奈米粒子非線性散射特性研究與應用

Study of Nonlinear Scattering of Nanoparticles and Applications

蘇東榆(Tung-Yu Su)

指導教授 ： 朱士維

國立臺灣大學/理學院/物理研究所/碩士(2012年)

https://doi.org/10.6342/NTU.2012.10096

摘要

近年來，許多材料科學的研究為了探討物質本身特性，而將重點放在物質的散射。散射為物質本身的特性，樣本不需要經過額外的加工處理，即可表現出其材料特性，帶領我們更進一步地了解物質本身。而在其中，非線性散射更能對物質本身之非線性特性進行研究，也成了當今研究材料非線性表現的主流方法。在本論文中，我們將詳細地討論在黑色素膠體溶液中的超瑞利散射(hyper-Rayleigh scattering)和在單顆金奈米粒子中的飽和散射(saturable scattering)這兩種非線性散射。
在第一部分，我們分析濃度類似於人體皮膚之黑色素膠體溶液的三倍頻訊號。有別於一般的三倍頻訊號，我們觀察到由黑色素膠體粒子誘發之非介面之三倍頻訊號。藉由分析其三倍頻強度對黑色素濃度的關係，我們得到線性相依性，證實該非介面三倍頻是由超瑞利散射產生。藉由分析此線性關係，我們得到黑色素膠體溶液之三階超極化率(3rd hyperpolarizability)。此項研究成果將有助於解釋含黑色素之生物樣本中三倍頻影像的診斷。
在第二部分，我們從理論與實驗角度深入探討金奈米粒子的飽和散射特性。在理論方面，我們利用時域有限差分法(finite difference time domain)來對金的非線性特性進行數值模擬。在實驗方面，我們利用多光源之共軛焦顯微技術來觀察單顆金奈米粒子的散射訊號。飽和散射在高激發強度(~107 W/cm2)的條件下，首次於實驗中被觀察到。若將強度持續加強(>109 W/cm2)，反飽和現象也會表現在單顆金奈米粒子的散射特性中，代表其更高階的非線性特性被激發表現出來。為了更進一步地了解其飽和散射之原理，我們利用三種激發波長(405奈米，532奈米與671奈米)和四種不同尺寸(40奈米，50奈米，80奈米與100奈米)之金奈米粒子樣本，來研究其對激發波長與粒子尺寸的相關性。就分析上來說，因為其金屬之散射與侷域表面電漿(localized surface plasmon resonance)息息相關，因此我們將這些相關性與表面電漿引起之吸收頻譜比較，進而證實飽和散射是由表面電漿子震盪之飽和造成。藉由分析這些相關性，單顆金奈米粒子的線性與非線性超極化率也同樣地被分析出來。
除了分析非線性特性之外，我們將金奈米粒子的飽和散射特性應用在飽和激發(saturated excitation)超解析度顯微技術上，並且得到解析度小於100奈米之散射影像。藉由時域有限差分法的數值分析，我們更進一步地確認其解析度的大幅進步是源自於非線性效應產生之"熱點"效應。這項技術不僅是世界上首次得到突破繞射極限的散射影像，而且將超解析顯微技術從傳統上侷限的螢光訊號拓展到散射訊號。而這項研究所衍生的應用，將可推廣至電漿奈米結構生醫影像領域。除此之外，所有利用到表面電漿特性之造影技術將會因為這項飽和特性而受惠，並改進其解析度。
最後，我們成功地利用金奈米柱在飽和時，兩軸的散射光強度會被互相影響的方式，將長軸的散射光飽和來控制短軸的散射光。這項技術是第一個利用雷射光去控制奈米結構的散射特性。藉由此項技術的幫助，未來將可利用飛秒雷射發展出超快速的光調變器，將電子訊號的調變時間縮短一大步。

關鍵字

金奈米粒子 ； 非線性光學 ； 超解析顯微技術

並列摘要

Being a natural behavior, scattering attracts much attention in studying the characteristic of materials. Without extra re-process of material, scattering exhibits intrinsic information of materials and broadens our view of understanding from macroscopic down to microscopic. In addition, nonlinear scattering is able to reveal more information and opens a window to study the nonlinear properties of materials. In this work, two kinds of nonlinear scattering are studied: hyper-Rayleigh scattering (HRS) in melanin colloids and saturable scattering of a single GNP (GNP).
For the first topic, we studied third harmonic generation (THG) of melanin solution with concentrations similar to melanocytes in human skin. In contrast to conventional observation of THG at interface, bulk THG was detected inside the solution due to the formation of melanin hydrocolloids. A linear relationship between melanin concentration and THG intensity was found, suggesting THG originated from high-order hyper-Rayleigh scattering. By analyzing this linear relationship, third-order hyperpolarizability of melanin hydrocolloids was determined to be three orders larger than that of water. This result will be useful for interpretation of THG signals in skin and other tissues containing colloidal particles
For the second part, we investigated, both theoretically and experimentally, the saturable scattering in a single gold nanoparticle (GNP) for the first time. In theoretical part, we used finite difference time domain (FDTD) method to simulate the nonlinear properties by inserting nonlinearities in gold material. In experimental part, multi-color confocal microscopy was used to observe the scattering of a single GNP. As a result, by a resonant excitation, saturable scattering was observed with moderate excitation intensity (~107 W/cm2); with even higher excitation intensity (>109 W/cm2), reverse saturable scattering was observed, indicating the existence of higher order nonlinear properties. To completely comprehend the mechanism of this saturable scattering, we applied three kinds of excitation wavelengths (405nm, 532nm and 671nm) and four kinds of GNP with different diameters (40nm, 50nm, 80nm and 100nm) to demonstrate the wavelength dependence and size dependence. Since the scattering of GNPs is significantly enhanced by localized surface plasmon resonance (LSPR), we compared these dependencies with the spectral properties induced by LSPR and found that they match the spectra, revealing that the saturation is dominated by plasmon resonance. Besides, by fitting the dependencies, linear and nonlinear hyperpolarizability of a single GNP were also deduced.
Beside for analyzing the nonlinearity of GNPs, this saturable scattering had been applied to optical super resolution imaging techniques by saturated excitation microscopy (SAXM). Images with a sub-100nm resolution had been achieved, breaking the optical diffraction limit of scattering signal for the first time. With the aid of FDTD simulation, “nonlinear hot spot” effect was confirmed to be an origin of this extraordinary enhancement of resolution. This result expands the horizon of super resolution imaging from fluorescence to scattering. The expected applications range from biomedical imaging to the functional inspection of plasmonic nanostructures. Moreover, all imaging modalities that utilize plasmonic properties, such as apertureless near-field microscopy, will be benefitted from the saturation effect for further resolution enhancement.
Finally, we successfully used the properties of saturable scattering in a single gold nanord to control the plasmonic scattering by laser light. This modulation of light scattering controlled by another light enhances the response time of electric modulation down to several picoseconds, pushing the limitation digital calculation nowadays.

並列關鍵字

Gold nanoparticles ； nonlinear optics ； superresolution microscopy

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