Cooperative and dipole-dipole effects in the fluorescent spectra of two identical three-level atoms at high photon densities

We have considered the stimulated one-photon spectra for a system consisting of two identical three-level atoms interacting simultaneously with a strong pump field and a weak signal field. The atoms consist of an upper excited state and two lower states and they interact through their dipole-dipole interaction and radiate to each other as well. The strong laser field depletes one of the lower ground states (metastable state) by bringing the electrons into the excited state from where the electrons decay through the signal field into the lower ground states of the system. Using the Green function formalism in the limit of high photon densities of the laser field, the spectral function for the signal field which describes the stimulated one-photon process of the symmetric and antisymmetric modes arising from the dipole-dipole and cooperative radiative interactions between the two atoms, has been calculated and compared with that of the single atom. In the absence of the dipole-dipole interaction between the atoms, the spectra consist of the main peak at the frequency of the signal field and two pairs of sidebands, which are symmetrically located from the position of the main peak. In this case, the difference between the single and two-atom spectra is caused by the cooperative radiative interaction between the atoms and amounts: i) to increase the lifetime of the main peak by a factor of two from that of the single atom; ii) to split the one pair of sidebands of the single atom spectrum into two pairs of sidebands; iii) to cause the intensity of the two pairs of sidebands to vary as a function of the Rabi frequency of the laser field; and iv) to induce strong asymmetries in the lineshapes of the sidebands. In the presence of the dipole-dipole interaction, the spectra consist of a main peak at the frequency of the signal field shifted by the magnitude of the dipole-dipole interaction and six sidebands whose positions and intensities are governed by the strength of the dipole-dipole interactions between the atoms. The lineshapes of all peaks are described by asymmetric Lorentzian lines and the extent of the asymmetry depends on the strength of the dipole-dipole interaction. The relative intensity of the main peak and those of the sidebands are either positive or negative depending on the strength of the dipole-dipole interaction. The amplification of the sidebands is found to be more pronounced in the two-atom spectra than in that of the single atom. The computed spectra are presented graphically for different values of the Rabi frequencies, dipole-dipole interactions and for different detunings.