Plasmonics that confined in a nanocavity has become an emerging research area in optics, which has shown an extraordinary ability to allow visualization of the inner structure of a single molecule with sub-nanometer resolution (Nature 498, 82-86 (2013); Nature 531, 623-627 (2016)) and chemical identification a single molecule in real space (Nat. Nanotech. 10, 865-869 (2015); Nature 2019, 568, 78-82). In all these appealing applications, the distribution of plasmon plays a decisive role. Conceptually, it is known that the plasmons in the nanocavity should be inhomogeneous. However, the actual field distribution in the nanocavity in the longitudinal direction, probably the last puzzle piece, has never been experimentally obtained, causing heated debates on the mechanism for achieving super-high resolution Raman images in literature.

In this work, we present a rational design to experimentally overcome this great difficulty, which enables us to measure the field distribution in a nanocavity with ~2 ångström spatial resolution in the longitudinal direction. It is done by placing a self-assembly viologen molecular monolayer with a specific probe moiety in a nanocavity (Fig. 1a). The position of the probe moiety is continuously shifted along the viologen molecule, differing by two two methylene units (Fig. 1b). The field distribution in the nanocavity can be directly determined through the proper deconvolution of the experimentally measured Raman signals. We surprisingly find a large field inhomogeneity (~5.2 fold between largest and smallest field, Fig. 1c) inside the self-assembly monolayers, which cannot be explained by the widely adopted continuous media approximation for molecular monolayer. Consequently, a plasmon comb forms owing to the self-focusing of the individual molecule (Fig. 1a), leading to a plasmonic force that can result in a trapping potential around 10 k_BT to stabilize a small molecule.

Our findings not only provide the solution to a long-standing problem in the field and enrich our fundamental understanding of plasmonics, but also have strong implications for a variety of applications, such as photo-selective bond dissociation, sub-nanometer chemical recognition, and optical force mediated assembly of nano-objects.

This work was recently published in Nature Nanotechnology:

Observation of inhomogeneous plasmonic field distribution in a nanocavity

Chao-Yu Li, Sai Duan, Bao-Ying Wen, Song-Bo Li, Murugavel Kathiresan, Li-Qiang Xie, Shu Chen, Jason R. Anema, Bing-Wei Mao, Yi Luo, Zhong-Qun Tian, and Jian-Feng Li

Nature Nanotechnology (2020), https://www.nature.com/articles/s41565-020-0753-y