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Physics Today

Researchers Report Evidence for Lasing Without Inversion

Barbara Goss Levi

Published Sep 1, 1995• 2 citations
Publisher

Abstract

We have come to think that lasing requires an inversion of population between atoms in an excited state and those in a lower state: Normally the probability of absorption from the lower state equals the probability of emission from the higher state, so that the only way to have more photons coming out than going in is to start with more atoms in the upper state. It would be useful to decouple the probabilities of emission and absorption, so that one could lower the loss associated with absorption without reducing the emission.

DOI
10.1063/1.2808154

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We have come to think that lasing requires an inversion of population between atoms in an excited state and those in a lower state: Normally the probability of absorption from the lower state equals the probability of emission from the higher state, so that the only way to have more photons coming out than going in is to start with more atoms in the upper state. It would be useful to decouple the probabilities of emission and absorption, so that one could lower the loss associated with absorption without reducing the emission.
[6]. This state is called trapped state or nonabsorbingstate. Therefore, if one managed to prepare all atoms at lower levels in such trapped state any small amount ofatoms at upper level should be sufficient for amplification since there should be transitions from the upper state tothe state which is orthogonal to the trapped one. Apparently, such inversionless amplification implies some hiddeninversion, namely, with respect to the empty untrapped state. In general, population inversion is required between
When an atomic system interacts with light, it can amplify the electromagnetic radiation when gain exceeds loss. This requires that transitions from the upper level to the lower level prevail over the transitions from the lower level to the upper level. If one deals with a two-level system, the only way to obtain amplification is to create a state of population inversion, i.e. the upper level must be more densely populated that the lower level. "Lasing without inversion" may be possible when part of the atoms are trapped, which removes them from the subspace of states with which the upper level can have transitions. This chapter determines to what extent population inversion is necessary to achieve amplification. Independently, amplification without inversion and lasing without inversion were predicted by Harris using a different type of atomic interference. The amplification threshold has a number of features that are reminiscent of the laser threshold.
This review paper is devoted to amplification and lasing without population inversion involving atomic transitions in gas media. We start by discussing the main motivation in inversionless lasing research, namely, the generation of short-wavelength laser light. Then, we review the basic physics of inversionless lasing in two-level and, eventually, in three- and multilevel atomic configurations. Finally, we summarize the current state of the art of LWI experiments and indicate the main difficulties with respect to short-wavelength laser generation.