Observation of inhomogeneous plasmonic field distribution in a nanocavity

等离激元材料和器件中电场的强度分布是等离激元技术及其应用的重要基础。虽然针尖增强光谱成像技术的发展已经实现了亚纳米的横向空间分辨率，并发现了亚纳米级电场的不均匀性，但是迄今人们对电场的纵向场强分布仍然知之甚少。李剑锋教授课题组设计了一种具有~2Å空间分辨率的分子尺，利用金单晶基底和壳层隔绝金纳米粒子来构筑等离激元纳米腔，并通过分子尺的拉曼信号强度，精准地直接表征纳米腔中的纵轴方向上高度不均匀的场强分布。中国科学技术大学罗毅教授课题组利用基于量子场论的局域场光谱理论，精确地模拟得到了与实验相符的等离激元纳腔中的场分布，并发现了因分子自聚焦作用而引起的“等离激元梳”。该工作提供了一种通用有效的定量表征纳腔中场强分布的方法，完善了对等离激元学基础的理解，为超高空间分辨的拉曼光谱成像、光学力调控分子组装、单分子反应操控提供指导。 该工作是在李剑锋教授和中国科学技术大学罗毅教授共同指导下完成的。实验部分主要由李超禹（论文第一作者，已毕业博士）、温宝英（在读博士）、李松波（已毕业硕士）完成，复旦大学段赛研究员（论文共同第一作者）和陈舒（已毕业博士）进行了局域场光谱理论计算。谢立强（已毕业博士）和毛秉伟教授帮助完成了扫描探针显微镜实验。浙江师范大学周小顺教授和王亚浩老师提供了自组装膜表征方面的重要帮助。印度的Kathiresan、叶龙武教授课题组和浙江大学陆展教授课题组在分子合成方面提供了重要帮助。瑞士伯尔尼大学Wandlowski教授和田中群教授为该工作提供了指导。【Abstract】The progress of plasmon-based technologies relies on an understanding of the properties of the enhanced electromagnetic fields generated by the coupling nanostrucutres.Plasmon-enhanced applications include advanced spectroscopies, optomechanics, optomagnetics and biosensing. However, precise determination of plasmon field intensity distribution within a nanogap remains challenging. Here, we demonstrate a molecular ruler made from a set of viologen-based, self-assembly monolayers with which we precisely measures field distribution within a plasmon nanocavity with ~2-Å spatial resolution. We observed an unusually large plasmon field intensity inhomogeneity that we attribute to the formation of a plasmonic comb in the nanocavity. As a consequence, we posit that the generally adopted continuous media approximation for molecular monolayers should be used carefully.The progress of plasmon-based technologies relies on an understanding of the properties of the enhanced electromagnetic fields generated by the coupling nanostrucutres1,2,3,4,5,6. Plasmon-enhanced applications include advanced spectroscopies7,8,9,10, optomechanics11, optomagnetics12 and biosensing13,14,15,16,17. However, precise determination of plasmon field intensity distribution within a nanogap remains challenging. Here, we demonstrate a molecular ruler made from a set of viologen-based, self-assembly monolayers with which we precisely measures field distribution within a plasmon nanocavity with ~2-Å spatial resolution. We observed an unusually large plasmon field intensity inhomogeneity that we attribute to the formation of a plasmonic comb in the nanocavity. As a consequence, we posit that the generally adopted continuous media approximation for molecular monolayers should be used carefully.The Swedish National Infrastructure for Computing is acknowledged for computer time. S.D. is sponsored by Shanghai Pujiang Programme (grant no. 19PJ1400600). 该研究工作得到国家自然科学基金、国家重点研发计划、安徽省量子信息技术引导专项等的资助和支持