For a four-level atomic system with a doublet of closely spaced levels, we find that, owing to the coherence that results from the vacuum of the radiation field, population trapping at excited levels and probe gain with population inversion can be achieved with weak incoherent pumping. This gain is different from both the conventional lasing gain and gain without inversion in that there exists population inversion on probe transitions but the inversion is achieved by the vacuum-induced coherence.
© 2003 Optical Society of America
Jin-Hui Wu, Hui-Fang Zhang, and Jin-Yue Gao, "Probe gain with population inversion in a four-level atomic system with vacuum-induced coherence," Opt. Lett. 28, 654-656 (2003)