Coherent hole burnings induced without saturation field in a Doppler broaden four-level N-type atomic system

We display the phenomenon of the hole burning which appearing without saturation field under the effect of different parameters. Theoretically the effect of spontaneously generating coherence (SGC) on the probe polarization spectrum in a Doppler broadened four-level N-type atomic system are investigated when the two driven and probe fields are arranged in copropagating and counterpropagating configurations which play an important role in appearing the coherent hole-burning (CHB) phenomenon. The condition of appearing the hole burning in the polarization spectrum at the presence of spontaneously generating coherence (SGC) and the nonradiative decay rates is that the wave number fields are different.