度電漿蝕刻技術，製作了一系列不同共振腔結構的二維光子晶體共振腔，並研究光子晶體缺陷共振腔之共振態特性，以及光子晶體共振腔設計之靈活性與多樣性。
由於共振腔中光源之發光行為會受共振腔模態影響而改變，在論文的第二部分中，我們接著研究砷化銦自聚性量子點在光子晶體奈米共振腔中的螢光特性。我們首先研究群體量子點與共振腔的耦合效應。藉由本實驗室所架設的微螢光光譜系統，我們觀察到室溫下量子點在光子晶體共振腔中的發光強度是在塊材中的百倍，這是因為光子晶體共振腔不但可以提升量子點在半導體材料中的光引出效率，量子點本身的自發輻射效率也會因受光子晶體共振腔的影響而改變。我們藉由量子點的螢光強度與激發光強度的關係，觀察到了三倍的自發輻射效率的提升。因單缺陷光子晶體共振腔具有極小的模態體積，使量子點與共振腔能在室溫下達到弱耦合而提升量子點的自發輻射速率。
在研究完群體量子點在光子晶體中的發光特性後，我們更進一步研究單量子點與光子晶體的耦合效應。我們使用微螢光光譜系統，並搭配高解析度的光譜儀與高靈敏度的矽-電子耦合光偵測器，使我們可以在極低的溫度與極弱的雷射激發功率下解析出單量子點的單激子螢光光譜。在此實驗中，我們藉由比較量子點的發光強度隨激發雷射光強度的關係，發現與光子晶體奈米共振腔共振的量子點之自發輻射效率被提升了十倍。在單量子點在共振腔中的螢光偏極化特性上，我們發現單量子點的發光耦合到簡併的共振態時，量子點的螢光極化方向會因為與不同極化的共振態耦合而不同，我們認為這是因為量子點在共振腔中的空間隨機分布現象所導致。
由於光子晶體奈米共振腔具有提高自發性輻射速率與提升光引出效率的功能，因此非常適合用來發展高效率的量子點單光子元件。在論文的第三部分，我們將光子晶體共振腔應用到量子點單光子源元件的開發上。我們首先簡介單光子量測之原理與技術，接著我們將量子點的發光耦合到多缺陷之光子晶體共振腔中。藉由光子晶體能係的效應，量子點發光的自發耦合效率可以達到92％，共振態非簡併與特定極化特性使單光子極化率也高達95％，這些特性使光子晶體量子點單光子發射器非常適合來發展極化編碼式量子密碼傳輸。另外，我們也研究了量子點在光子晶體共振腔中單光子輻射之熱穩定性，並且提升元件工作溫度到60 K。藉由適當的共振腔品質參數與模態體積，光子晶體共振腔可同時具有極佳的自發輻射增強與熱穩定性。在光子相干性量測上，單光子信號的單光子反成束現象與低信號抖動可以從7 K維持到60 K。

關鍵字

砷化銦鎵量子點 ； 共振腔效應 ； 光子晶體共振腔

並列摘要

This dissertation investigates the coupling effects of InGaAs quantum dots (QDs)
in two-dimensional photonic crystal (PC) nanocavities. The main focus of this
dissertation is divided into three parts. The first discusses cavity effects in
two-dimensional PC defect nanocavities. The theory of the modification of
spontaneous emission (SE), extraction efficiency and polarization property is presented.
This cavity effects in defect PC nanocavities are investigated. The inherent flexibility
and diversity of cavity designs of PC nanocavities are demonstrated.
The luminescence properties of light sources in optical cavities are dominated by
the cavity’s resonant mode. The second part of this dissertation studies the coupling
effects for In(Ga)As QDs in PC nanocavities. A micro-photoluminescence setup is used
to characterize the luminescence of QDs in PC nanocavities. The PL intensity of
ensemble QDs in a PC cavity is enhanced by two orders of magnitude. This large PL
enhancement is attributed to the combination of improved extraction efficiency and the
increased SE rate due to the Purcell effect. A threefold Purcell enhancement is observed
at room temperature, and is dominated by the very small mode volume of the PC
nanocavities. Emissions from single QDs in PC nanocavities are also discussed. A
high-resolution spectrometer and high-sensitivity silicon-based charge-couple device
are used to resolve single exciton emission lines under low excitations at low
temperature. Monitoring the power-dependence of individual QD emissions reveals a
nearly tenfold light enhancement from on-resonance QDs. The polarization state of
individual QDs in single-defect PC nanocavities is also investigated. Either a pure dipole mode or a mixture of both eigenmodes can be excited by an individual dot. This
behavior is attributed to the random distribution of QD position in the nanocavities.
PC nanocavities exhibit a strong ability to enhance SE rate and improve light
extraction, supporting the development of highly efficient single-photon devices. The
third part of this dissertation presents PC nanocavities to develop QD-based
single-photon devices. First, a general introduction of photon correlation
measurements, which can be used to characterize the quality of a single-photon source,
is presented. Then, single-photon pulses are coupled into the multi-defect PC
nanocavities. The single-photon source features the effects of photonic band gap,
yielding a single-mode SE coupling efficiency of as high as β ~92%. Since the cavity
possesses a single nondegenerate cavity mode with a well-defined polarization state, a
linear polarization degree of up to p ~ 95% for single photon emission is found. The
appealing performance makes it well-suited for the practical implementation of
polarization-encoded schemes in quantum cryptography. The temperature stability of
single-photon emission for QDs in PC nanocavities is also studied. The feasibility of
the operation of PC nanocavities for single QDs in single-photon applications up to 60
K is demonstrated. With a proper quality factor and a small mode volume, this PC
nanocavity exhibits excellent SE enhancement and high thermal stability. Measuring the
photon correlation function of single QD emission yields clear photon antibunching
with a small timing jitter, which is maintained from T = 7 to 60 K. These results
demonstrate that PC nanocavities with an appropriate quality factor and mode volume
are important for developing thermal-stable single-photon sources.

並列關鍵字

photonic crystal nanocavities ； InGaAs quantum dots ； cavity effects

參考文獻

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