操控金屬奈米粒子之表面電漿子與其在近場光學和光纖生物感測器之應用
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2007
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金屬奈米結構的表面電漿子（surface plasmon）會使得金屬表面附近發生侷域場增強現象(local-field enhancement)。侷域場增強現象可使電磁場有效的被侷限和控制在奈米尺度的區域內。我們以Mie散射理論和有限時域差分法（finite-difference time-domain method）探討如何操控金屬奈米粒子的表面電漿子。對於單一的金屬奈米粒子，我們發現可以藉由在金屬外圍包裹介電質且控制介電質的介電係數，來控制金屬粒子的表面電漿子。接著，我們利用有限時域差分法，探討金屬奈米粒子因近場耦合作用所產生的表面電漿子效應。對於成對的金屬粒子對，若兩金屬粒子大小不同，藉由相互感應的方式，在金屬奈米粒子形成的間隙四周產生侷域極化電荷。此侷域化電荷可在間隙內產生高強度且高度侷限的侷域場。藉由改變不對稱粒子對粒子的半徑比、粒子之間距離和入射光方向，可以控制間隙內的侷域場強度。另外，我們也發現了成對的金屬奈米粒子排列出的陣列結構，其結構所激發的表面電漿子共振行為是可被控制和被預測。我們提出了簡化的開放式空腔模型來解釋它們的共振行為。在應用方面，我們研究了近場光碟 (near-field optical disk) 和光纖生物感測器 (fiber-optic biosensor) 如何利用金屬奈米粒子的表面電漿子效應來提升光碟片的解析度和感測器的敏感度。研究發現，近場光碟存在的隨機分佈的金屬奈米粒子，可形成許多隨機分佈的散射中心，將次波長紀錄點所產生的近場消散訊號轉換成可在遠場測量的傳播訊號。有限時域差分法的模擬結果顯示，約波長1/10的紀錄點大小可以被解析。我們利用了傅立業光學理論（Fourier optics approach）來理解近場光學超解析能力的光學機制。本文中，也仔細探討了金屬奈米粒子密度、分佈和散射效率如何影響近場光碟的解析度。最後，我們研究了金屬奈米粒子的侷域場增強現象與光纖生物感測器的螢光訊號增強現象（fluorescence signal enhancement）的關聯性。除去包裹層的光纖表面發出消散波，會使金屬奈米粒子表面產生強的侷域電場，而此侷域場可用來提高束縛在粒子表面的螢光分子的螢光訊號強度達到提高生物感測器敏感度的目的。我們利用了消散波照射單一金奈米粒子的散射理論計算出在金奈米粒子表面平均侷域電場強度提升的比例。計算結果顯示，侷域電場提升的比例在數量級上與實驗量測的螢光訊號提升比例是一致的。
High local-field enhancement appears in the vicinity of the surface of metallic nanostructures due to surface plasmon excitation and therefore electromagnetic fields can be confined and controlled in nanoscale region. The controllable and tunable surface plasmon of metallic nanoparticles is studied analytically and numerically using Mie scattering theory and finite-difference time-domain method, respectively. For a single coated metallic nanoparticle, the surface plasmon excitation can be controlled by changing permittivity of the coated material, and furthermore the higher-mode enhancement of the coated metallic nanoparticle is observed. The surface plasmon due to near-field coupling of metallic nanoparticles is studied numerically by finite-difference time-domain method. For a silver nanocylinder pair with different radii of silver nanocylinders, asymmetric polarization charges are induced by the mutual interaction between nanocylinders. asymmetric confined charges are induced around the nanoscale gap by the mutual interaction between the nanocylinders. The field intensity in the gap associated with the density of confined charges around the gap and it can be controlled by interparticle distance, radius ratio of asymmetric pairs, and the illumination direction of incident light.Besides, the controllable and predictable surface plasmon resonance is demonstrated in three-pair array structures. A simplified open cavity model is proposed to understand the cavity-like resonant behavior of pair array structures. Recently, local-field enhancement of metallic nanoparticles has been applied to enhance the resolution and the sensitivity of near-field optical disks and fiber-optic biosensors, respectively. The random distributed silver nanoparticles which embedded in AgOx layer of an AgOx-type near-field optical disk become high scattering centers and can transfer the diffracted evanescent components from subwavelength recording marks into the propagating components that can be detected in far-field region. The numerical result shows that the recording marks smaller than wavelength/10 are distinguishable. A Fourier optics approach is used as a theoretical background to understand the subwavelength resolution capability of near-field optical disks. The influences of the distribution and density of metallic nanoparticles, and the illumination frequency of incident light on the resolution capability of near-field optical disks are also discussed. Finally, the mechanism of fluorescence signal enhancement of a localized surface plasmon coupled fluorescence fiber-optic biosensor with gold nanoparticles is studied by the scattering of evanescent waves by a single gold nanoparticle. Local-field enhancement appears in the vicinity of a gold nanoparticle when the nanoparticle is illuminated by evanescent waves from the surface of the uncladded fiber and the fluorescence signals of fluorophores which is bounded on the surface of the nanoparticle can be enhanced by the enhanced localfield. Calculated result shows that the averaged-field intensity around the gold nanoparticle is enhanced few times of the field intensity without nanoparticle and the field enhancement of the theoretical calculation is consistent with the experimental result.
High local-field enhancement appears in the vicinity of the surface of metallic nanostructures due to surface plasmon excitation and therefore electromagnetic fields can be confined and controlled in nanoscale region. The controllable and tunable surface plasmon of metallic nanoparticles is studied analytically and numerically using Mie scattering theory and finite-difference time-domain method, respectively. For a single coated metallic nanoparticle, the surface plasmon excitation can be controlled by changing permittivity of the coated material, and furthermore the higher-mode enhancement of the coated metallic nanoparticle is observed. The surface plasmon due to near-field coupling of metallic nanoparticles is studied numerically by finite-difference time-domain method. For a silver nanocylinder pair with different radii of silver nanocylinders, asymmetric polarization charges are induced by the mutual interaction between nanocylinders. asymmetric confined charges are induced around the nanoscale gap by the mutual interaction between the nanocylinders. The field intensity in the gap associated with the density of confined charges around the gap and it can be controlled by interparticle distance, radius ratio of asymmetric pairs, and the illumination direction of incident light.Besides, the controllable and predictable surface plasmon resonance is demonstrated in three-pair array structures. A simplified open cavity model is proposed to understand the cavity-like resonant behavior of pair array structures. Recently, local-field enhancement of metallic nanoparticles has been applied to enhance the resolution and the sensitivity of near-field optical disks and fiber-optic biosensors, respectively. The random distributed silver nanoparticles which embedded in AgOx layer of an AgOx-type near-field optical disk become high scattering centers and can transfer the diffracted evanescent components from subwavelength recording marks into the propagating components that can be detected in far-field region. The numerical result shows that the recording marks smaller than wavelength/10 are distinguishable. A Fourier optics approach is used as a theoretical background to understand the subwavelength resolution capability of near-field optical disks. The influences of the distribution and density of metallic nanoparticles, and the illumination frequency of incident light on the resolution capability of near-field optical disks are also discussed. Finally, the mechanism of fluorescence signal enhancement of a localized surface plasmon coupled fluorescence fiber-optic biosensor with gold nanoparticles is studied by the scattering of evanescent waves by a single gold nanoparticle. Local-field enhancement appears in the vicinity of a gold nanoparticle when the nanoparticle is illuminated by evanescent waves from the surface of the uncladded fiber and the fluorescence signals of fluorophores which is bounded on the surface of the nanoparticle can be enhanced by the enhanced localfield. Calculated result shows that the averaged-field intensity around the gold nanoparticle is enhanced few times of the field intensity without nanoparticle and the field enhancement of the theoretical calculation is consistent with the experimental result.
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表面電漿子, 金屬奈米粒子, 近場光學, 近場光碟, 生物感測器, 有限時域差分法, 傅立業光學, surface plasmon, metallic nanoparticles, near field optics, near-field optical disk, fiber-optic biosensor, finite-difference time-domain method, Fourier optics