Plasmonic edge and breathing modes in aluminum nanotriangles

Abstract

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 optics 1 . 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 structures 1 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 disks 2 and squares 3 . 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.

Probing the Radiative Electromagnetic Local Density of States in Nanostructures with a Scanning Tunneling Microscope

“…Based on these symmetry considerations, two main sets of plasmonic modes for the triangular platelet are identified: edge and breathing modes, in accordance with previous classifications in the literature. [61][62][63][64][65][66][67] The edge modes contribute preferentially to the EM-LDOS above the vertices and edges of the triangle (Figure 4a), and the number of nodes in the induced charge density distribution of these modes increases with decreasing wavelength. [61][62][63] The low-order modes of this type appear at long wavelengths and include the dipolar plasmon resonance (located above 2500 nm), and the quadrupolar plasmon resonance at 1300 nm (excitation in the center of the edge, point 2).…”

“…[61][62][63][64][65][66][67] The edge modes contribute preferentially to the EM-LDOS above the vertices and edges of the triangle (Figure 4a), and the number of nodes in the induced charge density distribution of these modes increases with decreasing wavelength. [61][62][63] The low-order modes of this type appear at long wavelengths and include the dipolar plasmon resonance (located above 2500 nm), and the quadrupolar plasmon resonance at 1300 nm (excitation in the center of the edge, point 2). These edge modes are clearly radiative: consider for example the lowest order edge mode for which the charge density distribution is shown in Figure 4a (λ = 1300 nm); the radiative R-EM-LDOS is more than half the value of the total EM-LDOS (recall that the total EM-LDOS is divided by a factor of 1.5 in Figure 4).…”

“…In this case, the induced charge distribution is concentric on the platelet facets. This is characteristic of breathing or film-like modes [61][62][63][64][65][66] launched on a gold film by a vertical oscillating dipole and discretized by the reflections at the platelet boundaries. By symmetry, these breathing modes have no net induced dipole moment and are in general much less radiative as compared to the edge modes.…”