Investigation of the EIT and double resonance optical pumping effects in ladder-type transitions of ⁸⁷ Rb atoms

We present electromagnetically induced transparency (EIT) and double resonance optical pumping (DROP) spectra for the 5 S 1/2 →5 P 3/2 →5 D 5/2 transitions of 87 Rb atoms. Two-photon excitation to the 5 D 5/2 state is confirmed by detecting 420 nm fluorescence from the decay channel 6 P 3/2 →5 S 1/2 . We observe a dual structure in the transmission profile for the closed transition ( F =2→ F ′ =3→ F ′ ′ =4), which we attribute to competing DROP and EIT processes. A quantitative parameter K is introduced to characterize the DROP influence on EIT spectra. By adding a re-pumper field locked to the |5 S 1/2 , F =1⟩→|5 P 3/2 , F ′ =2⟩ transition, we demonstrate clear EIT signal with suppressed DROP effects. Two-photon absorption is also observed in both probe transmission and blue fluorescence spectra. The steady state solution of the density matrix equations derived from our five-level analytical model is in good agreement with the experimental observations. Moreover, lock-in detection technique is used in the weak probe regime to achieve a high signal-to-noise ratio. These findings provide a deeper understanding of the mechanisms governing EIT and DROP in multi-level atomic systems and offer practical strategies for achieving DROP-free EIT signals, which are essential for cascade EIT-based applications like Rydberg spectroscopy, quantum information, and quantum memory.