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Journal of Experimental and Theoretical Physics

Interference phenomena in Doppler broadened quantum transitions: Amplification of intense radiation without population inversion

A. K. Popov, V. M. Kuchin, S. A. Myslivets

Published Feb 1, 1998• 5 citations
Publisher

Abstract

We study the effect of nonlinear interference processes on quantum transitions in intense resonant electromagnetic fields with allowance for changes in level populations, relaxation processes, incoherent excitation, and Doppler broadening of the transitions, on the absorption, amplification, and refraction of the interacting fields. The theory is generalized to the case of nonlinear interference interaction of two intense fields in open and closed three-level quantum systems. Using the density matrix, we derive general expressions in the case of stationary interaction that make it possible to analyze the optical characteristics for various configurations of the interfering transitions by a simple substitution of parameters. The possibility of amplifying light without saturated population inversion in a resonant transition is discussed. We formulate the conditions for such amplification and use examples to show that under appropriate changes in the initial level populations and the intensity of the auxiliary light, the inversionless amplification coefficient does not decrease with increasing intensity of the amplified radiation. We also show that allowance for these accompanying processes greatly affects the choice of optimal conditions for interference in optical transitions. As an illustration we list the results of a numerical analysis of possible experiments.

DOI
10.1134/1.558450

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We study the effect of nonlinear interference processes on quantum transitions in intense resonant electromagnetic fields with allowance for changes in level populations, relaxation processes, incoherent excitation, and Doppler broadening of the transitions, on the absorption, amplification, and refraction of the interacting fields. The theory is generalized to the case of nonlinear interference interaction of two intense fields in open and closed three-level quantum systems. Using the density matrix, we derive general expressions in the case of stationary interaction that make it possible to analyze the optical characteristics for various configurations of the interfering transitions by a simple substitution of parameters. The possibility of amplifying light without saturated population inversion in a resonant transition is discussed. We formulate the conditions for such amplification and use examples to show that under appropriate changes in the initial level populations and the intensity of the auxiliary light, the inversionless amplification coefficient does not decrease with increasing intensity of the amplified radiation. We also show that allowance for these accompanying processes greatly affects the choice of optimal conditions for interference in optical transitions. As an illustration we list the results of a numerical analysis of possible experiments.
An influence of nonlinear interference processes at quantum transitions in strong resonance electromagnetic fields on absorption, amplification and refractive indices as well as on four-wave mixing processes is investigated. Doppler broadening of the coupled transitions, incoherent excitation, relaxation processes, as well as power saturation processes associated with the coupled levels are taken into account. Both closed (ground state is involved) and open (only excited states are involved) energy level configurations are considered. Common expressions are obtained which allow to analyze the optical characteristics (including gain without inversion and enhanced refractive index at vanishing absorption) for various V, (Lambda) and H configurations of interfering transitions by a simple substitution of parameters. Similar expressions for resonant four-wave mixing (FWM) in Raman configurations are derived too. Crucial role of Doppler broadening is shown. Theory is applied for numerical analysis of some recent and potential experiments.
The influence of nonlinear interference processes and of changes in populations in strong electromagnetic fields, and also of the Doppler broadening on resonant four-wave frequency mixing is investigated for a system of Raman-type transitions. The case in which a quantum system interacts with one weak field and two strong fields is considered. Each of the energy levels is assumed to be perturbed by just one strong field. It is shown that the Doppler broadening alters drastically the contributions of the populations of the various levels to the investigated process and that this can alter by several orders of magnitude and nonlinear susceptibilities averaged over the velocities and influence significantly the selection of optimal configurations of the transitions and of the experimental conditions for effective resonant four-wave conversion of the radiation frequencies. A numerical analysis of the derived expressions is used to account for the experimentally observed considerable differences between the spectral dependences of the output power and the conclusions drawn from perturbation theory.
Specific features of nonlinear interference processes at quantum transitions in near- and fully resonant Doppler-broadened optically dense media are studied. The possibility of all-optical switching of the medium to opaque or, alternatively, to absolutely transparent, or even to strongly amplifying states is explored, which is controlled by a small variation of two driving or probe radiations and does not require any change of the level populations. Optimum conditions for inversionless amplification of short-wavelength radiation above the oscillation threshold at the expense of the longer-wavelength control fields are investigated. The feasibility of overcoming the fundamental limitation on a velocity-interval of resonantly coupled molecules imposed by the Doppler effect is shown, based on quantum coherence.
The general nonlinear stationary theory of propagation of an elliptically polarized pulse that resonantly interacts with the 1/2-1/2 transition is developed taking into account relaxation processes. It is shown that interference of magnetic sublevels in the field of polarized radiation that optically pumps atoms results in inversionless amplification of radiation. Taking into account the Doppler broadening of spectral lines shows that there exists a certain optimal length of the resonant medium along which the radiation that emerges from the medium is maximally amplified.
An investigation is reported of resonant four-wave frequency mixing of the type ω 4 =ω 1 —ω 2 +ω 3 under coherent population trapping conditions. Such mixing may appear when radiations of frequencies ω 1,2 are strong and when ω 1 —ω 2 =ω 20 (ω 20 is the frequency of a dipole-forbidden transition). The density matrix method is used to calculate (taking quantum interference into account) the absorption coefficients, the refractive indices, and the nonlinear susceptibilities exactly for the strong fields and in the first order of perturbation theory for the weak fields. A study is made of the dependence of the power of nonlinear optical generation on various characteristics of the transitions and of the radiation, subject to the propagation effects. The cases of homogeneous and Doppler broadening of the resonant transitions are considered. The characteristics of transitions in the barium atom are used in numerical illustrations. It is shown that the efficiency of resonant nonlinear-optical conversion of radiation can be enhanced by utilising interference processes in quantum transitions, which represent coherent population trapping.
Resonant nonlinear-optical interference processes in four-level Doppler-broadened media are studied. Specific features of amplification and optical switching of short-wavelength radiation in a strongly-absorbing resonant gas under coherent quantum control with two longer wavelength radiations, are investigated. The major outcomes are illustrated with virtual experiments aimed at inversionless short-wavelength amplification, which also address deficiencies in this regard in recent experiments. With numerical simulations related to the proposed experiment in optically-dense sodium dimer vapor, we show optimal condition for optical switching and the expected gain of the probe radiation, which is above the oscillation threshold.