Optical pumping-assisted electromagnetically induced transparency

In this paper we report an observation of the two-photon absorption in a four-level system in hot ⁸⁷Rb vapor based on the proposal of Harris and Yamamoto [Phys. Rev. Lett. 81, 3611 (1998)]. We show that this effect is reduced in hot atoms due to the non-Doppler-free nature of this scheme. Then we report a phenomenon that could be used in the same application of Harris and Yamamoto. The main result is a great enhancement of electromagnetically induced transparency (EIT) effect in hot ⁸⁷Rb vapor caused by optical pumping. We find that when the single photon detuning is near zero the EIT signal is dramatically enhanced by an optical pumping field. More interestingly when the single photon detuning is larger the signal can be changed from a sharp Raman peak to a sharp EIT dip. The full width at half maximum of the peak and dip are narrow and subnatural.