Analysis of plasmon resonances on a metal particle

- The null-field method is used to model the optical response of a metal nanoparticle.
- A polar representation of resonances is developed.
- We give a phenomenological description of localized plasmon resonances.
- Plasmon resonance characteristics are computed using an efficient algorithm.
- The cases of silver spheres and spheroids are studied.

An analytical representation of plasmon resonance modes of a metal particle is developed in the basis of the null-field method and its modal expansion of the particle optical response. This representation allows for the characterization of plasmon modes properties, as their spectral position, bandwidth, amplitude and local field enhancement. Moreover, the derivation of a phenomenological equation governing such resonances relates them to open resonator behavior. The resonance bandwidth corresponds to the plasmon life-time, whereas its amplitude is related to the coupling coefficient with the incident excitation. An efficient algorithm is used to compute and characterize the resonance parameters of silver spheroids as function of the particle geometry. The normal modes present on spheres are split into different azimuthal resonant modes in the case of spheroids, with amplitude depending on the incident polarization and position dependent on the particle aspect ratio.