金屬奈米顆粒的光學性質之電磁模擬

Abstract

當電磁波照射在金屬奈米顆粒之上時，電磁波會與金屬上的自由電子產生耦合，產生侷域性表面電漿。侷域性表面電漿的產生，與金屬奈米顆粒的形狀、大小、結構、處在環境有關。由於這項獨特的性質，金屬奈米顆粒可應用於電場增益之上，為了能夠更廣泛地應用這項性質，我們必須更深入研究金屬奈米顆粒的光學性質。 本論文中，我們用DDA (Discrete Dipole Approximation)來模擬一些不同大小的金屬奈米立方體 (如：銀、金、鈉)的光學性質。我們發現，這些立方體的頻譜圖中有多個共振頻率產生，異於單顆金屬球的結果。共振頻率的發生與金屬的材料有很大的關係。銀和鈉的光譜圖中，波峰比較高、窄而且比較尖銳，而對其他的金屬 (如：鈀、鉑)，光譜波峰則比較低、寬，這是由於銀、金的光學吸收比鈀、鉑等金屬材料低。當立方體的尺寸夠小的時候，延遲效應的影響可以忽略不計，當立方體的尺寸大到一定程度時，我們勢必要考慮延遲效應的影響。 鈉雙球的光學性質與兩球之間的間距、球的大小有很大的關係。對L-mode而言，入射電場與球的軸心方向平行，電磁波朝著兩球中心入射，當兩球的間距變大時，共振波長變短，且共振波長由原先的兩個波長，變為只有一個共振波長。對T-mode 而言，電磁波同樣朝著兩球中心入射，而電場與球的軸心方向垂直，在這種情況下，當兩球的間距變大時 ，共振波長則變長。本論文中所模擬的金屬鈉的半徑為30 奈米，兩球的間距由5 奈米增長至30 奈米。

Localized surface plasmon resonances (LSPR) appears in a metallic nanoparticle when the nanoparticle interacts with the incident electromagnetic field and creates coherent oscillations of its conduction electrons. The LSPRs depend on the shape, size, composition of nanoparticles and even environment where the nanoparticles reside. Because of this unique property, plasmonic nanoparticles are useful for enhancement of fluorescence and Raman scattering of a single molecule. To understand the design rules for practical applications, the plasmonic properties of a single nanoparticle would need to be understood in greater details. In this thesis, plasmonic excitations in several metal (Ag, Au, Na) nanocubes with different sizes have been studied by both discrete dipole approximation(DDA) calculations and quasi-static theory analysis. We find that unlike a single metallic nanosphere, there are multiple plasmon peaks in the optical spectra of the metallic nanocubes. The positions of peaks of these plasmon excitations depend strongly on the constituent material. The plasmonic excitation features are very sharp in the Ag and Na nanocubes, but in the Pd and Pt nanocubes, they are very broad and weaker than the former, due to the strong damping caused by the large imaginary part of the dielectric constant of bulk Pd and Pt metals. We also find that the retardation effect is very small for the nanocubes whose edge size is less than 30 nm where the extinct spectra are nearly independent of the cube size. In contrast, for the larger nanocubes, the retardation effect is rather pronounced, and the plasmonic features are significantly red-shifted. The optical properties of a Na dimer are sensitive to the gap between the dimer spheres, the radius of the spheres, and the orientation of the dimer. If we first put the direction of polarization of incident electromagnetic wave parallel to the axis of Na dimer, the gap increases, the localized surface plasmon resonance decreases. And the number of peaks decreased simultaneously, from two peaks to one peak. We repeated the simulation again, but with the direction of polarization of the incident electromagnetic wave and the axis of Na dimer perpendicular to each other. At this mode, just one resonance frequency occurs. As the gap between Na spheres increases, the resonance frequency increases, too. In last mode in which the direction of incident wave was parallel to the axis of the Na dimer, as the distance of the gap increases, the resonance frequency increases, which is similar to the second mode. The radius of the Na sphere is 30nm in all cases, and the gap between the Na spheres varies from 5nm to 30nm.