  Cavity electromagnetically induced transparency in a coherently prepared
cavity-atom system is manifested as a narrow transmission peak of a weak probe
laser coupled into the cavity mode. We show that with a resonant pump laser
coupling the cavity-confined four-level atoms from free space, the narrow
transmission peak of the cavity EIT is split into two peaks. The two peaks
represent the dressed intra-cavity dark states and have a frequency separation
approximately equal to the Rabi frequency of the free-space pump laser. We
observed experimentally the dressed intra-cavity dark states in cold Rb atoms
confined in a cavity and the experimental results agree with theoretical
calculations based on a semiclassical analysis.
