  We study the effect of a control beam on a Lambda electromagnetically induced
transparency (EIT) system in 87Rb. The control beam couples one ground state to
another excited state forming a four level N-system. Phase coherent beams to
drive the N-system are produced using a double injection scheme. We show that
the control beam can be used to Stark shift or split the EIT resonance.
Finally, we show that the 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.

Quantum Physics;Atomic Physics