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Bulletin of the Russian Academy of Sciences Physics

On superluminal pulses in nonequilibrium media

A. N. Bugay, Sergey Vladimirovich Sazonov

Published Dec 1, 2008• 0 citations
Publisher

Abstract

The propagation and transverse dynamics of extremely short pulses in a nonequilibrium medium consisting of two-level atoms has been investigated. It is shown that, if the spectrum of such pulses does not overlap the characteristic frequency of quantum transitions, non-1D superluminal pulses with retained shape may exist.
DOI
10.3103/s1062873808120113

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We study theoretically the phenomenon of self-induced transparency for ultrashort pulses (videopulses) propagating in a multilevel quantum medium under conditions in which the method of slowly varying amplitudes and phases breaks down and the pulse spectrum substantially overlaps the quantum transitions. A special class of transitions with one common level is considered. We find that the dynamics of the videopulses in such a medium is described by a double sine-Gordon equation. We establish the conditions under which steady-state traveling 0 π -, 2 π -, and 4 π -videosolitons are formed. It is found that 4 π -videosolitons can propagate both in equilibrium media and in some nonequilibrium media, while 0 π -videosolitons can propagate only in nonequilibrium media. We study amplification processes in highly nonequilibrium systems and show that, depending on the initial state of the medium, 2 π -and qπ -pulses (0< q <1) with increasing amplitudes may be formed. We conclude that the amplification of an ultrashort pulse occurs due to an increase in the photon number density and to an increase in the frequency of each photon. Finally, we study the possibility of an electromagnetic autosoliton being formed in a nonequilibrium dissipative medium.
Spectral reshaping and nonuniform phase delay associated with an electromagnetic pulse propagating in a temporally dispersive medium may lead to interesting observations in which the group velocity becomes superluminal or even negative. In such cases, the finite bandwidth of the superluminal region implies the inevitable existence of a cutoff distance beyond which a superluminal pulse becomes subluminal. In this paper, we derive a closed-form analytic expression to estimate this cutoff distance in abnormal dispersive media with gain. Moreover, the method of steepest descent is used to track the time-frequency dynamics associated with the evolution of the center of mass of a superluminal pulse to the subluminal regime. This evolution takes place at longer propagation depths as a result of the subluminal components affecting the behavior of the pulse. Finally, the analysis presents the fundamental limitations of superluminal propagation in light of factors such as the medium depth, pulse width, and the medium dispersion strength.