Two-level atom in an optical parametric oscillator: Spectra of transmittedand fluorescent fields in the weak-driving-field limit - art. no. 033802

We consider the interaction of a two-level atom inside an optical parametri c oscillator. In the weak-driving-field limit, we essentially have an atom- cavity system driven by the occasional pair of correlated photons, or weakl y squeezed light. We find that we may have holes, or dips, in the spectrum of the fluorescent and transmitted light. This occurs even in the strong-co upling limit when we find holes in the vacuum-Rabi doublet. Also, spectra w ith a subnatural linewidth may occur. These effects disappear for larger dr iving fields, unlike the spectral narrowing obtained in resonance fluoresce nce in a squeezed vacuum; here it is important that the squeezing parameter N tends to zero so that the system interacts with only one correlated pair of photons at a time. We show that a previous explanation for spectral nar rowing and spectral holes for incoherent scattering is not applicable in th e present case, and propose an alternative explanation. We attribute these anomalous effects to quantum interference in the two-photon scattering of t he system.