Interference spectra in four-level atomic systems

摘要

We have calculated the interference spectra of a single electron in the excited state of a four-level atom interacting with three laser fields, where two of them are on two-photon resonance with the three levels forming a 'Λ' scheme while the third laser operates between the second ground and the second excited states of the atom. It is shown that in the low-intensity limit of all three laser fields, the third laser field destroys the destructive interference that occurs between the spontaneous radiative decay and the process arising from the combined field of all the three laser fields. The result is that the overall intensity of the peak at the centre of the line depends on the strength of the third laser field and it takes negative values indicating that light amplification without population inversion (LAWI) is likely to occur at the one-photon frequency. The antisymmetric two-photon state of the two lower levels as well as the two-photon process, where one laser photon is absorbed and another laser photon is emitted simultaneously by the second excited state of the atom, is found to exhibit LAWI at the one-photon frequency. The derived results are graphically presented and discussed.