Single-atom versus coherent pressure-induced extra resonances in four-photon processes

The spectrum of light emitted by a single atom in a thermal bath interacting with three electromagnetic fields is calculated using the dressed-atom picture. The emission spectrum consists of an elastic Raman-like component and dephasing-induced redistribution components that are inelastic in nature. In contrast to coherent four-wave mixing [as given by chi((3))], in which a pressure-induced elastic component (PIER 4) has been predicted and observed, the single-atom response does not contain any elastic pressure-induced components. We derive explicit expressions for the single atom and for the PIER 4 spectra that are not restricted to the impact limit.