散射式近場光學顯微鏡對奈米金粒子陣列之研究

Abstract

電漿子結構中，在其結構周圍的局部性強場所形成的特殊光學特性，通常用來增強螢光或散射訊號。然而，受限於光學顯微的限制，只有少數光學顯微技術擁由足夠的空間解析度，可直接觀測到場的分佈，固此特殊光學特性理論計算的研究很多，而實驗方面的分析缺十分的稀少。 本研究中，吾人以散射式近場光學顯微技術，直接觀察與分析電漿子結構中的特殊光學行為。散射式近場光學顯微鏡之架構以原子力顯微鏡為核心，藉由針尖與樣品間的電磁交互作用，將局部性的場訊號轉換為遠場的輻射訊號後被偵測分析，其橫向與縱向解析度分別可達 9 奈米與 10 奈米。藉由此散射式近場光學顯微鏡以及尖銳化的矽探針，吾人成功的觀測到間距為 10 奈米之奈米金粒子陣列的侷域光學特性。當中最特別的是，吾人觀察到在奈米金粒子的間隙中，被增強的侷域場訊號所形成的“熱點”以及其局域場方向皆成功的被觀測到，且由吾人所提出的所提出的電偶耦合模型來解釋這兩種特殊的近場特性。這個結果證明了散射式近場光學顯微鏡十分適合用以研究奈米尺度的光學特性，且預期有助於了解二維電漿子結構的光學特性，進而協助開發其在光學方面的應用。

Anomalous optical properties displayed by plasmonic structures are commonly attributed to the enhanced, local field within their corrugations. Though theoretical calculations of such field enhancements abound, experimental observations are relatively few, because only few optical microscopic techniques have enough spatial resolution. The scattering-type scanning near-field optical microscope (s-SNOM)—operating based on the electromagnetic interaction induced by a nanotip—transforms local field characteristics to far-field radiation for detection. The lateral resolution and vertical resolution of s-SNOM are 9 nm and 10 nm, respectively. The local optical characteristics of gold nanoparticle array with a gap of 10 nm between adjacent particles are resolved by s-SNOM with use of sharpened silicon tips. Specifically, the local, enhanced field—“hot spot”—is observed at the gap region and the local field direction is extracted as well. These two distinctive near-field traits are interpreted with a proposed dipole-coupling model. The findings corroborate that s-SNOM is a powerful analytical instrument to reveal optical characteristics in sub-10 nm scale and would be expectantly beneficial to the development of optical applications based on two-dimensional sub-wavelength plasmonic structures and their epistemological understanding.