Electromagnetically Induced Transparency In A Near-Resonance Coupling Field

The phenomenon of electromagnetically induced transparency (EIT) in Rb-85 atomic vapor is studied in a Lambda-type of four-level system with the coupling laser frequency detuned from the atomic resonance frequency. We find that the EIT intensity grows weaker as the coupling laser frequency is detuned from resonance, but the relative depth of the transparency window increases. The maximum depth appears when the coupling frequency is detuned at about +180 MHz, not at the resonance frequency. We demonstrate that this is a result of the combined effect of the two excited states, and present a theoretical analysis based on the four-level system, which agrees quite well with the experimental results.