The coherent exchange of optical near fields between two neighbouring dipoles plays an essential role in the optical properties, quantum dynamics and thus the function of many naturally occurring and artificial nanosystems. These interactions are challenging to quantify experimentally. They extend over only a few nanometres and depend sensitively on the detuning, dephasing and relative orientation (that is, the vectorial properties) of the coupled dipoles. Here, we introduce plasmonic nanofocusing spectroscopy to record coherent light scattering spectra with 5 nm spatial resolution from the apex of a conical gold nanotaper. The apex is excited solely by evanescent fields and coupled to plasmon resonances in a single gold nanorod. We resolve resonance energy shifts and line broadenings as a function of dipole distance and relative orientation. We demonstrate how these phenomena arise from mode couplings between different vectorial components of the interacting optical near fields, specifically from the coupling of the nanorod to both transverse and longitudinal polarizabilities of the taper apex.
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The authors acknowledge financial support by the Deutsche Forschungsgemeinschaft (SPP1839 ‘Tailored Disorder’, grant LI 580/12 and SPP1840 ‘QUTIF’ grant LI 580/13), the Korea Foundation for International Cooperation of Science and Technology (Global Research Laboratory project, K20815000003) and the German–Israeli Foundation (GIF grant no. 1256 and 1074-49.10/2009). M.E. thanks the Studienstiftung des Deutschen Volkes (German Scholarship Foundation) for a PhD scholarship and the Deutsche Forschungsgemeinschaft (project 401390650). The authors thank V. Smirnov for performing supporting finite element method calculations and H. Kollmann for providing high-resolution SEM images of individual gold nanorods. The authors thank N. Talebi for discussions and supporting numerical calculations.
Supplementary Figs. 1–15. Supplementary Table 1