奈米桿表面電漿共振模態分析

Abstract

在本研究中探討單顆三維奈米金桿、銀桿及雙顆金桿dimer結構在受到平面電磁波波源及電偶極波源時所產生的各種現象。其中將以Maxwell電磁理論作為基礎，利用多重中心展開法來做為數值計算的方法。在上述的奈米桿中，分析近場表面電漿子的共振現象，並且討論遠場電磁場的震盪行為，再進一步觀察奈米粒子受到電偶極波源時在遠端其局部的電場增益。 其研究的結果，隨著奈米粒子的細長比增加，奈米粒子所對應的表面電漿共振的波長也會增加。當平面波波源為正入射或雙電偶極放置在兩端時，此時為對稱型態，在奈米桿中只能激發奇數共振模態，而當平面波的入射角度遭受旋轉時，對稱性遭到破壞，此時偶數模態會因此而顯現出來，而在雙電偶極波源下，改變其電偶極的位置或是使雙電偶極相位落後，也可以同時達到對稱性破壞的效果。而在電偶極在奈米桿尖端時可以藉由奈米桿的特性將能量藉由奈米粒子傳遞到奈米粒子的另一端，此時在金屬材料為銀的奈米桿其電場增益還可以維持在0.8左右的強度。 在雙顆奈米金桿dimer結構時，若雙顆金桿尺寸相同，其dipole模態的共振波段會增加，而quadrupole模態只有些微的強度。但當尺寸相異時，利用長桿的quadrupole模態及短桿的dipole模態有接近的共振波長，可以產生明顯的Fano現象，其中在Fano dip處雙顆金桿之間會有強的耦合情形。

This thesis discussed plasmon phenomena of various nano-structures, especially plasmonic modes, including a single gold nanorod, a single silver nanorod and gold nanorod-dimer structures, excited by electromagnetic plane wave and/or electrical dipoles. Maxwell theory together with the multiple-multipole (MMP) method is used to analyze the near-field surface plasmon resonance (SPR) and the far-field radiation patterns as well as the local field enhancement factor. Results show that as the aspect ratio (AR) increases the corresponding wavelengths of plasmonic modes increase. For the case of normal incidence plane waves or symmetric bi-dipole, only odd modes are excited; while for the case of oblique incidence plane wave or asymmetry bi-dipole, both odd and even modes appeared. When dipole at the tip of nanorod, it can transfer energy to far end of nano-particle, in silver nanorods, the enhancement factors keep at 0.8. For the case of gold nanorod dimer structures, for dimer with two identical nanorods, the bandwidth of dipole mode increases, while the peak of quadrupole mode weakens. For dimer with two different nanorods, if the wavelength of the quadrupole of long rod is close to that of the dipole mode of short rod, there is also a Fano resonance associated with a Fano dip due to a strong coupling between two nanorods.