EAGER: Measuring near-field nanoplasmonics fields using super-resolved far-field optics

在化学测量和成像项目的支持下，加州大学洛杉矶分校的Weiss教授和Neuhauser教授正在开发一种新的成像工具，用于测量纳米结构附近的局部表面等离子体场强度。众所周知，有时当入射光照射到固定有小金属特征的表面时，金属中的电子可以一起来回振荡，形成“波”。所谓的“表面等离子体”。表面等离子体是一种重要的光学现象，已被广泛应用于许多现实世界的应用中，包括在旧建筑中看到的中世纪彩色玻璃窗的暗红色。这种颜色来自于可见光与嵌在玻璃中的金纳米粒子相互作用。为了更好地利用表面等离子体现象，了解它是如何分布在不完美的纳米结构周围是很重要的。现代科学的进步使科学家能够利用计算机模拟程序估计表面等离子体场强度的分布，但是直接测量这种场强度，特别是在制备不完善的纳米结构周围，是具有挑战性的，并且尚未完全实现。Weiss和Neuhauser教授正在开发一种方法，通过监测某些无机粒子的闪烁速率，直接测量小表面结构附近的表面等离子体强度。这种方法将使他们能够以非常高的空间分辨率绘制场强图。与目前正在开发的其他方法相比，它也非常快速和便宜。在这18个月的资助期内，两个小组都专注于(1)将无机颗粒放置在纳米尺寸的表面特征周围，(2)研究如何使用这些颗粒的放置来绘制局部电磁场强度。他们正在应用这种成像技术来研究分子如何在表面附近移动，或者在金属纳米颗粒上如何发生反应。两个小组的研究生都参与了实验和理论研究。两位教授还积极鼓励有才华的高中生参加研究生课程，特别是来自未被充分代表的少数族裔的学生。在大范围内同时超分辨等离子体场强度的能力是独特和可取的。这种方法将加深对等离子体系统的理解和控制，并将扩大等离子体系统的影响。Weiss和Neuhauser教授正在研究的新型探测技术利用量子点中闪烁统计量对电场强度的依赖，来分辨远低于衍射极限的等离子体场强。该方法消除了在金属纳米结构附近定位偶极子发射体的典型复杂性。基于时间相关密度泛函理论的量子点响应建模的理论框架在确定性或随机变异体中也被用于构建简化的构建块。基于计算简化的构建模块模型允许同时模拟大量量子点和等离子体结构，模拟正在进行的实验系统。通过优化本文开发的理论和实验工具，可以快速连续测量~100x100微米平方尺寸区域的详细电场图。这种成像方法如果成功，将使许多依赖于测量衍射极限以下场强的应用受益，从生物学到高速集成电路，再到光学计算。此外，为实验和理论研究开发的软件为分析和预测异质区域的场强提供了一个可接近的工具。Weiss教授和Neuhauser教授打算通过免费的软件包将他们开发的研究工具传播到更广泛的社区。

With support from the Chemical Measurement and Imaging Program, Professors Weiss and Neuhauser at the University of California-Los Angeles are developing a new imaging tool to measure local surface plasmon field intensity near nanometer-sized structures. It is known that sometimes when the incoming light illuminates a surface immobilized with small metal features, the electrons in the metal can oscillate back and forth together and form a "wave" ? the so called "surface plasmon". Surface plasmon is an important optical phenomenon and has been widely used in many real world applications, including the dark red color in medieval stained-glass windows that are seen in an old buildings ?the color comes from the visible light interacting with gold nanoparticles embedded in the glass. In order to better utilize the surface plasmon phenomenon, it is important to understand how it is distributed around imperfect nano-structures. Modern science advancement allows scientists to estimate the distribution of surface plasmon field intensity with computer simulation programs, but direct measurements of such field intensity, especially around imperfectly prepared nano-structures, are challenging and have not been fully realized. Professors Weiss and Neuhauser are developing a way to directly measure surface plasmon intensity near a small surface structure by monitoring the blinking rate of certain types of inorganic particles. This method would allow them to map the field intensity at a very high spatial resolution. It is also very fast and inexpensive as compared with other methods currently in development. During this 18-month grant period, both groups are focusing on (1) placing the inorganic particles around nanometer-sized features on a surface and (2) studying how the placement of these particles may be used to map the local EM field intensity. They are applying this imaging technique to study how molecules move near a surface or how a reaction happens on a metal nanoparticle. The graduate students in two groups are involved in both experimental and theoretical components of research. Both professors are also actively engaged in encouraging talented high school student to be enrolled in graduate programs, in particular from underrepresented minority groups. The ability to simultaneously superresolve plasmonic field strengths over a large region is unique and desirable. Such approach will deepen the understanding of and control over plasmonic systems, and will broaden the impact of plasmonics. The novel probing technology Professors Weiss and Neuhauser are working uses the dependence of the blinking statistics in quantum dots on the electric field strength to resolve plasmonic field strengths well below the diffraction limit. The methods negate complications typical of localizing dipole emitters near a metallic nanostructure. A theoretical framework based on modeling of the quantum dots response with time-dependent density functional theory in deterministic or stochastic variants is also used to construct simplified building blocks. A computationally simplified building-blocks based modeling then allow simulations of a very large number of quantum dots and plasmonic structures simultaneously, mimicking the on-going experimental systems. By optimizing the theoretical and experimental tools developed here, the detailed electric field map of ~100x100 micrometer-squared size regions may be measured in quick succession. The imaging method, if successful, could benefit many applications that rely on the ability to measure field strengths below the diffraction limit, ranging from biology, to high speed integrated circuits, to optical computing. Additionally, the software developed for the experiments and for the theory studies provides an approachable tool for analyzing and predicting field strengths in heterogeneous regions. Professors Weiss and Neuhauser intend to disseminate the research tools developed to a broad community through freely available software packages.