Laser Oscillation without Population Inversion in a Sodium Atomic Beam

Continuous wave (cw) amplification and laser oscillation without population inversion have been observed for the first time in a $\Lambda$ scheme within the sodium $D_1$ line. This is also the first demonstration in which the lasing medium was an atomic beam; this is an approach which, in addition to elucidating the physics, lays a foundation for extensions into the ultraviolet. Calculations using realistic atomic structure were critical to the choice of experimental approach. Observations agree with full density-matrix calculations and clearly show there was no population inversion.