| 摘要: | 本文研究表面電漿於隨機結構與孤立奈米粒子團的電磁場局域增強及偏振旋性操控特性。透過消光譜量測、廣義多粒子米氏散射模擬、超解析四波混合及數位化色度空間等方法，研究表面電漿於上述結構產生之模態分布、共振波長飄移、線性與非線性光學響應、偏振態旋轉，及其應用於奈米環境感測之潛力。
隨機結構包括以鈉-銀離子交換製做出含不同尺寸及填充率之銀奈米粒子複合玻璃與濺鍍方式製做島狀分布金薄膜。調控製程參數，前者可產生共振波長自380 nm至540 nm一系列的帶止濾波玻璃，而後者在接近滲流閥值時有最高的電磁場增強效應。由於薄膜於此閥值下各尺度自我相似之特性，利用四波混合的三階非線性光學效應，在大範圍隨機薄膜結構上以遠場光學方法標定出單一狹縫結構熱點，並於120 nm × 70 nm的尺度範圍追跡熱點中心的波長響應。
在孤立奈米粒子團的研究對象包括:單體、雙聚體及三聚體。這部分的研究排除了隨機結構或非孤立系統的平均效應，而能較清楚呈現各效應之成因及物理機制。在單體部分，藉由廣義多粒子米氏散射模擬光譜結合數位化色度空間，建立了一套利用色座標變化來量測奈米環境折射率變化的感測方式，以24位元的彩色攝影機可檢出折射率變化0.0021的環境改變，相當於傳統可攜式光譜儀解析能力的4.8倍。在雙聚體的研究上，發現此為能產生單一光學旋性的最基本單元。除探討狹縫模態單一光學旋性的成因外，亦評價以色度座標飄移檢測待測分子手徵特性之優劣。最後透過金奈米粒子三聚體，探討在奈米尺度任意操控偏振態的可能。沿三聚體的雙軸基底分別以線性及非線性光學四波混合上，探討其出射光的偏振態與波長間的關聯性。成功地的在孤立的三聚體操控產出偏振消光比 = 20的準線偏振光至 = 2.31的橢圓偏振光。搭配寬頻四波混合線性啁啾的特性，可簡易地以調整時間延遲來操控出射波長以及偏振特性，建立一波長偏振可調的奈米光源。;In this thesis, plasmonically enhanced local electromagnetic (EM) field and chiral polarization manipulation in random and isolated nanoparticles are studied. Based on extinction spectrum measurement, generalized multi-particle Mie (GMM) scattering simulation, super-resolved four wave mixing (FWM), and digitized chromatic space, one had dug out plasmonically modal distribution, resonant wavelength shift, linear and nonlinear optical responses, polarization rotation, and the potential for nano-environment sensing.
For random structures, Na+-Ag+ ion exchanged composite glass and sputtering deposited thin films with a variety of filling fractions (FF) are studied. By controlling fabrication parameters, composite glasses with resonant wavelength ranges from 380 to 540 nm are obtained. These can be used as a series of stop band filters. As for the random films, it is found that the highest enhancement for the local EM field occurs when the FF reaches to the percolation threshold. Due to the property of self-similarity at percolation threshold, a single nano-interstice associated with the plasmonic hot spot over a large range can be located using the 3rd order FWM effect in far field. Besides, the position of hot spots with various resonant wavelength can be traced within a region of 120 nm × 70 nm, reflecting the mophlogical features of the film.
Regarding to isolated nanoparticle clusters, plasmonic monomer, dimer, and trimer are studied. This part research rules out the ensemble averaged effect and thereby the physical mechanisms behind can be clearly seen. For plasmonic monomer, GMM simulation is combined with digitized color space, which establishes a new nano-environment sensing method. Without utilizing spectrometer, index variation as small as 0.0021 can be resolved based on the corresponding chromaticity shift measured by a 24 bit color CCD. Compared with conventional portable spectrometers, this resolution is more than 4.8 times. In the case of plasmonic dimers, we show that this is the very fundamental unit which can generate homo optical chirality. Apart from the discussion of the origin of homo optical chirality, we also assess the capability of identifying the handedness of chiral molecules by means of the enhanced chromaticity shift. Finally, for the case of plasmonic trimmers, the manipulation of polarization at nanoscale is studied. By pumping along 2 eigen-directions in combination of optical FWM, the polarization and the output wavelength can be controlled. We successfully obtained nearly linear polarized light with extinction = 20 and elliptical light with = 2.31, demonstrating the potential of constructing nanoemitters with tunable wavelength and polarization. |
