Near-Field Plasmon Coupling in Gold Nanochains Revealed by Photoemission Electron Microscopy

Quan Sun Research Institute for Electronic Science, Hokkaido University, Sapporo, Japan Creative Research Institution (CRIS), Hokkaido, Sapporo, Japan Han Yu Research Institute for Electronic Science, Hokkaido University, Sapporo, Japan Kosei Ueno Research Institute for Electronic Science, Hokkaido University, Sapporo, Japan Atsushi Kubo Institute of Physics, University of Tsukuba, Tsukuba, Japan Yasutaka Matsuo Research Institute for Electronic Science, Hokkaido University, Sapporo, Japan Hiroaki Misawa Research Institute for Electronic Science, Hokkaido University, Sapporo, Japan

One dimensional (1-D) metallic nanoparticle chains can support two surface plasmon resonance (SPR) modes, namely longitudinal mode (L-mode) and transverse mode (T-mode) with the polarization of the excitation light parallel and perpendicular to the chain axis, respectively. It has been reported that there is energy splitting of these two modes and the energy splitting is dependent on the gap width and the chain length (the number of particles within a chain).^[^1] Typically, L-mode is significantly red-shifted and T-mode is insensitive or slightly blue-shifted as the chain length is increased or the gap width is reduced. Previous studies have shown that the energy splitting between L-mode and T-mode is mostly resulted from near-field coupling based on far-field measurements and/or numerical simulations.^[1,2] In this study, we investigated the near-field properties of SPRs in 1-D gold nanochains using photoemission electron microscopy (PEEM) with near infrared femtosecond laser pulses as the excitation source, which has been recently demonstrated as a promising approach to probe the plasmonic properties of SPRs.^[3] The near-field mapping as well as the local field enhancement sites can be precisely obtained because of the high spatial resolution of our PEEM system (~ 4 nm). The near field spectra are obtained by tuning the excitation wavelength. The experimental results clearly show the energy splitting between the L-mode and T-mode SPRs and its dependence on the chain length and gap width. They have the same tendency as that observed from the far-field measurements. This proves that the plamson coupling within the nanochains is indeed mainly resulted from the near-field coupling. Furthermore, we observed the signatures of energy transport along gold nanochains from the near field in both PEEM experiments and FDTD simulations. The energy transport holds promise for the applications of plasmonic waveguiding^[2].

[1] S. A. Maier, M. L. Brongersma, P. G. Kik, H. A. Atwater, Phys. Rev. B 65, 193408 (2002).

[2] B. Willingham, S. Link, Opt. Express 19, 6450 (2011).

[3] Q. Sun, K. Ueno, H. Yu, A. Kubo, Y. Matsuo, H. Misawa, Light: Sci. Appl. 2, e118 (2013).

quansun@es.hokudai.ac.jp