Resonance fluorescence of two-level atoms under strong bichromatic excitation

We have measured the emission spectrum of two-level-like Ba atoms driven by two strong, equal-amplitude fields with frequency separation 2δ. The spectrum consists of a series of peaks with an essentially constant spacing δ and alternating linewidths. These features differ qualitatively from the characteristic triplet spectrum observed in the case of strong monochromatic excitation. Certain features of the observed spectrum such as its comblike structure can be motivated in terms of the energy spectrum of atom–bichromatic-field product states. Other features, such as the alternating linewidths, require more subtle analysis.