Velocity selective multiple two-photon dark and bright resonances in Potassium vapor

Abstract We report the observation of two additional sub-natural line width quantum interferences in the $D_2$ manifold of $^{39}K$ vapor, in addition to the usual single Electromagnetically Induced Transparency (EIT) peak. In a typical three level $\Lambda$-type system, only one EIT peak is observed. However, here we report observation of two additional line shapes riding on top of the absorption profile. The fact that the hyperfine splitting is smaller than the Doppler width in $^{39}K$ allows the probe and control beams to swap their transition pathways in different velocity groups of atoms even when their frequencies are kept constant. Our observations are in striking contrast to standard EIT measurements. These findings are in quantitative agreement with density matrix formalism taking into account velocity-selective two-photon resonances. Owing to the favorably low ground hyperfine splitting ($\Delta_{hf}$) in $^{39}K$, which allows a significantly large number of atoms with a Doppler shift greater than or equal to the $\Delta_{hf}$, the strength of these additional resonances is strong compared to that of other alkali atoms such as $^{87}Rb$, $^{133}Cs$ where these resonances can not be observed. The control photon detuning to atomic transition captures the nature of the coherence; therefore an unusual phenomenon of conversion from perfect transparency to enhanced absorption of the probe photon is observed and explained by utilizing the adiabatic elimination of the excited state in the Master equation. Controlling such dark and bright resonances leads to new applications in quantum technologies such as frequency-offset laser stabilization and long-lived quantum memory.