Ultranarrow CPO resonance in a \Lambda-type atomic system

TL;DR

This paper reports the experimental observation of ultranarrow resonances in a b1-system in metastable helium, caused by coherent population oscillations, expanding understanding of narrow resonance phenomena in atomic gases.

Contribution

It demonstrates a new type of ultranarrow resonance in a b1-system due to coherent population oscillations, with experimental and theoretical analysis.

Findings

Ultranarrow resonances observed in metastable helium at room temperature.

Resonances caused by coherent population oscillations in an open two-level system.

Predicted disappearance of the resonance with parallel polarizations.

Abstract

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 resonances, as narrow as the EIT ones, in a \Lambda-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 anti-phase oscillations. We also predict theoretically that in case of two parallel polarizations, the system would behave like a closed TLS, and the narrow resonance associated with these oscillations would disappear.