Ultranarrow resonance due to coherent population oscillations in a Λ-type atomic system

It is well known that ultranarrow electromagnetically induced transparency (EIT) resonances can be observed in atomic gases at room temperature. We report here the experimental observation of another type of ultranarrow resonance, as narrow as the EIT ones, in a Λ system selected by light polarization in metastable ⁴He at room temperature. It is shown to be due to coherent population oscillations in an open two-level system (TLS). For perpendicular linearly polarized coupling and probe beams, this system can be considered as two coupled open TLSs, in which the ground-state populations exhibit antiphase oscillations. We also predict theoretically that in the case of two parallel polarizations, the system would behave like a closed TLS, and the narrow resonance associated with these oscillations would disappear.