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Technical Physics Letters

The effect of internal reflections of the field on the parameters of a superradiant pulse

Yu. A. Avetisyan

Published Oct 1, 1999• 0 citations
Publisher

Abstract

Superadiation (SR) of an inverted two-level medium, described by one-dimensional Maxwell-Bloch equations without assuming that the amplitudes of the electric field vary smoothly along the sample, is studied. It is shown that as the initial inversion density increases, a transition is observed from the so-called “lethargic” field amplification regime, characteristic for the initial stage of SR, to a regime close to the ordinary exponential law. This accelerates the process and can lead to synchronization of counterpropagating SR pulses. Estimates are obtained for the average value of the delays of the oppositely propagating pulses and for their correlation criterion, which depends strongly on the ratio of the sample length and the half-wavelength of the resonance radiation. The possibility of realizing SR under conditions of strong phase relaxation of the active medium is discussed.

DOI
10.1134/1.1262629

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Superadiation (SR) of an inverted two-level medium, described by one-dimensional Maxwell-Bloch equations without assuming that the amplitudes of the electric field vary smoothly along the sample, is studied. It is shown that as the initial inversion density increases, a transition is observed from the so-called “lethargic” field amplification regime, characteristic for the initial stage of SR, to a regime close to the ordinary exponential law. This accelerates the process and can lead to synchronization of counterpropagating SR pulses. Estimates are obtained for the average value of the delays of the oppositely propagating pulses and for their correlation criterion, which depends strongly on the ratio of the sample length and the half-wavelength of the resonance radiation. The possibility of realizing SR under conditions of strong phase relaxation of the active medium is discussed.
We consider a pencil-shaped active medium consisting of two- level initially excited atoms and described by 1D Maxwell- Bloch equations. With the help of these equations solution without slowly varying envelope approximation we shown, that internal field reflections from the ends of the sample transform the so-called 'lethargic' regime of super-radiance (SR) pulse amplification to the amplification regime similar to usual exponential law. iT can lead to the pulse development acceleration and counter-propagating waves synchronization. A simple formula for SR pulse delay time evaluation is presented. The criterion of the counter- propagating waves synchronization is discussed.
The superradiance (SR) of initially inverted two-level active medium in a cavity is considered. We discuss the results specific for the rigorous simulation of the SR pulse (obtained on the basis of 1D Maxwell-Bloch equations solution without slowly varying envelope approximation (SVEA) in the spatial variable): the strong dependencies of the counter-propagating waves correlation and pulse development acceleration not only on the module of the cavity mirrors reflection coefficient, but on the integer- factor of the ratio of the sample length and half of the resonant wavelength. The simple modification of the SVEA model approximately describing the above features is considered.
The reflection of short optical pulses from the boundary of a resonant medium is investigated theoretically by the coupled system of the Maxwell and optical Bloch equations. In order to describe the reflected wave, the slowly-varying-envelope approximation in space is not exploited. The coherent reflection is shown to be in connection with Dicke superradiance. It takes place if the relaxation time of the polarization is longer than the superradiation time of the atoms in an optically thin boundary layer of the medium. That is why much of the relevant information can be obtained already in the thin-medium limit, which is considered separately. The conditions of strong reflection from an extended medium are similar to the ones found for a thin sample. The transition into the incoherent and stationary regime is discussed.
An orderly account is given of the processes of coherent propagation, amplification, and generation of pulses in polyatomic resonant media. The characteristic features of the propagation of solitons in resonant media of finite length are considered. Comparisons are made of the exponential and lethargic amplification of pulses in resonant media. It is shown that an allowance for the reflection of the field from the boundaries of an active medium gives rise to a qualitatively different dependence of the parameters of the pulses on the reflection coefficients under superradiance and lasing conditions, which makes it possible to introduce a criterion to distinguish lasing from superradiance. An analysis is made of the characteristics of collective superradiance, which is a new case of coherent generation of pulses. Experimental and theoretical investigations of rf superradiance emitted by systems of nuclear spins are reviewed.
Using the semiclassical approach the simulation of the light pulse generation by a thin layer of quantum two-level dipoles has been performed. The state of each dipole is governed by Bloch equations for individual density matrix elements under condition of phase relaxation absence. Identical dipoles are located at the nodes of 2-D grating with corresponding sizes Lx, Ly and are considered as pointlike sources of classical field. At the initial moment all the dipoles are in an inverted state Initiation of the pulse was carried out by either incoherent spontaneous decay (superfluorescence mode simulated by setting of small random polarisation with *-correlation) or coherent initial state. Based on the numerical solution the kinetics of dipoles characteristics and evolution of the radiation directivity pattern have been calculated. The influence of the initial state and short-range Coulomb dipole-dipole interaction on the pulse characteristics have been analysed. Tending of axis X to be a direction of primary amplification of pulse with increasing of inequality Lx > Ly has been traced. The comparison of the results obtained with the results of paraxial approximation that uses the concept of slowly varying along axis X amplitudes of the electric field and medium polarisation has been done.