單晶鋁奈米結構表面電漿子共振之光學特性

Abstract

近來，金屬奈米結構與電磁波作用產生之表面電漿子共振特性受到許多人的關注，因其光場可突破繞射限制並產生強近場，對奈米光學元件應用相當有發展性。相較常用於電漿子元件之金和銀,鋁對紫外光吸收較小，使其可操作在紫外光頻率區間，且價格便宜，又適用於成熟的半導體製程技術中，大大提升其在奈米電漿子元件應用端的發展。 在本論文中,我們首次以分子束磊晶之單晶鋁作為表面電漿子元件之材料，利用聚焦離子束微雕來製作週期性奈米凹槽與奈米洞。我們系統性地利用暗視野散射及雙光子螢光共焦顯微術，觀察其線性與非線性之光學響應。在散射光譜實驗中，週期性奈米凹槽產生的表面電漿量子波，與單一奈米洞形成的侷域性表面電漿共振之散射光頻率分別隨著凹槽週期與洞直徑大小而改變，並利用有限時域差分法模擬並驗證其光譜特性。在非線性雙光子螢光實驗裡，鋁之雙光子螢光強度隨著奈米洞與激發雷射產生之電漿子耦合效率作消長，並在空間解析強度分布圖中觀察到偶極模態及非對稱激發產生的四極暗模態。除此之外,我們針對鋁之雙光子螢光偏振特性做了詳細的分析，測量奈米洞雙光子螢光與激發光之偏振相依性中，觀察到鋁奈米洞會保持激發雷射之線偏振特性,因奈米洞結構為輻射對稱,我們排除幾何結構可能產生之調制現象。又因使用之單晶鋁(110)表面並非輻射對稱,我們認為鋁之雙光子螢光發光效率與偏振特性,皆不受到晶體結構的影響。藉由表面電漿子基本特性的探討,相信我們的單晶鋁奈米結構在紫外光與非線性表面電漿共振系統中會有更佳的應用性。

Surface plasmon resonance in metal nanostructures with strong near field confinement and enhancement are of great interest since they are highly potential for nanophotonic devices. Compared to gold and silver, aluminum has the intrinsic property extending the resonant frequency into the ultraviolet region. As a plasmonic material, aluminum also has the advantages for being low-cost and compatible with semiconductor fabrication process. For the first time, we use molecular beam epitaxy grown single-crystalline aluminum film to fabricate high-definition nanostructures by focused-ion beam milling. We systematically study their linear and nonlinear optical characteristics with the confocal dark field scattering and the two-photon photoluminescence (TPPL) microscopy. The dark field scattering from surface plasmon polariton and localized surface plasmon resonance in periodic nanoslits and single-nanoholes respectively are presented and carefully compared with finite-difference time-domain numerical simulations. We also demonstrate that the TPPL yield varied with resonant condition of the nanoholes. By spatial-resolved TPPL mapping on the nanoholes, the images of dipolar and asymmetrically excited dark quadrupolar modes are observed. Additionally, we measured the dependence of excitation to emission polarization and observed that the TPPL partially preserving the excited laser polarization is the intrinsic properties of aluminum since there is no structural preference in polarization direction with the radial symmetric nanoholes. For the sample surface is non-radially symmetric aluminum-(110) plane, it can be concluded that the linear polarized TPPL is independent of crystallinity, which have not been reported for aluminum nanostructures. Based on these experiments, our single-crystalline aluminum nanostructures are promising for many UV and nonlinear plasmonics.