从单电浆纳米粒子散射非线性测量
Summary
Saturable and reverse saturable scattering were discovered in isolated plasmonic particles and adopted as a novel non-bleaching contrast method in super-resolution microscopy. Here the experimental procedures of detecting and extracting nonlinear scattering are explained in detail, as well as how to enhance resolution with the aid of saturated excitation microscopy.
Abstract
Plasmonics, which are based on the collective oscillation of electrons due to light excitation, involve strongly enhanced local electric fields and thus have potential applications in nonlinear optics, which requires extraordinary optical intensity. One of the most studied nonlinearities in plasmonics is nonlinear absorption, including saturation and reverse saturation behaviors. Although scattering and absorption in nanoparticles are closely correlated by the Mie theory, there has been no report of nonlinearities in plasmonic scattering until very recently.
Last year, not only saturation, but also reverse saturation of scattering in an isolated plasmonic particle was demonstrated for the first time. The results showed that saturable scattering exhibits clear wavelength dependence, which seems to be directly linked to the localized surface plasmon resonance (LSPR). Combined with the intensity-dependent measurements, the results suggest the possibility of a common mechanism underlying the nonlinear behaviors of scattering and absorption. These nonlinearities of scattering from a single gold nanosphere (GNS) are widely applicable, including in super-resolution microscopy and optical switches.
In this paper, it is described in detail how to measure nonlinearity of scattering in a single GNP and how to employ the super-resolution technique to enhance the optical imaging resolution based on saturable scattering. This discovery features the first super-resolution microscopy based on nonlinear scattering, which is a novel non-bleaching contrast method that can achieve a resolution as low as l/8 and will potentially be useful in biomedicine and material studies.
Introduction
等离子体的研究已经引起了极大的兴趣，因为它在许多不同的领域1-4的应用。一种在等离子体中研究最多的领域是表面等离子体激元，其中的传导电子夫妇在金属和电介质之间的界面的外部电磁波的集体振荡。表面等离子体激元已探索其在亚波长光学，生物光子学，以及显微镜5,6潜在的应用。在由于局部表面等离子共振（LSPR）金属纳米颗粒的超小体积的强场增强吸引，不仅是因为它的优异的敏感性，颗粒大小，颗粒形状，以及周围介质 7的介电性质的广泛关注，因为它能够提高固有的弱非线性光学效应11 -10的能力，而且还。 LSPR的特殊敏感性是生物传感和近外商投资企业有价值LD成像技术12,13。另一方面，电浆结构的增强非线性可以利用在光子集成电路中的应用，如光交换和全光信号处理14,15。众所周知的电浆吸收线性正比于在低强度电平的激发强度。当激励足够强，吸收达到饱和。有趣的是，在较高的强度，在吸收再次增大。这些非线性效应被称为饱和吸收（SA），15-17和反向可饱和吸收（RSA）18。
据了解，由于LSPR，散射特别强烈的电浆结构。根据基本的电磁散射与入射强度的反应应该是线性的。然而，在纳米粒子，散射和吸收紧密通过米氏理论联系在一起，都可以通过电子邮件xpressed中的介电常数的实部和虚部条款。下，一个单一的GNS表现为下光照偶极的假设下，从一个单一的等离子纳米颗粒根据米氏理论的散射系数（Q SCA）和吸收系数（Q 绝对）可以表示为19
其中 x是2πa/λ，a是球体的半径，并且m 2 为 ε 米 /εð。这里，ε 米 和 εD分别对应于金属的介电常数和周围的电介质，分别。因为散射系数的形式是类似的第Ë吸收系数，因此它被期望遵守饱和散射在一个电浆纳米粒子20。
最近，在一个孤立的电浆粒子非线性饱和散射证实首次21。值得注意的是，在深度饱和，其实散射强度略有下降，当激发强度增加。更值得注意的是，当激励强度继续后散射趋于饱和增加，散射强度再次上升，呈现出反饱和散射20的效果。 Wavelength-和尺寸相关的研究表明散射21 LSPR和非线性之间有紧密的关系。电浆散射的强度和波长的依赖性非常类似于那些吸收，这表明这些非线性行为基础的共同机制。
在应用方面，它是很好KNOWN该非线性有助于改善光学显微镜的分辨率。在2007年，饱和励磁提出（SAX）显微镜，其可以通过经由激励光束22的时间正弦调制提取饱和信号提高分辨率。 SAX显微镜是基于这样，对于一个激光焦斑，强度在中心处比在外围更强的概念。如果信号（或者荧光或散射）呈现饱和特性，饱和必须从中心开始，而线性响应保持在外围。因此，如果有一种方法，以仅提取饱和部分，它将只离开中心部分，同时抑制周边部，从而有效地增强了空间分辨率。原则上，存在的SAX显微镜没有更低分辨率极限，只要深达到饱和并且有由于强烈照明没有样品的损伤。
已经表明，该resolutioÑ荧光成像可以通过利用SAX技术来显著增强。然而，荧光遭受的漂白效果。结合散射非线性的发现和SAX的概念，是根据散射超分辨率显微镜可以实现21。相对于传统的超分辨率显微技术，散射为基础的技术提供了一种新颖的非漂白对比法。在本文中，一步一步的描述给出概述获得和提取等离子散射的非线性要求的程序。识别通过改变入射强度引入散射非线性的方法进行了描述。详情将提供解开这些非线性如何影响单个纳米粒子的图像，以及如何空间分辨率可以相应地由SAX技术来增强。
Protocol
Representative Results
Discussion
在协议中，有几个关键步骤。首先，制备样品时，纳米颗粒的密度不应太高，以避免颗粒之间电浆耦合。如果两个或多个颗粒非常接近彼此，耦合导致LSPR波长偏移到更长的波长，从而显著降低非线性。然而，这种成像技术实际上映射，代替微粒本身电浆模式的分布。因此，可以预期，与适当的激发波长中，耦合电浆模式也可以显示强散射的非线性，并且可以具有增强的分辨率进行成像。第二，这…
Divulgations
The authors have nothing to disclose.
Acknowledgements
This work is supported by Ministry of Science and Technology under NSC-101-2923-M-002-001-MY3 and NSC-102-2112-M-002-018-MY3. This research is also supported by the Japan Society for the Promotion of Science (JSPS) through the “Funding Program for Next Generation World-Leading Researchers (NEXT Program),” initiated by the Council for Science and Technology Policy (CSTP) and JSPS Asian CORE Program.
Materials
| microscope body | Olympus, Japan | BX-51 | |
| objective lens | Olympus, Japan | UPlanSapo, 100X, NA 1.4 | |
| 80-nm gold colloid | BBI Solutions, UK | EM.GC80 | |
| supercontinuum laser | Fianium, United Kingdom | SC400-2-PP | |
| broadband dielectric mirrors | Thorlabs, USA | BB1-E02 | |
| field emission SEM | JEOL, Japan | JSM-6330F | optional |
| spectrometer | Andor Technology, UK | Shamrock 163 | |
| charge-coupled device | Andor Technology, UK | iDus DV420A-OE | |
| acousto-optic modulators | IntraAction Corp., USA | AOM-402AF1 | |
| lock-in amplifier | Stanford Research Systems, USA | SR-830 | |
| MAS-coated slide glass | Matsunami Glass, Japan, | S9215 |
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