Optical absorption of nanoparticles described by an electronic local interband transition

It is reported that the optical absorption of a nanoparticle with an electronic structure described by a single local interband transition depends on the strength of the transition and the geometrical shape and size as well as the refractive index of the surrounding environment. A simple and generic formula describing these dependences is derived based on quasistatic theory and the classical Lorentz model. Mie calculations and dipole theory are used to study the optical properties of the described nanoparticles. Interestingly, it is illustrated that by increasing the strength of the transition, the optical absorption peak shifts to the blue. In contrast, by increasing the refractive index of the medium surrounding the nanoparticle, the absorption peak limitedly shifts to the red until it asymptotically approaches the energy of the local interband transition. Similarly, changing the geometrical shape of the nanoparticle from a sphere to a disk leads to a bounded spectral red-shift of the absorption peak. The findings are verified for Ni- and Al-nanoparticles.