Plasmons in metallic nanoclusters exhibit nonharmonic phenomena

Metallic nanoparticles, frequently applied to improve the efficiency or sensitivity of various antenna-reactor systems, utilize the plasmonic effect due to the unusual ability of plasmons to transmit energy at the nanoscale. The absorption and emission of light by plasmons are especially high in metallic nanoscale clusters with ${10}^{7\ensuremath{-}10}$ electrons, but not in small clusters with a lower number of electrons. We demonstrate that the exceptional properties of plasmons at the nanoscale are conditioned by a large anharmonicity of plasmon oscillations induced by strong Lorentz friction, which significantly deviates local charge oscillations from the harmonic regime in this scale of metal spatial confinement. Due to this anharmonicity, plasmons can maintain their oscillations despite strong damping associated with efficient energy transfer, even when such large damping would cause a harmonic oscillator to become overdamped and stop. We also identify a sum-rule-type formula for plasmon polaritons in metallic nanochains arising from the anharmonicity of plasmons.