Contrasts of electromagnetically-induced transparency in unperturbed and perturbed cold atoms of 87Rb

We experimentally measured the relative contrast of an electromagnetically-induced transparency (EIT) spectrum with two cw lasers in a I >-type system of magneto-optically trapped Rb-87 atoms, which is a perturbed cold atomic medium. The contrast depends nonlinearly on the power of the coupling beam. Experimental results showed good agreement qualitatively with the simulation results considering the trapping magnetic field gradient, which induced detuning of the coupling laser. We used a free expanding cold atomic cloud that had a temperature of (36 +/- 1) A mu K. To observe the EIT spectrum in the non-perturbed medium, we used two pulsed lasers. We observed that the relative contrast in the unperturbed medium was three times higher than that in the perturbed medium under our experimental conditions.