金奈米粒子飽和與反飽和散射的非線性分析與應用

Study of Saturable and Reverse Scattering in a Single Gold Nanoparticle by Saturated Excitation Microscopy

李炫(Hsuan Lee)

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.2014.02469

摘要

電漿子在光學領域一直是非常熱門的研究之一，在極為前端的科學期刊常可以看到相關的文章；而在應用上也是非常的廣泛，例如在生醫影像，生物探測，及光開關都可以看到電漿子的涉足。由於非線性效應的發現，電漿子所產生的電磁場的強度與密度被更為提高，進而增廣了其應用性，但其在影像上的解析度提昇目前僅限於近場顯微術。遠場光學顯微鏡的解析度受繞射所限，一般只能達到約波長的一半。過去二十年中，發展了許多新穎的超解析顯微技術，主要利用螢光的開關或是飽和等非線性光學特性來達到超高解析度，然而螢光具有光漂白的問題，無法做長時間的觀察。近年來，近年來，金奈米粒子的飽和散射性質的發現讓電漿子學的應用更為推廣，也已經被證實能夠應用在其中一種超解析度顯微術，飽和激發顯微術上，進而達到超越光學繞射極限，且可以長時間產生穩定的散射訊號，不像螢光的影像訊號會在短時間內逐漸減弱。除了飽和散射以外，金奈米粒子的反飽和散射性質也在更強的激發光下被發現，而其在共軛焦顯微術上的影像卻跟一般的粒子影像有所差異：呈現一種同心圓而非單一圓形的影像，且外圈的影像寬度非常窄。如果此極窄的外圈可以跟中心訊號分開的話，將能進一步提昇光學影像的解析度，然而，此外圈的影像無法與中心影像以共軛焦顯微術分開。因此，此論文的主軸是希望將反飽和散射結合飽和激發顯微術，希望能抽取出外圈的非線性訊號以增進解析度。
在本論文中，將先從飽和與反飽和散射影像出發，並探討金奈米粒子的飽和激發影像對於不同激發光強度的變化做討論，最後跟理論計算和螢光的情況做比較。此實驗結果與分析方法顯現出經由飽和激發顯微術的訊號能比一般的共焦點顯微術得到更多的非線性資訊，繼而能更精確地量測金奈米粒子的非線性曲線，而推測出金奈米粒子飽和散射性質。除此之外，也能經由該結果而選定適當的激發強度，獲得解析度比以前更佳的金奈米粒子影像。

關鍵字

非線性 ； 超解析度

並列摘要

Optical microscopy, since its invention some 400 years ago, has become an essential tool in many disciplines. When considering optical imaging, there are some important factors, including contrast, resolution, magnification, imaging speed, penetration depth, noninvasiveness, etc. Among them, contrast is arguably the most important one. Without contrast, nothing can be observed no matter how good are other factors. During the last century, most of the major developments in optical microscopy are related to contrast, such as dark field, phase contrast, differential interference contrast, fluorescence, etc.
During last decade, there were significant breakthroughs in resolution of optical microscopy, relying on the nonlinearity of fluorescence. To avoid the photobleaching issue of fluorescence, we recently discovered saturable scattering in an isolated plasmonic particle, which can broaden the application of plasmonics in the fields of optical communication, sensing, and imaging. Combined with saturated excitation (SAX) microscopy technique, our discovery provides a new contrast agent for superresolution microscopy, with resolution down to λ/8. In addition, we also found that scattering from a single plasmonic particle exhibits reverse saturation behavior at higher excitation intensities, where very narrow side lobes and significantly reduced width of main lobe in the point spread function are observed, on a single particle basis. It will be very interesting to extract the reversed saturated part and to further enhance optical resolution by completely separating the side lobes from the main lobe. However, such separation is not possible with conventional confocal microscopy scheme. Here we explore the combination of reverse saturable scattering and SAX. With exerting modulated excitation light, the nonlinear signals can be extracted by Fourier transform to realize SAX microscopy. Different from the SAX images of fluorescence, the nonlinear components of scattering show many dips at higher excitation intensity. Based on quantitative analysis of amplitude and phase of different frequency components in SAX, the phase change and resolution enhancement of signal are observed after every dip. The results suggest better resolution enhancement and higher signal contrast can be achieved by SAX microscopy in reverse saturable scattering region. The results can be used for achieving better resolution in imaging and investigating the nonlinear properties more precisely.

並列關鍵字

nonlinear ； super-resolution microscopy

參考文獻

1. N. Fang, H. Lee, C. Sun, X. Zhang, "Sub-diffraction-limited optical imaging with a silver superlens," Science 308(5721), 534-537 (2005).

2. B. Lee, S. Kim, H. Kim, Y. Lim, "The use of plasmonics in light beaming and focusing," Progress in Quantum Electronics 34(2), 47-87 (2010).

3. S. Lal, S. Link, N.J. Halas, "Nano-optics from sensing to waveguiding," Nature Photonics 1(11), 641-648 (2007).

4. S. Kawata, Y. Inouye, P. Verma, "Plasmonics for near-field nano-imaging and superlensing," Nature Photonics 3(7), 388-394 (2009).

5. K. Kneipp, Y. Wang, H. Kneipp, L.T. Perelman, I. Itzkan, R. Dasari, M.S. Feld, "Single molecule detection using surface-enhanced Raman scattering (SERS)," Physical Review Letters 78(9), 1667-1670 (1997).