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Modern Physics Letters B

THERMAL-NOISE INDUCED RADIO FREQUENCY SUPERRADIANCE IN RESONATOR

Yu.F. Kiselev, A.S. Shumovsky, Vyacheslav I. Yukalov

Published Oct 1, 1989• 10 citations
Publisher

Abstract

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.

DOI
10.1142/s021798498900176x

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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.
We discuss the coherent behavior of a polarized, nuclear or electron, spin system for which the magnetic dipole radiation emitted in the radio-frequency region, has approximately quadratic dependence on the number of spins. An effective method of describing these phenomena is provided by computer simulation of a microscopic model of the spin system. Important aspects of this numeric simulation are described, together with a comparison with the theoretical predictions. The behavior of the transverse component of the magnetic moment, M+(t){M}^{+}(t)M+(t), in super-radiant conditions is studied. In addition, the role of dipole-dipole interactions in super-radiation phenomena is investigated in detail. It is shown that some important features of super-radiation cannot be described with the Bloch equations.
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 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.
ABSTRACT It is shown that the magnetic field local inhomogeneous, caused by the stochastic spin distribution over the volume,effects significantly on the nuclear superradiation process. Keywords: Dicke superradiance, nuclear spin system 1. INTRODUCTION The possibility of the Dicke superradiance' in the nuclear spin system, placed in the magnetic field, was explained byBlombergen and Paund in 1954.2 Experimental researches of this effect related to more later time.3'4 The theoretical explanation of this experiment, based on the solution of the Maxwell - Bloch equations system was proposed (seealso7) in works.5'6 Further development of the nuclear spin superradiance theory is contained in works.8'4 A semi-classical approach,5'6 reproducing main features of the observed effect, contained a number of artificial assumptions. In particular, observation of the single pulses regime with high values of the initial spin polarizations couldn't beexplained with taking in account of Larmor frequency scanning, that took place in the experiment. To obtain thecorrespondence with the experimental data we had to artificially select the transverse relaxation time value.In the present paper we throw out these assumptions and show that the role of additional relaxation mechanismcan play line width, caused by the local magnetic field. Here we are not pretend to give the exact description of thementioned experiment,4 but only point to the sufficiently features of the phenomenon considered.
The aim of this review is twofold. Firstly, a general approach is presented which can provide a unified description of the dynamics of radiating systems of different natures. Both atomic systems and spin assemblies can be treated within the same mathematical method, which is based on a pseudospin (or spin) representation of the evolution equations. The approach is applicable to all stages of radiation dynamics, including the most difficult initial quantum stage, where coherence is not yet developed. This makes it possible to study the process of coherent self-organization from the chaotic quantum stage. Secondly, the approach is illustrated by applying it to the description of several coherent phenomena. Different types of superradiance are characterized: pure superradiance, triggered superradiance, pulsing and punctuated superradiance. The theory of such interesting effects as triggering dipolar waves, turbulent photon filamentation, collective liberation of light, pseudospin atomic squeezing, and operator entanglement production is presented.