Two electromagnetically induced transparency windows and an enhanced electromagnetically induced transparency signal in a four-level tripod atomic system.

We present the experimental observations and theoretical calculations of two electromagnetically induced transparency (EIT) windows and an enhanced EIT signal in a four-level tripod system of D1 line of 87Rb atoms. The two EIT dips produced by a strong coupling beam and a weak trigger beam were observed in the absorption spectrum of probe field, when the frequency detuning of the trigger beam was different from that of the coupling beam. When the frequency detuning of the trigger beam was near or equal to that of the coupling beam, the total depth of the EIT dip created by the coupling beam clearly became larger. The experimental measurements were in agreement with theoretical calculations. [ABSTRACT FROM AUTHOR]