三角柱之旋轉對領結形金奈米週期陣列造成效應之探討

Abstract

本論文以理論及實驗手法探討領結形奈米結構(Bowtie nanostructure)中三角柱 的方向在表面電漿共振上的效應。透過電子束微影製程製作不同旋轉角度的金奈 米領結型結構，並且透過時域有限差分法模擬其局域性表面共振行為，再分別以 表面增強拉曼散射光譜及暗場顯微散射光譜分別測量其近場及遠場的光學性質， 與實驗相互比對印證。除了一般廣泛見於二極表面電漿共振(Dipolar resonance)的 角模態(Tip mode)外，邊模態(Edge mode)也會在三角柱旋轉的過程中被激發。邊模 態的激發及其隨著旋轉角度的變化可以由金屬表面電荷密度來觀察，透過對角模 態及邊模態的電場及表面電荷密度分析，兩者在旋轉過程中所產生的趨勢也將被 討論。亦基於所歸納的趨勢：相對共振波長的差距相對於旋轉角度的餘弦呈現接 近指數遞減關係，建立電漿量角器(Plasmon protractor)。最後，我們提出一個針對旋轉領結型奈米結構的電漿子混成模型(Plasmon hybridization model)來解釋不同旋轉角度中表面電漿耦合的現象。透過此研究，旋轉角度與領結型奈米結構中電漿共振的關係已被建立，希望對於往後領結型奈米結構的應用，如在表面增強拉曼散射光譜方面，能夠有所幫助。

The effects of rotation angle on the surface plasmon coupling of nanoprisms were analyzed using finite-difference time-domain simulations and were first verified experimentally through both near-field and far-field optical techniques in present study. In addition to the widely-discussed dipolar resonance in regular bowtie nanostructures, defined as tip-mode resonance in present study, the excitations of edge-mode resonance were discovered under certain rotation angles of nanoprisms. The transitions between modes also took place during rotations. To justify the theoretical predictions, the gold bowtie nanoantennas were fabricated with different rotation angles using electron-beam lithography with highly controlled geometries of nanostructures, and two different excitation wavelengths were used as incident sources of Raman spectroscopy on fabricated rotated bowtie nanostructures with different rotation angles, to provide near-field evidences for the excitation and evolution of different resonance modes. The dark-field scattering microspectroscopy was also adopted for the detection of the far-field responses. Based on the discovered trend, a plasmon protractor was created with the near-exponential decay relationship between the relative resonance wavelength shift and cosine of the rotation angle. A plasmon hybridization model was also proposed for rotated bowtie to explain the coupling between nanoprisms during rotation.