In this paper the master equation is derived for the atomic reduced density operator, by means of projection operator technique, starting from the Liouville equation. The nondegenerate two-photon fluorescence spectrum for an atom with two-levels in a squeezed vacuum with broad band has been studied by means of the above equation. We find that there is a new type of oscillation phenomenon, in which there are both super and lower thresholds. Between the two thresholds the atom will exhibit oscillation and the two-photon fluorescence spectrum exhibits single peak. Outside the region between the two thresholds the oscillation will not happen and the two-photon fluorescence spectrum possesses novel negative peak or peaks. The shapes of the spectra depend strongly on the natural two-photon linewidth. However, the spectra are not phase-sensitive, which is unexpected.
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