Gain and coherence effects induced by strong cw-laser coupling in potassium–rare-gas mixtures

We describe theoretical and experimental investigations of steady-state gain without population inversion that can occur on the D₁ transition of potassium, upon strong cw-laser excitation of the D₂ transition. A Raman-driven four-level model, in which a noninverted population in the 4²{P}1/2$ level is established via collisional transfer from the 4${}²P3/2 level, is in good qualitative agreement with a variety of experimental results. These results include, among other studies, an examination of the impact of coherence dephasing on the realizable gain, as well as measurements of atomic energy-level populations for a case of line-center pumping that clearly displays gain. The measured relative populations for this case are n(4P1/2)/nₜₒₜₐₗ=0.098±{}0.034 and n(4P3/2)/nₜₒₜₐₗ=0.19±{}0.08, where nₜₒₜₐₗ is the total population. These measurements indicate that the probed level 4P1/2 population is not inverted with respect to the ground state. {} 1996 The American Physical Society.