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Soviet Journal of Quantum Electronics

Dynamics of superradiance of a system of nuclear spins

Anatolii V Andreev, A É Tsyganov

Published Nov 30, 1991• 0 citations
Publisher

Abstract

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.

DOI
10.1070/qe1991v021n11abeh004518

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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.
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.
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.
Coherent effects in polarized systems of nuclear or electron spins using a microscopic model are studied, but not the Bloch equations. It is shown that the spin system in a resonance circuit can emit a coherent superradiant pulse when the system is initially in an essentially unstable state, at a time much shorter than the spin dephasing time T2 when the coupling of the system with the circuit is sufficiently strong, and at the time longer than T2 when this coupling is weak. The important role if dipole spin-spin interactions is elucidated.
This paper deals with a theoretical interpretation of the experiments on detection of radio frequency superradiation in a system of nuclear spins, conducted by the authors previously.1–3 The model Hamiltonian in the pseudospin representation is used for consideration. Dynamic equations are obtained for pseudospin and photon operators with allowance for oscillators of losses and thermal noise. The radiation generating process is shown to go on as follows: thermal noise initiates maser generation that results in a superradiation pulse. The importance of taking account of thermal noise is stressed by numerical estimations.
The superradiation of a system of nuclear spins is investigated taking into account the broadening of the NMR spectrum due to the stochasticity of the local magnetic field. The effect of the local field generated by nuclear spins randomly distributed in space is investigated. In this case it is possible to achieve better agreement with the experimentally observed characteristics of this phenomenon. Specifically, the possibility of a monopulse regime with a high initial inversion of the spin polarization is explained.