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Short Wavelength Coherent Radiation: Generation and Applications

Non-Reciprocity of Autoionizing Interferences: Lasers Without Inversion

Stephen E. Harris

Published Jan 1, 1988• 0 citations
Publisher

Abstract

Interferences of autoionizing lines may reduce or eliminate absorption of lower level atoms. Stimulated emission shows no such interferences, thereby allowing laser gain without population inversion.
DOI
10.1364/swcr.1988.soa414

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The absorption and stimulated emission of coherent electromagnetic radiation tuned close to an autoionizing state is investigated with the aim of clarifying some of the aspects behind the proposals of inversionless gain involving such atomic configurations. This is carried out within the framework of individual atomic evolution rather than by the use of density matrix equations, thus providing better insight into the fundamental processes of absorption and emission. In particular, we find that stimulated emission does not overcome absorption in spite of quantum interference of the Fano type being present. It is also shown that gain, in this configuration, although it can take place without population inversion between the involved levels, requires in any case a strong departure from thermal equilibrium conditions. Thereby a strong pumping mechanism, or equivalently a strong excitation power, is needed to reach the lasing threshold. This raises some questions on the relevance of the population inversion indicator of efficient gain implicitly assumed in similar configurations and in many related works.
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In the last decade, laser-induced autoionization has been an active subject of Strong laser-atom interaction. The main interest in the subject arises from many interesting high-field effects in the problem, notably the stabilization of the autoionizing state (AIS) and population trapping, which leads to possible applications in lasers without inversion.1 However, many decay mechanisms may destroy this interference effect to various extents. Many authors have addressed this problem, but few 2,3 have investigated the effects of high-order processes in strong-field autoionization.