金屬雙柱系統與同調性完美奈米吸收器的電磁特性之理論研究

表面電漿是金屬和介電質之界面的表面電荷密度波，可由入射之電磁波激發。因其具有亞波長及強局域場特性，現已被應用於如生物或化學分子之偵測辨識、表面增強拉曼光譜（Surface enhanced Raman spectroscopy），以及近場成像等。也正因其極佳之侷域性，搭配其與光速同數量級之傳播速度，電漿子電路之研究方興未艾。因此，許多研究團隊不斷提出各種幾何設計與材料組合，試圖找出最佳之表面電漿傳輸體系。最近金屬奈米柱陣列系統的引起人們的研究興趣，其中一模態電磁場強集中於柱子之間，稱為gap mode，具有長程傳輸距離，較適合作為傳輸表面電漿信號的波導模式。對於此類體系中最簡單的雙金屬柱系統，除最低階的gap mode之外，其餘各模態雖未必可直接應用，但其耦合方式及傳遞之特性，仍值得研究。本文藉有限元法之軟體工具COMSOL，計算出各階雙金屬柱之本徵表面電漿極化子模態。透過電場強度分佈、電荷分佈，以及頻散關係曲線，得知各階雙柱本徵模態，乃由各對應之單柱本徵模態，依「鍵結-反鍵結」（bonding-antibonding）之規則耦合而成。最後並以對稱性推論出所有可能之雙柱本徵模態。兩個同調的電偶極輻射源放置在一個負折射材料板兩側，其朝向負折射板的輻射能量可以通過中心放置的反調電偶極子吸收，從而作為完美奈米吸收器系統（Coherent Perfect Nanoabsorber），中間的反調電偶極子亦可由一介電質奈米球替代，其吸收之能量大小可以大於一個電偶極輻射源之總輻射能量。本文以數值模擬研究以完美電導體或完美磁導體為一等效鏡面的簡化體系，並得知若依據偶極輻射源的對稱性選擇適當完美導體邊界之種類，可以達到和原同調性完美奈米吸收器系統一樣的吸收效果，且此種簡化體系更易實現並應用。

關鍵字

表面電漿極化子；表面電漿子波導；頻散關係；傳播距離；電漿子雜化；同調性完美奈米吸收器；負折射；完美電（磁）導體；鏡面系統

並列摘要

Surface plasmon polariton (SPP) is surface charge density wave induced by electromagnetic waves at a metallo-dielectric interface. Because of its sub-wavelength confinement and local field enhancement properties, it has many applications such as bio- or chemical sensors, surface-enhanced Raman spectroscopy, and optical near-field imaging, etc. Since SPP can propagate with a fast speed within a sub-wavelength volume, scientists are trying to develop new device technologies supplementing traditional electronic circuits. Therefore, different systems have been studied in order to find the best carrier to transport SPP modes. Recently, interacting metallic nanowire arrays have received much interest because gap SPP might be able to serve as a good waveguide mode. Various coupled SPP modes exist in such a system and we need more analysis to distinguish and demonstrate them. In this work, we investigate all the SPP eigenmodes in the system of two coupled metallic cylinders based on finite element method. In particular, we study the relation between cylindrical SPP modes and bicylindrical SPP modes based on the calculations of the field pattern, charge distribution, and dispersion relation of each mode. By increasing the distance of the two cylinders, the bonding-antibonding coupling of single cylindrical SPP modes can be revealed. Consequently, all the eigenmodes of bicylindrical SPP can be inferred from the consideration of symmetry and the principle of bonding and antibonding. The problem of energy divergence in perfect lens has been well known for a long time. Recently, a concept of coherent perfect nanoabsorbers (CPNAs) using a slab made of double negative (DNG) metamaterial has been introduced. If we put an anti-parallel dipole or a dielectric nanoparticle at the center of the slab, it can perfectly absorb energy radiated from the outside two dipole sources. In this work, we investigate a simplified system with a mirror made of perfect magnetic conductor or perfect electric conductor on the bisecting plane of the original system. We demonstrate it can work as good as the original CPNA system if we use an appropriate mirror, depending on the direction and type of the outside dipole source. Such kind of simplified system makes this idea more realizable for applications.

並列關鍵字

Surface plasmon polariton ； plasmonic waveguide ； dispersion relation ； propagation length ； plasmon hybridization ； coherent perfect nanoabsorber ； negative refraction ； perfect electric (magnetic) conductor ； mirror system

參考文獻

[1] H. Raether, Excitation of Plasmons and Interband Transitions by Electrons (Springer-Verlag, 1980).

[2] L. Tonks, "The birth of plasma," American Journal of Physics 35, 857 (1967).

[3] I. Langmuir, "Oscillations in Ionized Gases," Proceedings of the National Academy of Sciences 14, 627 (1928).

[4] C. T. Walker and G. A. Slack, "Who Named the -ON's," American Journal of Physics 38, 1380 (1970).

[5] D. Pines, "Collective Energy Losses in Solids," Reviews of Modern Physics 28, 184 (1956).

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金屬雙柱系統與同調性完美奈米吸收器的電磁特性之理論研究

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