金奈米粒子之可逆電漿散射開關及其在超解析影像上之應用

Reversible switching of plasmonic scattering of single gold nanoparticle and its application to superresolution imaging

吳學侑(Hsueh-Yu Wu)

指導教授 ： 朱士維

國立臺灣大學/理學院/物理研究所/碩士(2014年)

https://doi.org/10.6342/NTU.2014.02797

摘要

當帶有特定能量之光入射至奈米金屬粒子表面時，此入射光所帶之電場振盪激發其奈米金屬粒子表面之自由電子，形成侷域表面電漿與此電場達成共振(localized surface plasmon resonance)將造成非線性光學效應顯著地增強，使得各種應用如光電積體電路、奈米雷射、生物感測、或是近場超解析顯微技術等更為多樣化並增加其應用之價值。其中金奈米粒子的飽和吸收(saturable absorption)和反飽和吸收(reverse saturable absorption)便因其在光學訊號處理的潛力而被大量研究。然而，目前觀察飽和吸收與反飽和吸收之方法大多為Z-scan，因此所觀測得到的為一大群金奈米粒子之集體效應，而非單顆奈米粒子之行為。若我們想觀測的目標為單顆金奈米粒子並應用於光學顯微技術上，原先的Z-scan方法中所量測之穿透訊號將由於繞射而缺乏成像對比。因此，為了量測單顆金奈米粒子之非線性光學效應，我們使用了反射式共軛焦顯微鏡(confocal microscope)來觀測金奈米粒子的散射光，在利用折射率與玻璃相近之鏡油去除介面反射後，單顆金奈米粒子之散色光可被清楚地辨別，進而量測其非線性效應。在此論文中，我們發現了兩種單一金奈米粒子的非線性特性： 1. 飽和散射(saturable scattering)與反飽和散射(reverse saturable scattering), 2. 可逆之散射開關(reversible switching of scattering)
關於第一部分的發現，我們使用了多種不同波長的激發光以及不同尺寸的金奈米粒子以觀察它們的非線性散射行為，單顆奈米粒子的飽和散射(saturable scattering)和反飽和散射(reverse saturable scattering)行為首次在實驗中被直接觀測。實驗中飽和散射首先在激發強度約105 W/cm2的條件下發生，若將強度持續加強至106 W/cm2，反飽和現象也會出現，代表不同的非線性特性隨激發強度變強而表現出來。為了更進一步地了解其飽和散射之原理，我們利用四種不同的激發波長(405奈米，532奈米，561奈米與671奈米)和共五種不同尺寸(40奈米，50奈米，60奈米，80奈米與100奈米)之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共振(localized surface plasmon resonance)息息相關，因此我們將波長對尺寸相關性與表面電漿引起之吸收頻譜比較，進而證實飽和散射受侷域表面電漿振盪之影響。和其他文獻中的飽和吸收和反飽和吸收反應也被使用來做比較，發現穿透率/散射強度之特性非常相似，證明非線性吸收與非線性散射可能具有相同的物理機制。
另一方面，近年來在光學上最重要的突破之一為各種超解析顯微技術的發現，這些技術的發展使我們終於能突破繞射極限之限制。然而，因這些技術的原理基於螢光之開關或飽和，故螢光分子於高強度光照下之光致褪色(photobleaching)嚴重地影響了這些技術的實用性。因此，運用不具有光致褪色現象的光散射(scattering)之超解析顯微技術成為我們研究的目標。在此目標下，不久前我們已成功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70奈米之散射影像。接著在此論文之研究中我們發現若同時以兩道不同波長之光激發金奈米粒子，當其中一道光達到飽和散射時另一道光之散射將被抑制80%。接著將此特性應用在超解析度顯微技術上，並搭配反褶積(deconvolution)方法得到解析度小於60奈米之散射影像，此方法成功解析出極相近之兩顆奈米粒子，證明了其可行性。這項新技術和前面所提及之飽和激發超解析度顯微技術為世界上唯二能得到突破繞射極限的散射影像之方法，將超解析顯微技術從傳統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研究與生醫影像領域，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。除了各種造影技術之外，另一個金奈米粒子之散射壓制的應用方向即為超快並且超小之光電開關

關鍵字

金奈米粒子 ； 飽和吸收 ； 飽和散射 ； 超解析度 ； 非線性光學開關

並列摘要

When light interacts with plasmonic nanostructures, it couples to free-electron excitation near the metal surface, and resonance appears to significantly enhance local-field. The resonance therefore allows inherently weak nonlinear optical effects to take place with low excitation intensity, opening up new opportunities for versatile applications to photonic integrated circuits, nano laser, biosensing, and near-field superresolution imaging techniques with these nano-sized plasmonic particles. One of the most studied nonlinearities in plasmonic materials is saturable absorption (SA) and reverse saturable absorption (RSA) of gold nanoparticles (GNP) since the capability of SA and RSA of GNP to be applied to optical signal processing. However, most of the previous SA and RSA experiments were performed by z-scan technique as ensemble measurements, so there was no absorption knowledge of single GNP could be identified. To determine the optical response of single GNP, we noticed that the extraordinary strong scattering of single GNP can be easily detected under a microscope, with a dark-field or a reflection confocal scheme. In addition, the absorption and scattering in plasmonic nanoparticles are linked via Mie theory. Therefore, what we studied is this work is to characterize the nonlinearity of scattering from single plasmonic nanoparticle.
In our work, the nonlinear scattering of single gold GNP is investigated. A multi-color confocal microscope is applied to observe the scattering comes from a single GNP, and we find the scattering of GNP exhibits saturable scattering (SS) and even reverse saturable scattering (RSS) behaviors with increasing excitation intensity. Studies with different excitation wavelengths and different intensities reveal that the SS and RSS are dominated by localized surface plasmon resonance (LSPR). Interestingly, similar phenomena of saturation and reverse saturation have also been discovered in nonlinear absorption with ensemble of metal nanoparticles by z-scan methods, implying that SA and RSA share the same physical origins with nonlinear scattering. However, different from previous ensemble measurements, our result provides the nonlinear interaction within single nanoparticle domain. The point spread functions (PSFs) corresponding to SS and RSS are also presented, which offer us more insights about the dependencies of SS and RSS to the excitation light intensity.
On the other hand, one of the biggest breakthroughs in modern optics is that the physical limitation, which is due to diffraction of light, on the resolution of conventional optical microscopy has been overcome by the appearance of superresolution techniques. However, the photobleaching of fluorophores during intense excitation becomes a severe problem shared by current superresolution techniques, which are based on switching or saturation of fluorescence. Therefore, the bleaching-free, nonlinear scattering of GNPs is an attractive alternative to enhance optical resolution. Previously, we have applied saturated excitation (SAX) microscopy to the scattering of GNPs and achieved resolution beyond diffraction limit. Now in this work, we have further demonstrated a brand new reversible switching phenomenon based on SS within single GNP and its application to superresolution microscopy. In the experiment, two beams, where both wavelengths are within the plasmon band, are applied to be focused on the GNP. While scattering from one beam becomes saturated, it is found that the scattering from the other beam can be simultaneously suppressed. More than 80% suppression ratio has been accomplished with excellent reversibility and repeatability, and this reversible switching can be used to improve optical resolution beyond the diffraction limit. Combined with a donut-shaped beam plus deconvolution, the suppression of scattering imaging (SUSI) reveals unambiguous separation of closely packed GNPs with λ/9 full-width-at-half-maximum resolution. Also, no bleaching was observed after our elongated illumination. On the other hand, the suppression of scattering in single gold nanosphere (GNS) also provides a potential candidate for achieving ultra-small, ultrafast, and all-optical switching/modulation.
To summarize, our work opens up new possibilities in nonlinear plasmonic scattering, non-bleaching superresolution microscopy, as well as ultra-small active plasmonic components. Applications widely range from biomedical imaging, functional inspection of plasmonic nanostructures, and to all imaging modalities that utilize plasmonic properties, such as apertureless near-field microscopy. In addition to applications focus on imaging, another potential application of our finding is to achieve an ultra-small, ultrafast photonic switch using single GNP.

並列關鍵字

gold nanoparticle ； saturable absorption ； saturable scattering ； superresolution ； nonlinear optical switching

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