奈米Epsilon鏡像結構與金屬性氧化鋁鋅材料 之表面電漿特性

Abstract

在本論文中，我們提出一新穎金奈米Epsilon鏡像(mirror-image nanoepsilon，MINE)結構去達到高侷域及高增強的近場於間隙(gap)鄰近，並完整且有系統地研究其表面電漿行為以應用於多樣化的電漿領域。在外觀上，它像是字母Epsilon, ϵ, 對鏡映像而成的特殊結構；在功能上，它具有奈米棒二聚物(nanorod-dimer)及奈米環(nanoring)兩結構獨特的光學效應: 避雷針效應(lightning-rod effect)、電漿耦合效應(plasmon coupling effect)、高度可調的電漿共振(highly tunable plasmon resonance)和大的表面體積比(large surface to volume ratio)。有趣地，奈米Epsilon鏡像結構的運作思維可想像成將奈米環框在奈米棒二聚物周圍，奈米環提供額外的自由電荷經由像橋樑般的奈米棒將環中的電荷拉入尖銳的棒狀結構中，再透過奈米棒之避雷針效應與電漿耦合效應迫使電荷集中，使間隙鄰近的近場被有效地侷限與放大。此外，該結構之電漿模態可由奈米棒二聚物及奈米環兩基本結構的電漿模態相互作用(interaction)與雜化混成(hybridization)而產生，並依它們的電荷分佈定義出兩個主要的電漿模態: 對稱模態(symmetric mode)與反對稱模態(anti-symmetric mode)。對稱模態因它的電荷對稱性可導致較集中且強勁的近場而較具應用潛力。除了探討基本的電漿模態外，我們也針對奈Epsilon鏡像結構的幾何因子與所形成之二聚物是如何影響它的電漿特性作深入的數值與實驗分析。從這些分析結果顯示出它的光場強度與電漿共振特性與幾何因子高度相關及它所形成的二聚物因耦合效應產生極高應用價值的耦合對稱模態(coupled symmetric mode)，使高能量、高密度的熱點(hot spot)結構陣列成為可能。所有研究成果使我們可以更加瞭解像這樣複雜的混合電漿結構，並有能力可導引與設計更優越的結構條件來達到多樣且實際的奈米光學元件。最後，我們也嘗試使用此新穎結構去評估仿生物的粒子捕獲應用，模擬與實驗結果顯示出該結構可穩定地捕獲微小的聚苯乙烯球，凸顯出奈米Epsilon鏡像結構極具有生物化學等應用之潛能。 除了新穎金奈米Epsilon鏡像結構被深入研究外，我們也嘗試發展新穎透明導電氧化電漿材料於電漿子光學(Plasmonics)。傳統的電漿金屬材料在光學頻率的波段上，因載子的躍遷與散射效應(carrier transition and carrier scattering)導致許多的損耗(loss)，然而這些損耗將限制了許多如變換光學(transformation optics，TO)、近零Epsilon (epsilon-near-zero，ENZ)等元件應用的可行性。因此，尋找一交替低損耗的電漿材料取代傳統金屬於特定應用是一個重要的議題。在這個主題上，我們有系統地研究薄膜與圖樣化的氧化鋁鋅材料在不同結構、不同退火條件下的電漿行為。在適當的製程條件與近紅外光區域，我們發現氧化鋁鋅薄膜具有高度可調性、金屬性及低損耗性的電漿特質；氧化鋁鋅奈米結構則呈現與金屬奈米結構相似的侷域性表面電漿共振特性。我們也在模擬與實驗上展示出氧化鋁鋅奈米結構具有高靈敏度的環境折射率感測，成功驗證氧化鋁鋅奈米結構於感測應用的可行性。透過氧化鋁鋅材料對特定氣體與生物化學敏感之本質，我們深信未來一高靈敏、快響應的電漿氧化鋁鋅光學感測器可被快速發展與高度期待。

In this study, we propose a novel mirror-image nanoepsilon (MINE) structure resembled the combination of two face-to-face nanoscale ϵ-shaped structures to achieve highly localized and enhanced near field at its gap and systematically investigate its plasmonic behaviors. The MINE can be regarded as the combination of two fundamental plasmonic nanostructures: a nanorod-dimer and nanoring. By adapting a nanoring surrounding a nanorod-dimer structure, the nanorod is regarded as a bridge pulling the charges from the nanoring to nanorod, which induces stronger plasmon coupling in the gap to boost local near-field enhancement. This MINE with mode coupling between the nanorod-dimer and nanoring leads to the inducement of hybridized plasmon modes. Among these plasmon modes, the symmetric mode in the MINE structure is preferred because its charge distribution leads to stronger near-field enhancement with a concentrated distribution around the gap. In addition, we investigate the influence of geometry on the optical properties of MINE structures by performing experiments and simulations. These results indicate that the MINE possesses highly tunable optical properties and significant near-field enhancement at the gap and rod tips. Besides, MINE dimers arrays with varying dimer gap are also explored. As a result, a coupled symmetric (CS) mode can be excited in small dimer gap under longitudinal polarization, which provides the practicality of a high density hot spot and uniform pattern in MINE dimers array. The results improve the understanding of such complex systems and are expected to guide and facilitate the design of optimum MINE structures for various plasmonic applications. Finally, the application of the particle trapping is numerically and experimentally performed using the unprecedented MINE structure. It is shown that the proposed MINE structure can achieve stable trapping on tiny particles for future bio-chemical applications. In addition to the novel MINE structure, we also focus on another topic about a novel plasmonic transparent conducting oxide (TCO) material in this dissertation. It is well known as conventional metals suffering from large losses in optical frequencies because of the electron transition and electron scattering losses, which limits the feasibility of plasmonic applications, such as transformation optics (TO) devices and epsilon-near-zero (ENZ) devices. Thus, searching low-loss alternative materials for plasmonic applications is an important topic. In the study, we systematically investigate the plasmonic behaviors of aluminum-doped zinc oxide (AZO) thin films and patterned AZO nanostructures with various structural dimensions under different annealing treatments. We find that AZO film can possess highly-tunable, metal-like, and low-loss plasmonic property and the localized surface plasmon resonance (LSPR) characteristic of AZO nanostructure is observed in the near-infrared (NIR) region under proper annealing conditions. Finally, environmental index sensing is performed to demonstrate the capability of AZO nanostructure for optical sensing application. High index sensitivity of 873 nm per refractive index unit (RIU) variation is obtained in experiment. Taking with the advantages of excellent sensing ability on gas or biochemistry using AZO material, we believe that highly sensitive and responsive optical nano-sensor can be expected in the future.