Abstract
In this experimental work we report our findings about a cascade ($\Xi $) transition ${{5{\rm S}}_{1/2}} \to {{5{\rm P}}_{3/2}} \to {{5{\rm D}}_{3/2}}$ of both $ ^{85,87}{\rm Rb} $ 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 ${{5{\rm D}}_{3/2}}$ level for $ ^{87}{\rm Rb} $ isotope. For $ ^{85}{\rm Rb} $, the resolution of ATS is limited by the relatively closer proximity of ${{5{\rm D}}_{3/2}}$ hyperfine components. The results are also verified through blue fluorescence detection by monitoring the ${{5{\rm D}}_{3/2}} \to {{6{\rm P}}_{3/2}} \to {{5{\rm S}}_{1/2}}$ 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.
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