Transition from Autler–Townes splitting to electromagnetically induced transparency based on the dynamics of decaying dressed states

The threshold for the transition between Autler-Townes splitting (ATS) and electromagnetically induced transparency (EIT) is studied through examining the dynamics of decaying dressed states, which are derived from the effective Hamiltonian. It is found that the threshold corresponds to the suppression of the Rabi oscillation of the populations by the relaxation as the coupling field becomes weak. Moreover, ATS and EIT belong to two different regimes, the former being in the non-perturbation regime, where there is coherent Rabi oscillation of the populations of the states coupled by the coupling field. By contrast, EIT is in the perturbation regime, and the transparency window in the EIT resonance can be explained as being due to the gain of the four-wave mixing process. Experiments are performed in cold rubidium atoms, where both the absorption and dispersion are measured, showing that EIT can be discriminated from ATS through Fourier transformation of the spectra. Compared to the statistical method proposed by Anisimov et al (2011 Phys. Rev. Lett. 107 163604), our method is more direct and is deterministic.