Top row: electrical field amplitude distribution for a 10 [nm] base, 20 [nm] perpendicular right-angled triangular particle, illuminated along the (11) direction. Two different plasmon resonances are investigated, corresponding to two different illumination wavelengths (left: 329nm, right: 458nm). The white arrows give the direction of the electric field at a specific time.

The different field topologies associated with each resonance can be understood in terms of polarization charges distributions, as illustrated in the bottom row. At the plasmon resonances polarization charges (color-coded in red and green) are set in motion around the nanoparticle (the overall particle remains neutral, though). These polarization charges are given by the divergence of the electric field. In the main resonance (wavelength 458nm, right) charges of a given sign build up at the sharp corner, while opposite charges are distributed on the entire circumference of the particle. This distribution oscillates over time, the sign of the accumulated charges on the sharp corner changing every half-period. For the next resonance (wavelength 392nm, left), both charge species accumulate simultaneously at the sharp corner: one specie accumulates at the very tip, while the specie of opposite sign is distributed along the adjacent sides. This dipolar-like charges distribution determines the field at the sharp corner and leads to a much faster decrease of the field intensity in the vicinity of the corner for that wavelength, compared to that of the main resonance at 458nm, which is associated with a point-like charges distribution.

This work has important implications for the utilization of plasmon resonant nanoparticles in the context of light guiding or of chemical enhancement (e.g. surface enhanced Raman scattering).

Images courtesy of Olivier J.F. Martin Nanophotonics and Metrology Laboratory, EPFL, Switzerland.