The optical properties of metallic particles at nanometric scale have raised a great interest in scientific community due to the high promising technological applications such as optical communication and storage or quantum optics¹. It is well known that optical properties of metallic nanoparticles are dominated by surface plasmons that are collective electron oscillations at a metal-dielectric interface which can be exploited to manipulate light. In a metallic nanoparticle these oscillations are confined by the boundaries of the particles, resulting in discrete modes of oscillations (plasmon modes) which can be tuned by shaping the geometry of the nanoparticle. Recently, a detailed analysis of surface plasmons in flat structures¹ has allowed classifying plasmon modes in two groups. The first group corresponds to the so-called edge modes which are localized at the periphery of the nanoparticle. These edge modes are well known in the literature and have been reported for several geometries, including nanorods and nanotriangles. On the other hand the second group of modes corresponds to the so-called breathing modes (or cavity modes) which are localized on the center of the nanoparticles. Nowadays breathing modes have just been reported and explained for simple structures as disks² and squares³. More complex structures remain to be understood.

In this work we study aluminum nanotriangles (edge length size ranging from 125 to 622 nm) by electron energy loss spectroscopy (EELS) coupled with a transmission scanning electron microscope (STEM) in order to understand the complexity of plasmon modes in this kind of structures. Behind this geometry, a rich variety of edge and breathing modes are observed ranging from 1 to 5 eV (figure 1a). Thanks to the high spatial and energy resolution of STEM-EELS technique, we were able to generate with high level of precision, plasmon maps for all modes (figure 1b). In order to understand the breathing modes in triangular nanoparticles, we propose an analytical model considering the interference of reflected waves at the boundaries of triangular cavities which allowed us to explain the symmetry of lobes shown in maps of figure 1b. Furthermore, plasmon modes dependence with nanotriangles size will be shown and interpreted based on our analytical model and rigorous theoretical simulations.

References:

(1) Schmidt, F.-P.et al. Universal dispersion of surface plasmons in flat nanostructures. Nat. Commun. 5, 3604 (2014).

(2) Schmidt, F.-P.et al. Dark plasmonic breathing modes in silver nanodisks. Nano Lett. 12, 5780–5783 (2012)

(3) Edson P. Bellido. et al. Electron Energy-Loss Spectroscopy of Multipolar Edge and Cavity Modes in Silver Nanosquares. ACS photonics. DOI: 10.1021/acsphotonics.5b00594

Figures:

Figure 1. (a) Deconvoluted EELS spectra acquired at different locations on a single Al nanotriangle as indicated through the color boxes in the inset containing the HAADF images. (b) EELS intensity fitted maps of edge and breathing modes sustained by a single Al nanotriangle. The notation for plasmon modes correspond to integer numbers coming from an analytical model.

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EMC Abstracts - https://emc-proceedings.com/abstract/plasmonic-edge-and-breathing-modes-in-aluminum-nanotriangles/