Narrow absorptive resonances in a four-level atomic system

We study the effect of a control beam on a Λ electromagnetically induced transparency (EIT) system in 87Rb. The control beam couples one ground state to another excited state forming a four-level -system. Phase coherent laser beams to drive the -system are produced using a double injection locking scheme. We show that the control beam can be used to Stark shift or split the EIT resonance. Finally, we show that, when the control beam is on resonance, one observes a Doppler-free and sub-natural absorptive resonance with a width of order 100 kHz. Crucially, this narrow absorptive resonance only occurs when atoms with a range of velocities are present, as is the case in a room-temperature vapour.