Casimir-Polder forces on excited atoms in the strong atom-field coupling regime

Based on macroscopic quantum electrodynamics in linear media, we develop a general theory of the resonant Casimir-Polder force on an excited two-level atom in the presence of arbitrary linear bodies, with special emphasis on the strong-coupling regime where reabsorption of an emitted photon can give rise to (vacuum) Rabi oscillations. We first derive a simple time-independent expression for the force by using a dressed-state approximation. For initially single-quantum excited atom-field systems we then study the dynamics of the force by starting from the Lorentz force and evaluating its average as a function of time. For strong atom-field coupling, we find that the force may undergo damped Rabi oscillations. The damping is due to the decay of both the atomic excitation and the field excitation, and both amplitude and mean value of the oscillations depend on the chosen initial state.