Unexpected Doppler-free resonance in generalized double dark states
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Doppler-free resonance was observed in rubidium atomic vapor coherently driven by two strong coupling fields in an intrinsically non-Doppler free geometry. Different configurations of drive and probe fields of the inhomogeneously broadened rubidium vapor were studied. Up to 35% enhancement of the probe absorption due to quantum interference was observed. The changes in absorption spectra in the co- and counter-propogating geometries were explained by the four level atom model.