Excitation spectrum of two-identical three-level atoms at high photon densities

A theory is developed concerning the excitation spectrum arising from the interaction between two-identical three-level atoms (molecules), one of which is excited in the presence of a strong resonant electromagnetic field (pump field). General expressions for the Green's functions are derived in the limit of high photon densities, which describe the excitation spectrum of the symmetric and antisymmetric modes respectively. The theory is applied to the two-atom Hanle-type resonance spectra. Detailed expressions for the spectral functions have been derived which describe the excitation spectrum of the symmetric and antisymmetric modes when the two-atoms are close together as well as when they are far apart. It is found that the expressions for the spectral functions include two main specific processes which lead to the amplification of the signal field: (1) the probability amplitude for the central peak of the laser field becomes negative when the resonance condition ω232 = Ω22 + Ω23 is satisfied, where ω32 = ω3 - ω2 is the energy separation between the two interacting excited states 2 and 3, and Ω2 and Ω3 are the corresponding energy shifts (Rabi frequencies) induced by the laser field. (2) Among the several sidebands, there are two pairs of sidebands which have positive and negative probability amplitudes respectively, and when certain conditions prevail, the sidebands with the negative amplitudes dominate. In this case, it seems that the amplification of the signal field is more pronounced for the antisymmetric rather than for the symmetric modes. The cooperative effects arising from the presence of the second atom are fully discussed.