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Physical Review A

Balance equations and threshold conditions for the inversionless laser in an autoionizing system

Jacob B. Khurgin, E. Rosencher

Published Sep 1, 1996• 4 citations
Publisher

Abstract

A rigorous density-matrix approach has been applied to the interaction between an autoionizing system and electromagnetic radiation. The balance equations have been derived and analytical expressions for the inversionless lasing threshold and saturation output have been obtained. Using the intersubband transitions as an example, the potential performance of the inversionless laser has been analyzed and found to be inferior to that of the conventional laser. \textcopyright{} 1996 The American Physical Society.

DOI
10.1103/physreva.54.2451

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Physical Review A•Apr 1, 1994•2 citations
A rigorous density-matrix approach has been applied to the interaction between an autoionizing system and electromagnetic radiation. The balance equations have been derived and analytical expressions for the inversionless lasing threshold and saturation output have been obtained. Using the intersubband transitions as an example, the potential performance of the inversionless laser has been analyzed and found to be inferior to that of the conventional laser. \textcopyright{} 1996 The American Physical Society.
A rigorous density-matrix-based theory of lasing based on optically induced or autoionization-induced coherence between the levels is developed. The balance equations are obtained, and the conditions for lasing threshold are derived as a function of pumping strength and relaxation rates. Connection between the lasing without inversion and Raman and parametric processes is established. The main conclusion is that true CW lasing without inversion can indeed be obtained but only in a system where pumping and relaxation rates are favorable for attaining conventional lasing with population inversion. Practical implications of it, especially for the intersubband lasers, are discussed.
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.
We analyze a laser system consisting of N closed three-level atoms confined in a single-mode cavity and derive the conditions for the onset of lasing without inversion or with inversion. We calculate the steady-state gain and lasing in the system and show that far above the lasing threshold, depending on the pumping and decay rates, the inversionless laser may operate in three different regimes, where the intensity of the laser depends linearly on N, is proportional to \ensuremath{\surd}N , or independent of N.
We analyze intensity and statistical properties of lasers without population inversion in a closed three-level V-type system. We derive the threshold condition and examine the intensity dependence on various system parameters. Unlike other inversionless laser systems that generate amplitude-squeezed light, the intensity fluctuation of the inversionless laser from the three-level V-type system is above the shot-noise limit in a wide parameter range studied here. \textcopyright{} 1996 The American Physical Society.
In this paper, a quantum theory of an inversionless laser is presented. The gain medium is considered to consist of three-level atoms in a closed \ensuremath{\Lambda} configuration, with a lower-level microwave coupling. By deriving an equation of motion for the reduced density matrix for the two modes of the field, gain and diffusion are studied. In particular, two mechanisms through which inversionless amplification takes place in the context of this model, namely, the suppression of absorption and mode coupling, are discussed. Stable conditions for inversionless amplification and the attendant phase diffusion are discussed and a comparison is made with conventional, inversion-based lasers. It is shown that lasing without inversion does not necessarily result in a reduced linewidth compared to a conventional laser.