Stimulated emission of a laser photon by the excited states of a three-level atom

Abstract

We study the stimulated emission spectra arising from the emission of a laser photon by two excited states of a three-level atom interacting with a laser field at low intensities. The lifetimes of the stimulated photons emitted by the two excited states are much longer than those emitted spontaneously, while the intensities of the induced peaks take negative values indicating that amplification occurs at low frequencies. The ratio of the intensities of the light emitted by the excited states |3 > and |2 > of the atom is proportional to (γ3/γ2)1/2, where γ3 and γ2 are the radiative decay rates of the spontaneously emitted photons by the excited states |3 > and |2 > into the ground state |1 > of the atom, respectively. An absorption spectrum is induced into the ground state of the atom by the laser field. The competition between induced absorption and stimulated emission at low frequencies without population inversion is considered in the low-intensity limit of the laser field. It is shown that for values of γ3/γ2 > 1 the relative intensity (height) of the induced peak takes positive values implying that the process of the induced absorption dominates. As the ratio γ3/γ2 increases, the height of the induced peak decreases and vanishes for values of γ3/γ2 400, the height of the induced peak becomes negative indicating that the process of the stimulated emission (amplification) is likely to occur at low frequencies. The computed spectra are graphically presented and discussed.