Quantum beats in two-photon resonance fluorescence

The two-photon resonance fluorescence spectrum of a three-level atom is shown to consist of the low frequency modes in addition to the high frequency ones in the limit of high photon densities. The spectral function for the low frequency modes consists of two lorentzian lines describing: the peak occuring at the renormalized beat frequency Δ+ and that of the zero-photon excitation at the frequency Δ-, where Δ±=Δ-3Ω2/2ωa±Ω2u/2ωa, u2=1+(2Δωa/Ω 2)2. Here, 2Δ is the energy splitting between the two excited states, ωa is the photon energy of the pump field and Ω is the Rabi frequency. The peak at the renormalized beat frequency Δ+ occurs provided that the condition (2Δωa/Ω2)2 > 1 is satisfied. The two-photon laser spectroscopy is expected to be a useful tool for the observation of the low frequency modes in question.