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標題: | 受圓偏振光照射之金奈米結構所產生的軌道角動量 Orbital angular momentum of gold nanostructures irradiated by circularly polarized light |
作者: | Jia-Yun Luo 羅佳芸 |
指導教授: | 郭茂坤(Mao-Kuen Kuo) |
關鍵字: | 軌道角動量,自旋-軌道作用,Poynting向量,表面電漿子波,圓偏振光,金奈米結構,介電奈米珠,金奈米桿,拓樸荷, orbital angular momentum,spin-orbit interaction,Poynting vector,orbital momentum,surface plasmon polariton,circularly polarized light,gold nanostructure,dielectric bead,gold nanorod,topological charge, |
出版年 : | 2020 |
學位: | 碩士 |
摘要: | 本論文探討金奈米結構受到圓偏振光照射後所產生的軌道角動量(orbital angular momentum),原本該圓偏振光的光子僅具有自旋角動量,不具軌道角動量。研究以Maxwell電磁理論為基礎,數值模擬運用多重中心展開法(multiple-multipole expansions method),求得光與物體交互作用後產生的電磁場,再藉以計算結構外域之坡印廷(Poynting)向量及軌道動量(orbital momentum)密度的三維流線(streamline)場,以及結構的表面電磁場分布,分析奈米結構受圓偏振光照射後,產生的自旋軌道作用(spin-orbit interaction)。 數值結果顯示,當金奈米結構(單顆、雙顆、三顆金奈米粒子)受到圓偏振光照射後,外域的實部Poynting向量流線及軌道動量密度流線會在結構表面產生彎曲纏繞行為,此現象顯示近場有軌道角動量的產生,並且軌道動量流線的纏繞現象隨顆數增加,變得更為複雜。此外,外域的虛部Poynting向量流線也呈現螺旋向外輻射的形狀,並伴隨有若干個奇異點。電場的空間相位分布則呈現特定的拓樸核數。 此外,當高介電材料之奈米結構受到圓偏振光照射時,雖材料不具吸收特性,但因結構與環境的折射率差異大,仍可產生顯著的自旋軌道作用。本研究以兩種不同折射率之介電奈米珠(聚苯乙烯及矽)為例,結果顯示後者(高介電材料)的Poynting向量及軌道動量密度流線場亦有明顯的纏繞現象,即結構近場有軌道角動量的產生。但前者則不明顯。 另外,當結構為一細長較小的金奈米桿(nanorod),以圓偏振平面波垂直長軸入射時,Poynting向量及軌道動量密度流線於結構兩頂端,產生明顯側向軌道角動量。若結構為一長金奈米桿,以圓偏振平面波沿其長軸方向入射,則Poynting向量及軌道動量密度的流線於靠近奈米桿表面處呈螺旋纏繞,但此結果為多種不同模態的表面電漿子波(surface plasmon polaritons;SPP)之組合。若改以不同型式的電偶極為激發波源,則可產生單一模態的SPP於長金奈米桿上傳遞,以兩振盪方向相互垂直且具90度相位差的電偶極組為例,可激發出模態1的SPP;其軌道角動量的拓樸荷為1。 This thesis studies the orbital angular momentum (OAM) induced by a gold nanostructure interacting with a circularly polarized (CP) light. Based on Maxwell's electromagnetic theory, the multiple-multipole expansions method is used to simulate the electromagnetic field generated by the light-matter interaction. Via the electromagnetic field, we analyze the distributions of surface electric/magnetic charge and current on the nanostructure and the 3D streamlines of Poynting vector and orbital momentum (OM) density in the exterior field to investigate the photon’s spin-orbit interaction (SOI). The numerical results show that when a gold nanostructure of cluster (monomer, dimer, or trimer) is irradiated by a CP light, the winding behaviors of the streamlines of the real part of Poynting vector and OM density around the nanostructure are observed. This phenomenon indicates that the OAM is induced by SOI via nanostructure. This phenomenon becomes more complicated as the number of cluster increases. In addition, the streamline of the imaginary part of Poynting vector exhibits a pattern of spiral radiation with several singularities. The spatial phase of electric field shows the number of topological charge. Although a lossless dielectric nanostructure has no optical absorption, the SOI still can be induced as a dielectric nanostructure of high refractive index is irradiated by a CP light. This could be attributed to that the difference of refractive index between the nanostructure and medium is large. Two kinds of dielectric bead (polystyrene, Si) in water as irradiated by a CL light are studied. The streamlines of Poynting vector and OM density of the silicon bead shows a significant winding behavior; a strong SOI occurs in the nearfield of silicon bead. In contrast, the winding behavior is not obvious for the polystyrene bead. For a gold nanorod (NR) of small aspect ratio irradiated by a CP plane wave propagating in a direction perpendicular to the long axis, the streamlines of Poynting vector and OM density indicate that lateral OAM in the proximity of two tops is generated. For an elongated NR irradiated by a CP plane wave propagating along the long axis, the surface plasmon polaritons (SPPs) of different modes can be generated simultaneously; the streamlines of Poynting vector and OM density exhibit the winding behavior along the elongated NR. If electric dipole is used as light source, SPP of a single mode can be excited. For example, the SPP of mode 1 can be excited by a pair of crossed electric dipoles with 90o phase difference; the topological charge is l= 1. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/55884 |
DOI: | 10.6342/NTU202001988 |
全文授權: | 有償授權 |
顯示於系所單位: | 應用力學研究所 |
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