Atomic dynamics in microcavities: absorption spectra by Green function method

The method of thermodynamic Green functions is applied to the determination of the susceptibilities and absorption spectra of atoms in microcavities. The method is introduced by its application to an atom in free Space, where the well known expression for the spectrum is very simply rederived. The c alculations remain simple for more complicated systems, and results are obt ained for atoms in single-mode cavities with lossy mirrors and with the add itional damping by emission into free-space modes that occurs in open-sided cavities. The expressions for some limiting cases of the general theory ar e compared with the results of previous work and contact is made with the d ressed-state theory. Special consideration is given to an atom placed in a planar microcavity; it is shown that non-lorentzian spectra may in principl e occur but that the parameter values of practical cavities produce Lorentz ian spectra with linewidths given by Fermi's golden rule.