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Physical Review Letters

Degenerate quantum-beat laser: Lasing without inversion and inversion without lasing

Marlan O. Scully, Shi-Yao Zhu, Athanasios Gavrielides

Published Jun 12, 1989• 960 citations
Publisher

Abstract

A single lasing mode driven by a three-level ``quantum-beat'' atomic configuration can show gain without population inversion or optical absorption into an excited state without spontaneous or stimulated emission.
DOI
10.1103/physrevlett.62.2813

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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".
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