Probing of Propagating Plasmons wth Two Near-Field Tips

Roy Kaner Chemical Physics, Weizmann Institute of Science, Rehovot, Israel Yaara Bondy Chemical Physics, Weizmann Institute of Science, Rehovot, Israel Guy Shalem Mechanical Engineering, Technion-Israel Institute of Technology, Haifa, Israel Yehiam Prior Chemical Physics, Weizmann Institute of Science, Rehovot, Israel

Unlike the case of coupled nanoparticles, surface propagating plasmons in thin metal films give rise to a much longer-range coupling between metallic nanocavities[1]. Localized surface plasmons from a cavity couple to propagating surface plasmons polaritons (SPP), which in turn excite localized plasmons in a neighboring cavity. Here we characterize this coupling by measurements and calculations of electric field distribution near the cavities.

We study circular nanocavities pairs (~200nm) in thin gold films at distances of 40-2000 nm. First, the sample is far-field illuminated (632.8nm, polarized along or perpendicular to the cavities axis) from below, and the electric field above is near-field mapped with spatial resolution of ~100nm by a gold coated NSOM tip (Nanonics MV4000). At large distances (part a, bottom) we observe patterns of interfering propagating SPPs which is modelled very well (a, top) by classical dipoles located at the cavities center. For small gaps (b), the ‘capacitor’ effect between the two dipoles occurs for parallel polariza­­­tion.

Next, we used two tips, one illuminating and scanning above one cavity, while the collecting tip is located up to a few microns away. The propagating field is measured as a function of height above the surface. Fig c depicts a typical rapid decay when the tip is raised above the surface.

Last, we observed the transmission spectrum for a range of cavity separation (d). The spectra were calculated by solving FDTD equations, and measured (not shown here). A discrete set of modes of plasmon propagation between the coup­­­led cavities is clearly visible, and its origin and potential applications will be discussed.

[1] Adi Salomon et al. Plasmonic coupling between metallic nanocavities, Journal of Optics, 16, 114012 (2014)

roy.kaner@weizmann.ac.il









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