Exploring hyperfine levels of non-Rydberg excited states in a Ξ system using Autler-Townes splitting

In this experimental work we report our findings about a cascade (Ξ) transition 5S_1/2→5P_3/2→5D_3/2 of both ^85,87Rb atoms under different laser detuning combinations. The relative power levels of two individual lasers are adjusted under a counter-propagating configuration so that the system exhibits Autler-Townes splitting (ATS). However, the ATS, which is otherwise difficult to detect in a room-temperature alkali vapor cell offering large Doppler background, is well resolved here by using a combination of modulation transfer and phase-sensitive detection techniques. The results show that the AT components clearly indicate the hyperfine structure of 5D_3/2 level for ⁸⁷Rb isotope. For ⁸⁵Rb, the resolution of ATS is limited by the relatively closer proximity of 5D_3/2 hyperfine components. The results are also verified through blue fluorescence detection by monitoring the 5D_3/2→6P_3/2→5S_1/2 non-degenerate decay arm. The technique is easy to implement and is able to reveal the hyperfine structure of the excited levels. However, the technique is not a good choice when an excited level with dense hyperfine structure is targeted.