Es. Fry et al., ATOMIC COHERENCE EFFECTS WITHIN THE SODIUM D1 LINE - LASING WITHOUT INVERSION VIA POPULATION TRAPPING, Physical review letters, 70(21), 1993, pp. 3235-3238
Atomic coherence effects within the sodium D1 line are shown to lead t
o the suppression of optical pumping, to the switching of light on and
off when the coherence effects are turned on and off, and especially
to lasing without inversion.