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Apr21-11, 09:34 AM
P: 16
(im)possibility of a two-level laser

Sorry for the late answer, and thank you for your replies which confirm my guess that things are somewhat more subtle than claimed by the above statement. For example, I asked a Ph.D. student who is engaged in quantum electronics about the diode laser that I proposed as a counterexample, and he agreed that people often regard a diode laser as a two-level laser, although there are rather energy bands than energy levels, so in this case the question about the existence of a two-level laser is rather a question about the acceptance of the notion.
After all, one should say "there is no optically pumped two-level laser".