Effect of Doppler broadening on the vacuum induced coherence-dependent probe field absorption in Y-type four-level system

Using the numerical result, the influence of the Doppler broadening on the vacuum induced coherence (VIC) related probe field absorption is discussed. It is shown that if there exists no Doppler broadening, only when VIC is absent can the electromagnetically induced transparency (EIT) phenomenon occur; VIC leads the probe field absorption to vary obviously and the gain to appear; in both cases with and without VIC, the absorption curve has a double-peak structure which is symmetrical about the probe detuning. If there exists the Doppler broadening, in both cases with and without VIC, EIT phenomenon can always occur; VIC still leads the probe field absorption to vary obviously and the gain to appear; no matter whether the VIC is present. The probe field absorption has the following characteristics: the absorption curve no longer has symmetry about the probe field detuning and changes gradually from the double-peak structure to the single-peak structure with Doppler broadening width (D) increasing; the probe field absorption does not monotonically increase or decrease with D increasing; the probe field absorption, when the probe and driving fields propagate along the opposite directions, is smaller than that when the probe and driving fields propagate along the same directions.