Double coherent population trapping in a laser-driven atomic medium via spontaneous emission generated coherence

Abstract The coherence phenomenon of atomic population trapping in an excited state of a medium is interesting regarding the basic physics of laser-matter interaction and applications to the generation of pulsed lasers. Here, we propose N numbers of five-level atoms as a gain medium in a weak magnetic field with three excited closely-spaced Zeeman-hyperfine energy levels and two lower energy levels. The excited levels are strongly coupled with a lower level via a coherent-field and to the ground energy level via vacuum field modes. The spontaneous emission spectrum, in general, produces four spectral lines associated with the decaying four dressed states of the atom. Based on parallel dipole moments and radiative decay rates, we find the condition of double coherent population trapping (DCPT) originating from the non-decaying states of the excited dressed states. The decay from the states generates coherence among themselves to eliminate two distinct spectral lines from the spectrum using the trapping condition. In each of the two non-decaying dressed states, 23% of the total atom population traps.