Pumping the Cs 852.1nm laser by photoassociating Cs-rare gas atomic pairs allows one to observe a four-level system morph into a two-level system. This occurs when the state 3-state 2 energy defect falls below ~2kT/3.
© 2011 OSA
J. D. Hewitt, J. D. Readle, and J. G. Eden, "Observing the Continuous Transformation of a Four Level Laser into a Two Level System," in CLEO:2011 - Laser Applications to Photonic Applications, OSA Technical Digest (CD) (Optical Society of America, 2011), paper CMH3.
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