金奈米環陣列表面電漿特性與感測應用之研究

Abstract

近年來，金屬表面電漿成為奈米光子學領域中一項非常重要的研究，主因製程技術的進步實現了金屬奈米粒子之侷域表面電漿共振現象。其中，金屬奈米粒子的結構與參數設計是在表面電漿領域中很重要的課題之一。最近，奈米環結構受到廣大的研究興趣與重視，因此結構環內與環外表面電漿的強烈共振耦合，可產生很強的侷域電場，除此之外，環內區域可提供額外的電場分佈，此特性對於需要大量增強電場的實際應用上非常的有益，例如感測應用。 在本論文中，我們主要著重在金奈米環結構的參數優化設計上，包括：環寬、薄板厚度與高寬比，使其產生高侷域電場增強特性與對介質環境的高靈敏度。在環寬的設計中，我們以實驗驗證了金奈米環結構可藉由增加環寬來提高鍵結模態的介質感測靈敏度，當金奈米環寬為199奈米時，我們量測的最高靈敏度為691奈米每單位折射率。在薄板厚度的設計上，我們觀察當縮小金奈米環薄板厚度時，可增強環內與環外的電場強度，此結果對於製作具有高場強的奈米環結構與感測應用上，提供了一套很有用的設計方法。在高寬比的設計上，我們發現較大高寬比的金奈米環在縱向與橫向偏振時，分別展現出類似於盤狀與孔洞的光學特性，此特性也分別使得環外與環內呈現出較大的場強提升，而在生物分子的感測實驗中，我們觀測到藉由增加金奈米環的高寬比，可提高鍵結模態的表面感測靈敏度。除此之外，為了探索鍵結模態的近場交互作用，我們特別去研究奈米環二聚物的光學特性，當兩個金奈米環靠近時，可發現其間距處會產生極大的表面電漿耦合效應，在間距為10奈米下，其間距處的電場強度與金奈米盤狀二聚物相比增強了23%，另外，此強烈的耦合效應也使得介質感測靈敏度會隨著間距縮小而呈指數提升，相較於金奈米盤狀二聚物的耦合偶極模態相比，金奈米環二聚物的耦合鍵結模態擁有更高的介質感測靈敏度。而藉由金奈米環結構的參數優化設計，我們期盼金奈米環能成為表面電漿感測器中的高靈敏度組件，對於未來在醫學診斷、疾病監控與藥物研發上具有極高的發展性。

Plasmonics has become a very critical research in nanophotonics in recent years because of the advance in fabrication, which realizes localized surface plasmon resonance (LSPR) on metal nanoparticles. The design of metal nanostructures is one of the critical aspects of plasmonics. Recently, nanoring (NR) structures have attracted great research interest since such nanostructures can produce high local fields arisen from strong coupling of inner and outer surface plasmons. In addition, they provide an additional electric field in the cavity region of the ring structure, which is particularly beneficial for practical applications that need high volumes of enhanced fields such as sensing application. In this thesis, we focus on design optimization of structural parameters of the gold NR structure including ring width, slab thickness, and aspect ratio for high local electric field enhancement and high sensitivity to surrounding media. For ring width design, we experimentally demonstrate the index sensitivity of bonding mode in gold NR can be improved by broadening the ring width. High sensitivity of 691 nm per refractive index unit is obtained for NRs with 199 nm ring width. For slab thickness design, the enhancements of electric field intensities at the inner and outer ring surfaces when reducing the slab thickness are investigated. This result renders a useful design principle to fabricate such nanoparticles with a highly enhanced field for sensing application. For aspect ratio design, the disc- and hole-like optical properties of high-aspect-ratio gold NR for longitudinal and transverse polarizations are found, which exhibit stronger field intensity enhancements at the outer and inner ring surfaces respectively. And we find that bonding modes show increased surface sensitivities when enlarging the aspect ratio of gold NR in biosensing experiment. In addition, for probing the near-field interaction of bonding modes, we investigate the optical properties of gold NR dimers particularly. As the gold NR particles approach each other, a very strong surface plasmon coupling in the gap region of gold NR dimers is observed, whose field intensity at the gap distance of 10 nm is enhanced 23% compared to that for gold nanodisk (ND) dimers with the same diameter. Furthermore, this coupling causes exponential increase in sensitivity to refractive index of surrounding medium with decreasing the gap distance. Compared with coupled dipole mode in gold ND dimers, coupled bonding mode in gold NR dimers shows higher index sensitivity. Via structure optimization of gold NRs, we expect that gold NR structure can serve as highly sensitive components of plasmonic sensor for the development of medical diagnosis, monitoring of diseases and drug discovery in the future.