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  • 學位論文

光學波段表面電漿環形矩超穎材料之研究

Plasmonic Toroidal Response at Optical Frequencies

指導教授 : 蔡定平

摘要


環形結構或甜甜圈結構在生物分子、病毒、蛋白質中極為常見,然而微波波段中純環形矩貢獻的共振模態則在近幾年中才被實現。環形矩共振在分子內交互作用與能量轉移議題上具有一席之地,然而環形矩難以耦合至遠場且不容易被觀察到。人造環形矩結構是否能夠在更高頻段響應環形矩的議題值得被探討,金屬在光學波段中的焦耳損耗與製程技術成為實現高頻環形矩的挑戰。 本文主要提出並研究兩種近古典的表面電漿環形矩超穎材料的「人工分子」於光學波段環形矩模態分析,第一種表面電漿環形矩超穎材料由四個開口朝上立式裂環共振器組成,第二種表面電漿環形矩超穎材料由兩個開口朝上、兩個開口朝下的裂環共振器組成。首先我們利用有限元素法模擬此兩種結構中的光學共振模態,藉由電偶與磁偶的耦合關係解釋環形矩共振模態與磁共振模態在兩種情況的模態反轉現象。接著我們提出類似電感電容電路的拉格朗日函數模型來量化環形矩分子內的耦合交互作用以及模態能量轉置現象。我們亦利用增益介質量化表面電漿環形矩超穎材料的光學損耗與焦耳損耗,並放大環形矩共振訊號至65 dB,藉由此表面電漿環形矩奈米結構的集體共振放出同調光,提供一種新型表面電漿雷射。最後利用電子束二次對準曝光技術在玻璃基板上製作四個開口朝上立式裂環共振器所組成的表面電漿環形矩超穎材料,在光學波段中實現環形矩。

並列摘要


Toroidal shapes are often found in bio-molecules, viruses and proteins, but only recently it was proved experimentally that toroidal structures can support exotic high-frequency electromagnetic excitations that are neither electric or magnetic multipoles. Such excitations, known as toroidal moments, could be playing an important role in inter-molecular interaction and directive energy transfer on the molecular level, but are weekly coupled to free space and are difficult to observe. Whether or not the toroidal dipoles will resonate at higher frequencies remains to be answered, since Joule loss of metals is higher at optical frequencies. In this paper, we present two new related classes of plasmonic metamolecules composed of purposely arranged of four U-shaped split-ring resonators (SRRs) that show profound resonant toroidal responses at optical frequencies. First, the toroidal and magnetic responses were investigated by the finite-element simulations. A phenomenon of reversed toroidal responses at higher and lower resonant energy has also been reported between this two related metamaterials which results from the electric and magnetic dipoles interaction. We propose a physical model based on coupled LC circuits to quantitatively analyze the inter-molecular interaction system of the plasmonic toroidal metamaterials. Using a model metamaterial system we show that coupling optical gain medium with toroidal molecules can enhance the single pass amplification on up to 65 dB of the toroidal resonance frequency. This offers an opportunity of creating the “lasing spaser” a source of coherent optical radiation that is fueled by toroidal plasmonic oscillations in the nanostructure. Finally, we experimentally demonstrate the toroidal dipolar resonance at optical frequencies in the U-shaped metamolecules, which are manufactured using a double exposure e-beam lithographic process.

參考文獻


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