利用時域有限差分法模擬金奈米粒子之飽和散射與耦合現象

散射是常見的物理現象。也因為如此，讓我們考慮到是否有其方法可控制散射。而近年來，金奈米粒子的吸收現象被廣泛的研究，甚至在近幾年發現到金奈米粒子的吸收會因為光強度的增加而達到飽和。考慮到吸收與散射皆為光學性質中常見之現象，提供給本研究一個方向: 金奈米粒子是否有飽和散射之現象、可否利用飽和現象去控制散射。本篇論文希望提供關於單顆金奈米粒子散射現象之理論基礎。因此我們利用時域有限差分法 (ﬁnite-diﬀerence time-domain，簡稱 FDTD) 去模擬二維金奈米球與金奈米圓柱之散射性質。由於 FDTD 是在時域中進行，故我們使用 recursive convolution 將描述金屬色散介電常數的 Drude critical point 模型的頻域轉為適合在時域中。此外，我們在自製的模擬程式中加入了三階項的極化率，目的是為了探討金奈米粒子中之非線性光學現象。本論文分成兩大重點: (1) 金奈米粒子 (金奈米球、金奈米圓柱) 的飽和散射現象 (2) 利用金奈米圓柱中兩軸散射的耦合來控制散射結果顯示，無論飽和或是耦合現象都來自高次項的極化率。我們可以利用提高入射光強度使高次項極化率效應顯現，並用來抑制散射。

關鍵字

金；奈米粒子；時域有限差分法；耦合；飽和；散射；電漿

並列摘要

In physics, scattering and absorption are general phenomenon. The saturable absorption has been observed in several kinds of metal nanostructures, and absorption can be saturable. Nevertheless, there has been no report for saturation of scattering before. The purpose of this work is to provide a theoretical basis for the saturable scattering, and use saturation to suppress scattering. We use ﬁnite-diﬀerence time-domain (FDTD) method, to characterize scattering in a single gold nanoparticle (GNP). To obtain spectral response with FDTD, Drude critical point model as well as recursive convolution are adopted to transform frequency domain into time domain. The correctness of this simulation is checked by comparing with experimental results of the scattering spectrum of GNPs with diﬀerent sizes. In addition, to examine the saturation eﬀect, third order susceptibilities χ (3) are included in our model. Correlated to the experimental observation,χ (3) of a single GNP can be deduced from our simulation. The results show that saturation is contribute by higher susceptibility, and we can enhance scattering of one mode to suppress scattering of the other mode in a single gold nanorod.

並列關鍵字

gold ； nanoparticle ； finite difference time domain ； coupling ； saturation ； scattering ； plasmon

參考文獻

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國際替代計量

利用時域有限差分法模擬金奈米粒子之飽和散射與耦合現象

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