Roles of degenerate Zeeman levels in electromagnetically induced transparency

Since the phenomenon of electromagnetically induced transparency ~EIT !@ 1# was first proposed and observed@2,3#, it has attracted great attention along with its applications and the related effects such as lasing without population inversion ~LWI !@ 4#, coherence population trapping ~CPT !@ 5#, enhancement of nonlinear wave mixing @6,7#, and modification of the refractive index @8,9#. Recently, it gains more popularity to employ laser-cooled atoms in studying these quantum interference phenomena @10‐14,17‐19#. The effect of Doppler broadening is eliminated in cold atoms with temperatures below mK. This allows the coupling and the probe beams to propagate in arbitrary directions and to have various kinds of polarization configurations in the experiments of studying these phenomena. The flexibility of the experimental arrangements and the degree of freedom of the studies are improved. In addition, the motion of cold atoms is very slow. One can have very high density sample of cold atoms for the studies and the collision perturbation is still negligible. This can greatly enhance the quantum interference effects. We therefore choose to use laser-cooled 87 Rb atoms to study the L-type EIT. In this work, an unexpected profile appears in the EIT spectrum which, in our knowledge, has not been demonstrated in the literature. This has prompted the motivation of the study. We vary the polarization configurations of the laser fields and obtain different EIT spectra. The result indicates that the degenerate Zeeman sublevels play important roles in the EIT spectra. In order to clarify the experimental data, we conduct the theoretical calculation that considers all the Zeeman levels. The results of the theoretical calculation are in good agreement with the experimental data. We provide a simple theoretical picture to explain the observed spectra. Our final step is to extend the theoretical calculation to predict spectra of room-temperature samples. Figure 1 illustrates the experimental arrangement. Cold