Extinction and Scattering Properties of High-Order Surface Plasmon Modes in Silver Nanoparticles Probed by Combined Spatially Resolved Electron Energy Loss Spectroscopy and Cathodoluminescence

Abstract

We investigated the optical properties of localized surface plasmons with different orders in individual silver nanotriangles with different sizes by electron energy loss spectroscopy (EELS) and cathodoluminescence (CL) in the same scanning transmission electron microscope. EELS and CL spectral imaging within the same nanotriangles give information about the extinction and scattering from the nanostructures. As measured from both techniques, the first two order modes showed similar spatial distributions. However, the appearances of slightly different resonant energies were confirmed in both lower and higher order plasmon modes. For the first two lower order modes, the resonant energy in CL was blue-shifted compared to that in EELS, which can be understood in a simple damped oscillator picture. This confirms the experimental and theoretical observations recently made on dipolar plasmons in gold. It also extends it to the second-order modes. The next higher order modes exhibit an apparent inverted shift, which could not fit in such a picture. On the basis of boundary element methods numerical simulations, we interpret this apparent inversion of shift as a combined effect of (1) the spectral mixing of higher order modes and (2) the change in the relative weights of higher order modes between both spectroscopy techniques, i.e., the much faster drop in higher order mode intensity in CL compared to EELS. Both effects are enhanced by the large damping in these imperfect, lithographed structures.