高品質因子及低增益閾值之受激輻射引致表面電漿子放大元件模擬特性研究

Abstract

光子傳輸可以提供快速、低功耗、無失真之訊號傳輸，能克服積體電路線寬 的微縮下訊號延遲、失真與耗能等問題。目前積體電路尺度發展至今已可以微縮 至奈米等級，若要以光路取代電路，勢必須將同調光源微縮至相同尺寸，然而傳 統的半導體雷射受限於光學繞射限制，其光學共振腔至少需半波長的長度，其體 積仍遠大於積體電路傳輸尺寸。然而，受激輻射引致表面電漿子放大元件（surface plasmon amplification by stimulated emission of radiation，SPASER ）利用增益介質引致表面電漿子放大達到雷射所需之回饋機制，且金屬粒子自成 腔體，故能將腔體微縮至遠小於波長之等級，突破傳統雷射之光學繞射限制，還 能將光源聚焦於奈米尺度，大幅改善傳統雷射僅能聚焦微米尺度之窘境[2]，因 此受激輻射引致表面電漿子放大元件(SPASER)可望取代傳統雷射成為未來奈米 光源之主流，故本論文針對該元件研究。 由於 SPASER 元件是利用表面電漿子取代傳統雷射之光子，因此在本文第二 章中，我們簡述了表面電漿共振模態的原理，並介紹此元件產生雷射的物理機制， 以及評斷該元件優劣之參數。本實驗使用有限元素法模擬單一粒子的 SPASER 元件，採用二維度模擬取得元件最佳化，其模擬結果也預期會與三維模擬結果相 同。 在第三章中，我們藉由不同幾何形狀之設計尋找最佳化結構，綜觀六種不同 形貌之受激輻射引致表面電漿子放大元件，三層半球殼結構因為其半球殼構造具 「高局域場」，能提供強大的回饋機制更易於產生受激輻射致使表面電漿子放大， 再加上「半球殼金屬僅為全包覆結構之一半」減少了金屬材料的能量吸收損耗， 此外，金屬殼層外多了一層增益材料的殼層，其增益介質比例相對較高且包覆於 金屬兩側，更能有效地從金屬兩側補償金屬殼層損耗，故三層金屬全包覆結構具 有最佳特性：高局域電場、低增益閾值、高品質因子及最高 Purcell factor，整體 而言，其特性顯著提升也遠高於目前文獻所刊載之結構。 第四章中，以最佳結構─三層金屬全包覆結構為基礎，調變摻雜增益介質的二氧 化矽殼層厚度，觀察該參數對電漿子雷射特性之影響。結果發現，隨著殼層厚度 越大，增益閾值| k|下降、品質因子上升、輻射之光源單頻性更佳、Purcell factor 也會提升，但是有效膜態面積卻相對變大，此外也有共振波長藍移之現象，我們 可以利用改變殼層厚度任意調變其共振波長，做為更廣泛之應用。

Photon transmission can provide high speed, low power dissipation and non-distortion for signal transmission which can overcome the problems caused by line width. IC has been developed to be at the scale of nanometer, and if we want to replace it with optical path, it will be necessary to keep them in the same scale. However, due to the diffraction limit in conventional laser, optical cavity should be at least half-wavelength but causing the volume exceed of IC’s transmission scale. To deal with this problem, SPASER has been developed for it’s surface plasmon amplification to achieve the feedback mechanism, making the cavity much smaller than the wavelength that breakthrough the traditional limit. Moreover, we can concentrate the source on the scale of nanometer that improve the embarrassment compared to the conventional laser. To sum up all above, we can notice that SPASER will play an important role in the near future, and in our study, we focus on this topic. The rest of the study is organized as follows. Because of the photons replaced by surface plasmon in a SPASER, we introduce the theory of the surface plasmon,the physical mechanism and relative parameter in chapter two. Besides, in this work, we simulate the SPASER with the finite element method (FEM) and optimize the structure in two-dimension which will be the same result expected in three-dimension. In chapter three, we research for the optimal structure by different geometric designs. Taking six different structures of SPASER into consideration, “Three-layer semishell SPASER” has high local field providing strong feedback effect on SP amplification. Beside, with the half-metal compared to full cover structure, we can reduce the power dissipation in lasing mode. We can conclude that “three-layer semishell SPASER” has the best property in comparison to the reference purposed before: higher local electric field, lower gain threshold, higher quality factor and the best Purcell factor. In chapter four, based on the optimal structure-“three-layer semishell SPASER”, we modulate the semishell thickness of gain medium. With the experiment, we can find that with the higher thickness of shell, the result show us the lower gain threshold, higher quality factor, higher Purcell factor.