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Physical Review Letters

Lasers without inversion: Interference of lifetime-broadened resonances

Stephen E. Harris

Published Feb 27, 1989• 1,276 citations
Publisher

Abstract

We show that if two upper levels of a four-level laser system are purely lifetime broadened, and decay to an identical continuum, then there will be an interference in the absorption profile of lower-level atoms, and that this interference is absent from the stimulated emission profile of the upper-level atoms. Laser amplification may then be obtained without inversion. Examples include interfering autoionizing levels, and tunneling systems.

DOI
10.1103/physrevlett.62.1033

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We show that if two upper levels of a four-level laser system are purely lifetime broadened, and decay to an identical continuum, then there will be an interference in the absorption profile of lower-level atoms, and that this interference is absent from the stimulated emission profile of the upper-level atoms. Laser amplification may then be obtained without inversion. Examples include interfering autoionizing levels, and tunneling systems.
I consider light amplification arising from the competition between absorption and stimulated emission of an excitation, which is induced by a laser field at low intensities interacting with a three-level atom. The lifetime of the induced excitation is 105–106 times longer than those of spontaneously emitted excitations. Conditions are established under which the stimulated emission prevails over the induced absorption, and the net negative intensity is 102–103 times greater than those of spontaneously emitted excitations, indicating that significant amplification occurs near the frequency of the laser field. PACS numbers: 32.80-t, 42.50.-p, 42.50H2
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.
In standard lasers, light amplification requires population inversion between an upper and a lower state to break the reciprocity between absorption and stimulated emission. However, in a medium prepared in a specific superposition state, quantum interference may fully suppress absorption while leaving stimulated emission intact, opening the possibility of lasing without inversion (LWI) [1] . Conventional LWI schemes generally strive to maintain a specific phase relationship between the lower-lying states that carry most of the population. We present a scheme [2] , [3] , that does not follow this tradition. It uses only the natural dynamics of a multi-level quantum system and requires no coherence between the excited and the lower electronic states; effectively, LWI comes "for free".