Comment on 'Theoretical study of single molecule fluorescence in a metallic nanocavity' [Appl. Phys. Lett. 80, 315 (2002)]

摘要

In a recent letter, Enderlein, reported enhanced single molecule fluorescence in a nanometric metal-coated dielectric bead. The enhancement of the fluorescence can be more than one order of magnitude for a thin silver coating layer down to 5 nm. They have used the standard numerical method for the calculations, but the size-dependent dielectric function for the thin silver layer was not included in the calculations. It is well known that the dielectric function of a small metal particle will become size dependent when the size of the metal particle is much smaller than the electron mean-free path of the bulk metal. The electron mean-free path for bulk silver is about 57 nm, which is one order of magnitude more than the thickness of the silver coating layer in Ref. 1. In this case, surface-electron collisions will give a dominating contribution to the dampuig mechanism of the plasmon response of the silver shell rather than electron-electron collisions. The larger damping as a result of more frequent electron-surface collisions diminishes drastically the field enhancement inside the nanocavity of the silver shell. Hence, the results in Ref. 1 will not be reliable.