空間控制陣列結構中局域性表面電漿子之研究

Abstract

本論文主要在探討具有空間控制的半導體陣列結構上，局域性表面電漿子(localized surface plasmons (LSPs))特性之研究。本論文的第一部份，我們以電子束刻版印刷術(electron beam lithography (EBL))製作出三組不同間距的鍍金矽奈米柱樣品，然後以鎢燈提供的白色光源照射該三組陣列結構，經由反射光譜的觀察，證實該三組陣列結構上的局域性表面電漿子的共振頻率能被調變；鍍金矽奈米柱間距越大則表面電漿子共振頻率越小(紅位移特性)。本論文的第一部份中，我們也探討了以二維有限元素法計算模擬陣列近場範圍內的總能量分佈。近場模擬結果除了顯示局域性表面電漿子的分佈情形和局域性表面電漿子頻率的紅位移特性外，還顯示陣列結構所特有的光柵特性－也就是說，不同間距的結構(相當於光柵)都有對應能在其中形成建設性干涉的特定光波長。在模擬分析中，由高階繞射項在近場範圍(包含角度範圍廣及高階繞射項所能涵蓋的所有角度)內所形成建設性干涉可以被完全預測出來。然而，反射譜無法讓我們觀察到上述的光柵效應，因為反射譜的觀察角度只有在入射角等於反射角的位置上(此角度正好是零階繞射項)。) 在建立能提供在空間上排列整齊的局域性表面電漿頻率的陣列結構之後，我們進行第二部份的研究，將量子點旋轉塗佈在蓋玻片上，再將三個不同間距的鍍金矽奈米柱陣列結構先後分別置於已塗佈完成量子點的蓋玻片上。在三個陣列上，針對相同的50顆目標量子點觀察其發光強度所受到的影響。我們發現在最密的陣列結構上量子點與金奈米柱可能接觸的面積最大，因而量子點螢光強度減弱的比例最高。而最疏的結構上，其局域性表面電漿子的頻率與量子點螢光頻率最接近，因而其螢光強度增強的比例最高。另外，我們發展出一個新的技巧，使我們在測量量子點螢光強度時也能先後取得陣列結構的光學影像(藉由移除濾鏡，將激發雷射光源聚焦在接近基版位置處，得到基版位置明亮而柱子頂端黑暗的光學影像以定位陣列的位置)、量子點的螢光影像(放回濾鏡，將激發雷射光源聚焦在量子點上)，然後，將兩張影像疊加，進而得以定出量子點與陣列的相對位置。藉由此項技巧，我們歸納出量子點在陣列樣品不同相對位置上發光強度的變化規則。當量子點位在金柱表面所形成的局域性表面電漿區域內或是位在兩金柱中點附近的建設性干涉區，螢光強度皆能增強。在螢光強度能增強的兩個情形中，量子點位在金柱表面所形成的局域性表面電漿區域內，因為量子點螢光能與局域性表面電漿子產生耦合效應， 使得量子點螢光具有較短的生命期。而當量子點太靠近金柱(<10 nm)甚至接觸到金柱時，螢光強度減弱機會比較大。最後，再由二維有限元素法，以電磁模型為基礎，在量子點螢光頻率條件下，計算總能量在陣列結構近場空間中分佈的情形。我們得到與實驗一致的結果。

This dissertation is devoted to study the localized surface plasmons (LSPs) in spatially controlled array structures. In the first part, three periodic Au coated Si nano rod (SiNR) arrays on Si substrate with different distances between the SiNRs were fabricated by electron beam lithography (EBL). The LSPs is induced when these spatially controlled array structures were irradiated by an un-polarized white light from a tungsten halogen lamp, we found that the trends of reflectance spectra indicate that the LSP frequency can be spatially controlled by manipulating the distances between the SiNRs of the arrays. In addition, the experimental results were compared with 2D numerical simulations based on the finite element method. The simulation result of each array in the near field regions can reveal subtle characteristic of the intensity distributions including the grating effects (the higher order diffractions) and more LSP modes than in the far field reflection observation. After the investigation of the spatially controlled LSPs, we started the second part of the research, fluorescence signals of quantum dots (QDs) influenced by the three array structures of the Au coated SiNRs. The QDs were spin-coated on a cover glass. And then, the three array structures were put upside down on the cover glass in sequence. We further developed a new technique which can obtain the optical image of the array structures without losing information of the QD locations at the time of QDs fluorescence measurement. We removed the filter and focused the excitation laser on the substrate to obtain the optical image of the array structures (the substrate is therefore bright, and the top of the SiNR is dark). And then, we put back the filter and focused the excitation laser on the QDs to obtain the fluorescence image of the QDs. After superposing the two images, the relative locations between the QDs and the Au coated SiNRs were defined. The same 50 QDs were successively observed on the three array structures (one by one). On the densest gold coated SiNRs array structure, the highest QDs fluorescence quenching rates are observed. And on the sparsest array structure which provides the LSP frequency closer to the emission band of QDs, the highest QDs fluorescence enhancement rates are observed. The QDs fluorescence enhancement effects are observed on the locations proximity enough (but not touched by the Au) to the Au coated SiNRs, and on the locations near the mid point of two Au coated SiNRs (where the constructive interference is formed). And we found again that when the QDs contact with Au coated SiNRs, instead of enhancement, a non-radiatve process may occur, leading to QDs fluorescence quenching. Finally, both in the near field region, 2D numerical simulation results are consistent with the experiment results.