Interference effects on the spontaneous emission spectrum from two coupled upper levels

Interference effects between decay processes from two upper levels, which are coupled by the same vacuum modes to a lower level and are driven by a strong coherent field, on the spontaneous emission spectrum have been studied. The interference could lead to population trapping in the upper levels, two dark lines in the spectrum and spectral narrowing. The spectral narrowing depends on the ratio of the two upper levels' decay rates and the Rabi frequency of the driving field. One can change the spectrum by varying the Rabi frequency, an experimentally controllable parameter.