The aspect ratio of a geometric shape refers to the ratio between its sizes in different dimensions. By changing the length and width, not only the energy but also the character of the resonance can be tuned. The resonance energies are strongly dependent on the shape of the nanoparticles (NPs). In particular, nanorods with high aspect ratios show a continuously decreasing resonance energy with increasing length.
An article which relates to aspect ratios of nanoparticles and its effect on the surface plasmon resonance:
X. Lopez-Lozana, H. Barron, C. Mottet and H. Weissker (2014). Aspect-ratio- and size-dependent emergence of the surface-plasmon resonance in gold nanorods – an ab initio TDDFT study, Physical Chemistry Chemical Physics, (16) 1820-1823, DOI: 10.1039/C3CP53702A.
Another interesting aspect to keep in mind is related to the anisotropicity of the plasmonic modes: the presence of an AR bigger than 1 implies a certain degree of anisotropicity in the nanoparticles, which is reflected on its plasmons. Therefore, with the right orientation of the particles and the right polarization of the light source is possible to selectively excite one desired plasmon.