Variation of probe absorption with intensity of radiation fields in three energy-level system

Abstract

The variation of probe absorption with the intensity of coupling and probe field in a cascade three energy-level atomic systems is observed based on the density matrix equation. It shows that in the case of weak probe field, probe absorption presents electromagnetically induced transparency (EIT) near the resonant frequency. The depth of transparent window increases with the increase of the intensity of the coupling field until complete transparent. After that, the width of EIT window increases and changes to Aulter-Townes double peak finally. While with the increase of the probe intensity, the contrast of EIT declines rapidly, and EIT will disappear when Rabi frequency of the probe field exceeds the decay rate of the transverse relaxation. If the coupling field is off-resonance, the double peak of probe absorption is no longer symmetric to the resonant point and the intensity of two peaks is no equal also. With the increase of the probe field, the intensity of two peaks converts each other, and at last forms one wide peak by overlapping together. At room temperature, EIT phenomenon can still be observed if adopting Doppler-free configuration and weak probe field. But if the probe field is strong enough, besides the absorption curve is broadened, a small peak which comes from Doppler effects will appear at the bottom of the EIT window also. Thus if we engage in the research work related to EIT, we ought to adopt Doppler-free configuration and weak probe field.