Mesoscopic nanoshells: geometry-dependent plasmon resonances beyond the quasistatic limit.

The plasmon response of a spherical metallic shell becomes significantly more complex as its size is increased beyond the quasistatic limit. With increasing size and decreasing aspect ratio (r1/r2), higher order multipolar modes contribute in a more dominant manner, and two distinct core-shell geometries exist that provide the same dipole plasmon resonance, with differing relative multipolar contributions in their overall spectral response. With further increase in particle size, the geometric tunability of the core-shell structure disappears, and in the infinite radius limit the plasmon response is consistent with that of a thin metallic film.