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Soviet Physics Uspekhi

Optical superradiance: new ideas and new experiments

Anatolii V Andreev

Published Dec 31, 1990• 34 citations
Publisher

Abstract

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.

DOI
10.1070/pu1990v033n12abeh002664

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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.
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.
A theory is proposed to account for the superradiance emitted by a system of nuclear spins interacting with a resonant circuit. A study is reported of the dependences of the parameters of the superradiance pulses on the main parameters of the medium. A comparison is made of these dependences with the corresponding expressions in the optical superradiance case. The transition from superradiance to maser operation is investigated. It is shown that the calculations are in full agreement with the experimental curves.
The present state of theoretical and experimental research on collective spontaneous emission (superradiance) in a system of radiators (atoms, molecules, or nuclei) is reviewed. The distinction between superradiance and the amplification of spontaneous emission is discussed. There is also a discussion of conditions under which the effect can be observed and of various theoretical methods for describing superradiance: the quantum single-mode and multimode models and the semiclassical approach. Theoretical papers on superradiance in systems with dimensions smaller than the wavelength of the radiation and also in extended systems are reviewed. It is shown that superradiance may occur in weakly amplifying media. A situation in which the superradiance is oscillatory is described. The possible use of superradiance to generate coherent emission in the x-ray and ? ranges is discussed. The superradiance accompanying Raman scattering of light in atomic and molecular media is studied. The theoretical results are compared with experimental observations of superradiance in the optical range. In an appendix, the nature of the phase transition in a system of radiators interacting through an electromagnetic field is discussed.
The main part of this review is devoted to the comprehensive description of coherent radiation by nuclear spins. The theory of nuclear spin superradiance is developed and the experimental observations of this phenomenon are considered. The intriguing problem of how coherence develops from initially incoherent quantum fluctuations is analysed. All main types of coherent radiation by nuclear spins are discussed, which are: free nuclear induction, collective induction, maser generation, pure superradiance, triggered superradiance, pulsing superradiance, punctuated superradiance, and induced emission. The influence of electron-nuclear hyperfine interactions and the role of magnetic anisotropy are studied. Conditions for realizing spin superradiance by magnetic molecules are investigated. The possibility of nuclear matter lasing, accompanied by pion or dibaryon radiation, is briefly touched.
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.