Angular momentum and two-photon detuning dependence of reflection spectrum on degenerate two-level systems in Cs vapour

The absorption and reflection spectrum of probe light, interacting with a degenerate two-level system of any transition in the Cs D1 line driven by standing-wave, is experimentally studied. The effect that absorption changes from a transparency dip to an absorption peak when the travelling coupling field is replaced by the standing-wave field can only occur in the hyperfine transition F_g↔F_e ≤ F_g (F_g and F_e are the total angular momentum of the ground and the excited levels respectively), and in this case, a reflection of probe can be generated. It is also examined that the absorption coefficient and reflection efficiency in the transition of F_g > F_e are much higher than that in F_g = F_e. For the transition of F_g < F _e, where coherent population trapping cannot be established, the absorption does not change the sign, and there is no reflection to be observed when the coupling is switched from a travelling to a standing-wave field. As a consequence, only the degenerate two-level systems, where the population is trapped in the ground states, can exhibit steep absorption and reflection of the probe light with a standing-wave coupling. Furthermore, the reflection spectra with an asymmetric profile as the function of one- and two-photon detunings are obtained, which is theoretically explained by the phase mismatch compensation during the process of four-wave mixing in an atomic system. This shows that under the condition of coherent population trapping the nonlinear reflection can be improved with one- and two-photon detunings in a degenerate two-level system.