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Physical Review Letters

Experimental demonstration of light amplification without population inversion

Wytze E. van der Veer, R. J. J. van Diest, A. Dönszelmann, H. B. van Linden van den Heuvell

Published May 24, 1993• 160 citations
Publisher

Abstract

We report the observation of light amplification without population inversion. A linear superposition of an m=1 and an m=-1 magnetic substate is populated coherently. Absorption from this superposition to a higher state is canceled by interference of channels, a so-called dark resonance. An inverse transition is used for amplification of light. The temporal behavior of the amplified pulse and the gain are studied as a function of the energy splitting due to a magnetic field. In a vapor of cadmium-112 gain factors of up to 4.3 are observed.

DOI
10.1103/physrevlett.70.3243

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We report the observation of light amplification without population inversion. A linear superposition of an m=1 and an m=-1 magnetic substate is populated coherently. Absorption from this superposition to a higher state is canceled by interference of channels, a so-called dark resonance. An inverse transition is used for amplification of light. The temporal behavior of the amplified pulse and the gain are studied as a function of the energy splitting due to a magnetic field. In a vapor of cadmium-112 gain factors of up to 4.3 are observed.
We report a lasing without inversion (LWI) experiment on the basis of coherent population trapping, realized in low-pressure cadmium vapor. A single-pass inversionless amplification by more than a factor of 10 has been measured, a significant improvement over previous experiments. In addition, we report synchronously pumped laser oscillation without inversion. Using a simple model to fit the gain/absorption curves as a function of magnetic field, we have obtained values for the populations in the system, showing that there is no inversion in the atomic basis. Combined with a simple optical method to measure absolute densities of a gas, this model results in a completely optical method to determine the absolute populations and density in the system.
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.
Light-induced absorption with population inversion and amplification without population inversion in a coherently prepared closed three level V-type system are investigated. This study is performed from the point of view of a two color dressed-state basis. Both of these processes are possible due to atomic coherence and quantum interference contrary to simple intuitive predictions. Merely on a physical basis, one would expect a complementary process to the amplification without inversion. We believe that absorption in the presence of population inversion found in the dressed-state picture utilized in this study, constitutes such a process. We derive approximate analytic time-dependent solutions, for coherences and populations that are compared with full numerical solutions exhibiting good agreement. Steady-state quantities are also calculated, and the conditions under which the system exhibits absorption and gain with and without inversion, in the dressed-state representation are derived. It is found that for weak input probe-laser field absorption with inversion and amplification without inversion may occur, for a range of system parameters. These take place at resonance and the generalized Rabi ac Stark-shifted frequencies, in agreement with exprimental investigations, and at beat frequencies, depending on the relevant parameters.
We show that in a four-level atomic system with electromagnetically induced transparency, a coherent pump field can induce light amplification without or with population inversion. As the pump field intensity increases, the atomic system evolves from normal dispersion and amplification without population inversion into anomalous dispersion and amplification with population inversion. We report an experiment on light amplification in cold rubidium atoms and present experimental measurements that agree with theoretical calculations based on the four-level model system.
We show that in a four-level atomic system with electromagnetically induced transparency, a coherent pump field can induce light amplification without or with population inversion. As the pump field intensity increases, the atomic system evolves from normal dispersion and amplification without population inversion into anomalous dispersion and amplification with population inversion. We report an experiment on light amplification in cold rubidium atoms and present experimental measurements that agree with theoretical calculations based on the four-level model system.