Electromagnetically induced transparency and ultra-narrow absorption in a ladder-type three-level atomic system

The absorption feature of a ladder-type three-level atomic system associated with double-color laser fields is investigated by adopting the theory of density matrix equation. It is shown that due to quantum coherence caused by the interaction between coupling field and atom, transparency of probe absorption appears at the position of resonant absorption when the Rabi frequency of the coupling field is comparable to the one of the probe field. The depth and width of transparency window increase with the intensity of the coupling field. The phenomena of ultra-narrow absorption can appear by adjusting the intensity and resonance detuning of the coupling field. It is particularly worth mentioning that if the Rabi frequency of coupling field is smaller than the optical relaxation rate, we can achieve a transformation from electromagnetically induced transparency (EIT) to absorption by changing the resonance detuning of the coupling field from negative to positive. Both EIT and absorption have ultra-narrow linewidth. At the same time, quantum coherence makes a dispersion-like absorption and we can design an optical switch based on this point.