Radiative decay of plasmons in a metallic nanoshell

Retarded plasmon eigenmodes in metallic nanoshells are theoretically analyzed, and both plasmon eigenfrequencies and plasmon decay rates are calculated. Spherelike and voidlike plasmon modes are considered and their behavior with geometrical parameters is analyzed. Special attention is given to the problem of radiative decay of different plasmon modes supported by such systems. It is concluded that by varying the shell-layer thickness, the voidlike plasmon decay time can be varied over more than two orders of magnitude throughout the femtosecond range. For shell layers thinner than the characteristic skin depth, the voidlike plasmon modes exhibit subfemtosecond radiative lifetimes and hence they become more radiative than spherelike ones. For shell-layer thickness exceeding the characteristic skin depth, the decay time of the voidlike plasmons becomes of the order of tens of femtoseconds, yielding ultrahigh local-field enhancements. We predict local-field enhancement factors that exceed 60 and 150 in gold and silver nanoshells, respectively. These results are supported by calculations of absorption cross sections of these shells to external light. The results are applied to qualitatively explain strong coupling of plasmons with light in nanocellular metallic films, recently observed in light reflection experiments.