Higher harmonics in collective resonance fluorescence

The Stark representation of a strong external resonant field and the third order of perturbation theory of the fluorescence field are used to show that the probability of emission of the (ω_a + 4Ω) harmonic (ω_a and Ω are the atomic and Rabi frequencies) of collective resonance fluorescence of a pair of two-level atoms is equal to γ²/384, where γ is the width of the spontaneous emission line of a single atom.