Lasing Without Inversion
Usually, the gain medium of a laser works on the basis of a population inversion. (Essentially, that means (for a simple system) that the upper laser level has a higher population than the lower laser level, so that stimulated emission can provide a positive net gain.) In the 1990s, however, it was shown that optical amplification and consequently lasing without inversion are possible by using an additional optical or microwave field which induces a quantum coherence in the atoms of the gain medium. The basic idea is to provide two different pathways for atoms to get from the ground state to the excited state – a direct one and another one via a third energy level –, and to induce a quantum coherence, so that the quantum-mechanical probability amplitudes for both processes cancel. In effect, that suppresses the reabsorption and thus makes it possible to obtain a positive net gain even with a small population in the upper state.
In principle, lasing without inversion could help, e.g., in realizing lasers operating on very short wavelengths, where a population inversion is difficult to achieve. However, to date it is unclear whether this theoretical potential can really be turned to practical advantages. So far, lasing without inversion has to be considered as a definitely interesting quantum-mechanical effect, but not as a practically important technique.
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|||S. E. Harris, “Lasers with inversion: Interference of lifetime-broadened resonances”, Phys. Rev. Lett. 62 (9), 1033 (1989), doi:10.1103/PhysRevLett.62.1033|
|||M. O. Scully and M. Fleischhauer, “Lasers without inversion”, Science 263, 337 (1994), doi:10.1126/science.263.5145.337|