Magnetically assisted optical gain in Zeeman degenerate two-level systems of cold atoms

We report on the observation of magnetically assisted narrow band probe beam amplification in Zeeman degenerate closed two-level system associated with cold cesium atoms. In the experiment the closed cesium D2 hyperfine transition Fg=3→Fe=2 is coherently excited by a probe and an orthogonally polarized coupling beam in the presence of a transverse dc magnetic field. Single pass gain of 80% is observed for moderate values of the magnetic field and coupling beam power. The gain mechanism can be understood in terms of population transfer between the different Zeeman ground-state levels induced by the external magnetic field. We have developed a simple theoretical model using a simpler atomic transition Fg=1→Fe=0, which accounts reasonably for the main experimentally observed results.