Observation of a critical concentration in laser-induced transparency and multiphoton excitation and ionization in rubidium

We report the behavior of Autler-Townes splitting and production of a four-wave mixing (FWM) field in rubidium in the context of laser-induced transparency. Gain saturation of the FWM and simultaneous suppression of Autler-Townes splitting above a critical concentration are interpreted in terms of the odd-photon destructive interference effect. The results demonstrate that, when multimode lasers are used, odd-photon destructive interference significantly limits the high-efficiency and high-intensity FWM generation promised by early studies of laser-induced transparency.