A study on the ultranarrow spectra in a driven quasi-Λ four-level system

We present a new Quasi-Lambda-model-four-level system based on a general three-level A-model and investigate the resonant fluorescence spectroscopy in detail. There are three ultra-narrow spectral lines in the spontaneous emission spectra from upper level to lower-lying levels of the two emission channels, which occur over a wide range of the parameters. The appearance of the three ultra-narrow spectral lines is closely related with Rabi frequency of the two coherence driving fields; and the larger the Rabi frequency, the narrower the lines. The effect of line narrowing does n't occur when there is only a coherence driving field. While the incoherent transition and the collisional decay between the energy levels destroy the feature of the spectral narrowing which may be considered to be the result of the multi-channel quantum interference.