Radiation by atoms in resonant cavities

This paper reports that the field in a closed optical cavity with no absorption present can oscillate only at a sequence of discrete normal mode frequencies. When absorption is present, each of these normal mode frequencies is broadened into a band of finite width. An excited atom, placed within such a cavity, radiates only into those modes with which it possesses appreciable coupling. In an approximate sense then, if the frequency with which it radiates is ν0, and its own spectral decay width is Γ, it can only couple to modes possessing appreciable spectral density within the band from ν0 - Γ to ν0 + Γ. (Those must furthermore be modes with non-vanishing amplitudes at the atomic position). The atomic decay width Γ, however, is not in general a fixed or predetermined quantity. Its value depends on the availability of modes into which a photon can be emitted. While the width Γ selects a narrow frequency band of cavity modes for the photon to enter, the number and amplitudes of those selected modes in turn determine the value of Γ. Radiative decay is, in this sense, a truly environmental effect. It depends sensitively on the boundary conditions obeyed by the electromagnetic field at vast distances from the excited atom