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Soviet Journal of Quantum Electronics

Interaction between radiation and quantum systems with relaxing sublevels

Tamara M. Il'inova, M P Il'inov, R. V. Khokhlov

Published Jun 30, 1972• 2 citations
Publisher

Abstract

A theoretical analysis is given of the propagation of resonance radiation in a medium with relaxing sublevels. When the time constants of changes in the optical field are comparable with the relaxation times, the relaxation of sublevels gives rise to several special features in the behavior of steady-state π pulses. The analysis explains several effects observed in various experimental situations, including the spectral "hole burning" effect.

DOI
10.1070/qe1972v001n06abeh003295

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A theoretical analysis is given of the propagation of resonance radiation in a medium with relaxing sublevels. When the time constants of changes in the optical field are comparable with the relaxation times, the relaxation of sublevels gives rise to several special features in the behavior of steady-state π pulses. The analysis explains several effects observed in various experimental situations, including the spectral "hole burning" effect.
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The results are given of a theoretical investigation of the interaction of a laser pulse with a molecular medium. Allowance for the vibrational relaxation processes is made within the diffusion approximation framework which makes it possible to describe relaxation as the diffusion of particles in continuous electron energy bands. A simultaneous solution is obtained of the diffusion equations for the medium and the equations for the radiation field in the special cases of slow and fast vibrational relaxation. It is shown that the fast relaxation, which prevents gain saturation, increases the signal energy compared with the case of slow relaxation in an amplifying medium and reduces this energy compared with such relaxation in an absorbing medium. Expressions are obtained for the functions which describe the distributions of the band particles established by the investigated signal. The analysis made shows that the diffusion model can be applied to the interaction between laser pulses and molecular media.
The propagation of two ultrashort radiation pulses interacting via an intermediate level is analyzed by numerical integration of a system of equations describing coherent interaction between radiation and a three-level medium. It is shown that the propagation of the pulses depends strongly on their areas on entry to the medium and on the relationship between the dipole moments of transitions between neighboring levels. Substantial temporal narrowing was established for fairly high-power pulses (4π and 6π pulses) and this may be utilized to control their durations. Investigations are also made of the influence of relaxation processes, delay of pulses relative to one another on entry to the medium, and nonresonant dispersion during pulse propagation under the specified conditions.
A theoretical model for the propagation of coherent light pulses in resonant media where a large number of atoms cooperate in spontaneous emission (superradiance) is discussed. The medium is supposedly free of inhomogeneous broadening, and no lossy relaxation mechanisms are taken into account. The conditions for the emission of all spontaneous radiation in the direction of pulse propagation are described. A semi-classical model is used and, at each point in the medium, the propagation field and the spontaneous damping field are considered separately. The equation governing the evolution of a superradiant system in the presence of an applied pulse is derived. A mechanism of energy transfer from the traveling pulse to the medium is described which includes superradiant spontaneous emission effects. As a result of the neglect of all lossy relaxation processes, all energy stored in the medium is eventually returned to the pulse by coherent spontaneous emission. Therefore, in this theory, all pulses travel through the medium without energy loss. Numerical results for square input pulses are discussed which show two possible types of propagation behavior: (a) Low input intensities produce a negative tail to the input (positive) pulse and sharpening of the tail with the advance in the medium; (b) higher input intensities produce, after sufficient penetration, a steady-state pulse envelope somewhat similar to the $2\ensuremath{\pi}$ pulses of self-induced transparency. The steady-state solutions of the equations of motion are found numerically and the pulse shapes are computed for different degrees of superradiance. In the limit of strong superradiance, very asymmetric pulse envelopes of large area are found. For negligible superradiance, approximately $2\ensuremath{\pi}$ hyperbolic-secant pulses are obtained. The effects of inhomogeneous broadening are briefly considered, and the consequences of important superradiant damping in photon-echo experiments are qualitatively discussed.