Manipulating excited state hyperfine level populations in an atomic laser through electronic spin polarization: controlling upper laser level degeneracy and small Signal Gain.

Anisotropic coherent radiation has been generated from an isotropic medium, in the absence of an external magnetic field, by the spin polarization of an atomic excited state. Lasing on specific hyperfine lines of the 6p2 P32 →6s2 S12 (D2 ) transition of Cs at 852.1 nm has been realized by photoexciting Cs-rare gas thermal collision pairs with a circularly-polarized (σ+ ) optical field. Subsequent dissociation of the transient Cs-rare gas B2 Σ12+ diatomic molecule selectively populates the F = 4, 5 hyperfine levels of the Cs 6p2 P32 state. Not only does electronic spin polarization of the upper laser level yield circularly-polarized coherent emission, but the effective degeneracy (g2 ) of the 6p2 P32 state is altered by the non-statistical hyperfine state population distribution, thereby permitting control of the laser small signal gain with an elliptically-polarized pump optical field. The D2 laser efficiency and output power correlate directly with the molecular orbital structure of the Cs-rare gas B2 Σ+ state in the region of internuclear separation at which the diatomic complex is born.