Control of coherence in a ladder type system with double resonance optical pumping and electromagnetically induced transparency

It is shown that a repump laser (named repump after the convention used in the Laser Cooling experiment) can be effectively used to externally control the existing coherence in a ladder type atomic level coupling scheme comprising of 5S_1/2(F = 2) → 5P_3/2(F') → 5D_5/2(F′) states of ⁸⁷Rb atom. The repump laser connecting the F = 1 → F' = 2 levels results in a substantial increment in the Electromagnetically Induced Transparency signal, with simultaneous reduction in Double Resonance Optical Pumping, throughout the entire Doppler width. Also, the optimization of EIT (both line width and strength) could be achieved by adjusting the intensity and the de-tuning of the repump laser, which successfully reduces incoherent pumping and related radiation trapping in the system. Of the two kinds of repump options, namely by connecting (i) the normal hyperfine levels and (ii) the degenerate Zeeman sublevels, the option (ii) has been found to be the more effective one for controlling the EIT in terms of 'Gain' and simultaneous preservation of narrow (often sub-natural) line width.