  Casimir-Polder interactions between an atom and a macroscopic body are
typically regarded as due to the exchange of virtual photons. This is strictly
true only at zero temperature. At finite temperature, real-photon exchange can
provide a significant contribution to the overall dispersion interaction. Here
we describe a new resonant two-photon process between an atom and a planar
interface. We derive a second order effective Hamiltonian to explain how atoms
can couple resonantly to the surface polariton modes of the dielectric medium.
This leads to second-order energy exchanges which we compare with the standard
nonresonant Casimir-Polder energy.
