近場偵測與光學控制表面電漿干涉

Abstract

本論文利用在單晶黃金金箔上製作出的奈米雙狹縫與四狹縫結構，藉由掃描式近場光學顯微鏡（Scanning Near-Field Microscopy, SNOM）觀察在狹縫間的大面積金板區產生表面電漿的干涉，並且使用有限元素分析法進行模擬分析。我們藉由具有偏振特性的光，使金屬內的自由電子產生電偶極震盪，在金屬-空氣介面產生表面電漿波。本論文先對不同夾角的金屬奈米雙狹縫觀察表面電漿量子的波動性，根據表面電漿對偏振的選擇性，我們可以由模擬結果看到，對於偏振方向垂直分角線與平行分角線的激發，有著強度上的極大差異，因此本論文將針對垂直分角線的偏振進行實驗。藉由本實驗的狹縫設計，除了相互夾角30度以及60度的雙狹縫，我們也使用長為8微米的四狹縫，調整激發光偏振以及波長，觀察各結構所產生的表面電漿干涉。經由近場量測與模擬計算，我們發現表面電漿干涉週期（調制週期）隨激發波長增加而增加，也隨夾角增加而增加，我們使用平面波干涉理論模型計算，也得到吻合實驗與數值模擬的結果。最後我們討論，表面電漿干涉的相干性，表面電漿波的相干性與激發波長有關。本實驗能夠控制固定狹縫結構的干涉圖樣，藉由調變偏振，能夠得到點狀的干涉圖樣以及線狀的干涉條紋，能簡易的調變表面電漿波的空間分布，藉由表面電漿波在近場增強特性，並期望未來能在非破壞性的光學檢測及奈米光學的應用上有所貢獻。

The surface plasmon polaritons (SPPs) are evanescent electromagnetic surface waves coupled to the collective longitudinal oscillations of the free-electron gas in a metal propagating along a metal-dielectric interface. Thus, we utilize the subwavelength slits and the corresponding polarization of light to excite SPP. We investigate the double-slit structure with variation angle between the slits, the simulation result shows the maximum intensity of modulation is achieved for the polarization direction perpendicular to the axis of the tilt angle. There are two different slits structure were designed for simplify the experiment, other than double slits with 30 and 60 degree, the four slits produce the convenience for observation of parallel or perpendicular slits interference on the large (8 micrometer) gold surface. The experimental near-field image obtained by Scanning Near-Field Microscope was in a good agreement with numerical results and analytical results. Finally, the coherence of near-field interference was in discussion, and the coherence varies depend on the incident wavelength. We demonstrate the manipulation of surface plasmon polaritons, which can be utilized in nano-lithography, trapping and selective growth of nanoparticles, and other fields of nanotechnology.