Laser oscillation without population inversion in a sodium atomic beam.
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Continuous wave (cw) amplification and laser oscillation without population inversion have been observed for the first time in a scheme within the sodium D1 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. 1996 The American Physical Society.