Effects of spontaneously generated coherence on the resonance fluorescence spectrum of a laser-driven four-level diamond-shape atomic system

The resonance fluorescence spectra of a laser-driven four-level closed-loop diamond-shape atomic system are investigated in this paper. The four decay channels in the system interfere with each other in different ways according to several distinct arrangements of the two intermediate atomic levels, and hence produce the spontaneously generated coherence (SGC) effects. The numerical calculation shows that the amplitude, the linewidth and the number of sidebands of the resonance fluorescence spectrum are deeply modified by the SGC effects; accordingly, interesting effects such as the spectral-line narrowing, spectral-line suppression, spectral-line enhancement and even the complete quenching of fluorescence are generated under certain conditions. Moreover, we propose a new kind of level structure, the seesaw structure, in which the SGC effect can also be attained without the rigorous conditions of degenerate levels. In the end, we discuss the possibility of demonstrating our scheme in real atomic systems and summarize our results.