Control of spontaneous emission spectra and simulation of multiple spontaneously generated coherence in a four-level atomic system

We investigate the fluorescence spectrum for a four-level atomic system, three levels of which are coupled with two laser fields in a ladder configuration, and the upper-most level decays to the fourth level via spontaneous emission. The results show that a few interesting phenomena such as spectral-line narrowing, spectral-line enhancement, spectral-line suppression, and spontaneous emission quenching can be realized in our system and can be manipulated via adjusting the proper parameters. It is found that, in the dressed-state picture, this system is equivalent to the system with multiple spontaneously generated coherence studied by Joshi et al. [A. Joshi, W. Yang, M. Xiao, Phys. Lett. A 325 (2004) 30].