Plasmonic properties of metallic nanoshells in the quantum limit: from single particle excitations to plasmons

Plasmonic nanoshells show great promise in a wide range of quantum applications due to the tunability of the plasmon frequency across a broad spectral range. With the aim of giving a clear detailed analysis of the onset of plasmon generation and subsequent sustenance of plasmons on nanoshells in the quantum limit, we provide a fully quantum mechanical description of nanoshells using real-time and real-space time-dependent density functional theory. On the basis of the aspect ratio (AR), we identify three types of nanoshells; thin (AR ≥ 0.6), medium (0.35 ≤ AR < 0.6), and thick (AR < 0.35), each showing different types of plasmon modes. Small nanoshells with a thin metallic layer show symmetrically or antisymmetrically coupled plasmons, which agree with semiclassical predictions. However, in nanoshells with medium and thick metallic layers, we identify two types of quantum core plasmons modes, each mixing with symmetrically or antisymmetrically coupled plasmons. For semiclassical modes, all of the electron transitions; monotonic and oscillatory, are near the Fermi level, whereas for quantum core plasmon modes, electrons tend to transition to higher energy levels. We also discuss the dependency of optical properties on aspect ratio, overall size, ground-state electron density profile, spillout, material, and symmetry of nanoshells.