Theoretical study of electromagnetically induced transparency in a five-level atom and application to Doppler-broadened and Doppler-free Rb atoms

We report theoretical studies of a Lambda- type five- level atomic system. The density matrix equations are set up and solved numerically to obtain the probe absorption line shape of Rb D-2 transitions for cold ( Doppler- free) and room temperature ( Doppler- broadened) atoms. Simulated spectra for Doppler- broadened systems lead to four velocity- selective dips along with an electromagnetic induced transparency ( EIT) peak as observed earlier from the co- propagating pump - probe spectroscopy of Rb D-2 transitions. Effects of pump power and spontaneous decay rate from the upper levels on the simulated spectra are also studied. For cold atoms a very pronounced EIT peak is observed when the pump frequency is on resonance with one allowed transition. We find that lower pump power leads to a much sharper EIT signal in this case. A simulated dispersion curve shows a rapid variation of the refractive index that may lead to a sharp reduction of the group velocity of photons.