Coherent Spontaneous Emission Research Articles

Emission from atoms in linear superpositions of center-of-mass wave packets.

We study theoretically the electromagnetic field emitted by atoms prepared in linear superpositions of several internal states, each of which is attached to a different center-of-mass wave packet by the preparation process. It is shown that the motion and mutual separation of the wave packets can be monitored either by observing the coherent spontaneous emission in a heterodyne experiment or by measuring the energy-absorption rate from a weak probe-laser beam. We also show that a three-level system involving three wave packets coherently emits photons, forming a linear superposition of states of opposite wave vectors. A heterodyne scheme for detecting the photons in this state is proposed.

Ground level signal strength of electromagnetic waves generated by pulsed electron beams in space

A theoretical study has been made of the signal strengths at ground level of waves generated by pulsed electron beams in space. Such beams might be generated by pulsed electron guns aboard either a satellite or a rocket. The radiated energy is first calculated by an improved version of a theory based on coherent spontaneous emission. This theory evaluates the electric and magnetic field strengths and power fluxes in the far field by applying asymptotic expansion techniques. With this information at hand, the power flowing out within a cone whose apex is located at the gun position is calculated. Finally, the intersection of the rays in this cone with the Earth's surface is determined by using Snell's law considerations. Ground signal levels are calculated for typical ionospheric conditions as a function of pulsing frequency for fixed beam voltage and for voltage adjusted for resonance between the waves and the particles. For short beams, the ground level signal strengths are relatively insensitive to the wave particle resonance condition, but for longer beams the associated peaking of the signal level begins to be observed. Finally, these results are compared against ambient noise levels to determine under which circumstances these ground signals can be detected.

Three-dimensional simulations of harmonic radiation and harmonic lasing

Characteristics of the harmonic emission from free electron lasers (FELs) are examined in the spontaneous, coherent-spontaneous and stimulated emission regimes. The radiation at both odd and even harmonic frequencies is treated for electron beams with finite emittance and energy spread. In the spontaneous emission regime, the transverse radiation patterns including the transverse frequency dependences, are given. How this expression is modified to include energy spread and emittance is described. In the coherent-spontaneous emission and stimulated emission (lasing) regimes, the interaction of the radiation fields with the electrons must be treated self-consistently. Here, a single-frequency distributed transverse source function for each electron is used in the harmonic version (HELEX) of the 3D code FELEX to model the harmonic radiation. This code has recently been modified to simultaneously model the fundamental and harmonic interactions for multiple-pass oscillator simulations. These modifications facilitate the examination of FELs under various operating conditions. When the FEL is lasing at the fundamental, the evolution of the harmonic fields can be examined. This evolution is unique in the sense that the electron beam (which is bunched by the fundamental optical field) radiates at the harmonic frequencies in the presence of the harmonic radiation circulating in the cavity. As a result, enhancements of the harmonic emission can be observed. Finally, harmonic lasing can occur in cases where there is sufficient gain to overcome cavity losses and lasing at the fundamental can be suppressed. The characteristics and efficiency of these interactions are explored.

On the direct measurement of nuclear γ-resonance parameters of long-lived (≥1 s) isomers

A method for the direct measurement of the γ-resonance width for nuclei with excitation energies E 0 ≤ 100 keV and lifetimes τ ≥ 1 s is discussed. Instead of scanning the energy, as is conventional in Mössbauer spectroscopy, it is proposed to measure the time behaviour of the spontaneous coherent emission of γ-quanta by a nuclear ensemble after the cutoff of the exciting radiation flux.

Suppression and spectral splitting of the amplitude noise due to mode beatings in a single-frequency injection laser

Investigations of intensity fluctuations in a single-frequency injection laser showed that they are due to beatings between coherent field and superluminescent spontaneous emission. It has been experimentally found that in the region of developed laser emission, the level of fluctuations is decreased with an increase of the output power, and so the relative spectral shift of external cavity superluminescent mode frequenices is dependent on the laser output power. The explanation of this phenomenon was made on the basis of the mechanism of a nonlinear interaction of a field in the active region of a laser diode. It has been shown that the fluctuation level is over the shot noise level by less than 4 dB when the laser output power is sufficiently large, about 6 mW.

Theory of the optical klystron

The optical klystron is a modification of an undulator which can be used to improve the gain of an FEL oscillator or to enhance the coherent spontaneous emission originating from electron bunching. It is most useful when long enough straight sections are not available or when the energy spread is extremely small.

Kinetics of nuclear super-radiance

We derive the minimum requirements for super-radiant gamma-ray emission by applying the semiclassical theory of coherent spontaneous emission to the case of nuclear transitions in a crystal host. We find that, if a pure sample of a storage isomer can be rapidly transferred into an inverted population for a recoilless transition, less than 1013 nuclei should suffice for an experimental demonstration of nuclear super-radiance.

Radiation from long pulse train electron beams in space plasmas

A previous study of electromagnetic radiation from a finite train of electron pulses is extended to an infinite train of such pulses. The electrons are assumed to follow an idealized helical path through a space plasma in such a manner as to retain their respective position within the beam. This leads to radiation by coherent spontaneous emission. The waves of interest in this region are the whistler slow (compressional) and fast (torsional) Alfvén waves. Although a general theory is developed, analysis is then restricted to two approximations, the short and long electron beam. Formulas for the radiation per unit solid angle from the short beam are presented as a function of both propagation and ray angles, electron beam pulse width and separation and beam current, voltage, and pitch angle. Similar formulas for the total power radiated from the long beam are derived as a function of frequency, propagation angle, and ray angle. Predictions of the power radiated are presented for representative examples as determined by the long beam theory.

Coherent spontaneous emission from a modulated beam injected in a magnetized plasma

The linear stage of the coherent emission of a modulated beam injected in a cold plasma is investigated. The feature of the radiation produced is shown to be governed by the ratio ω2p/ω2c of the background plasma. This kind of emission provides an efficient way to convert the kinetic energy of the beam particles into electromagnetic wave energy: The power given to waves averaged over periods of the beam modulation is found to be an appreciable part of the initial beam power. The transfer can be maximized in the electron whistler range (Cerenkov emission) for electron beams whose characteristics are designed to met the ‘‘focalization condition’’ (double pole in the dispersion relation). The most suitable mode is the normal Doppler shift ion cyclotron mode for ion beams. Application to relevant ionospheric plasmas is considered.

Low frequency radiation characteristics of a modulated electron beam immersed in a magnetized plasma

The possibility of using a modulated electron beam injected in a magnetized plasma as a low frequency antenna (frequency lower than the gyrofrequency) is explored. The physical mechanism proposed to insure this property is spontaneous coherent emission. Derivation of the power lost by the beam is done with emphasis on the particular conditions named "focalization condition" encountered in the whistler range of frequency. In conclusion the limits of the model are discussed together with the perspectives opened by this work.

THE COHERENCE OF RADIATION AND INCREASE IN ENTROPY

A model of radiation by multi-atoms is discussed. System A, composed of N atoms with two energy levels, is coupled with a single-mode radiation B. A and B are in turn coupled with two thermal reservoirs D and C respectively, and we also assume that all atoms which are located within a wave length of the radiation field have the same phase. Then under the approximation of long wave, rotating wave and mean fields, by introducing Markov approximation after using the method of density matrix, we have derived the of equations motion for systems A and B respectively. These equations are concerning with the following processes: the spontaneous emission of single atom, the absorption and induced emission stimulated by thermal radiation, the induced emission of N atoms stimulated by single mode coherent state radiation as well as the coherent spontaneous emission of this multi-atoms system. The latter two processes are actually the two kinds of superradiance originated from Dicke model and had been investigated previonsly. We have analysed these five processes by numerical calculations. Besides, the influence of these processes upon the evolution of atomic system attaining thermal equilibrium and the increase of entropy is also discussed. It is pointed out that the correlations inside a multi-atoms system would not be destroyed by coherent spontaneous emission, in contrast with the incoherent one which leads to the change of entropy in atomic system. However, only when the entropy of the two reservoirs has been taken into account, will it be possible to demonstrate the mono-tonic increase in total entropy.

A source of nonclassical radiations exhibiting photon antibunching

We show that coherent spontaneous emission by a two-level atomic system in a Dicke state placed in a resonant cavity gives a non-classical radiation exhibiting photon-antibunching. It is assumed that the “effective” population of the ground state atoms is initially large compared to unity.

Theory of the interaction of a single-mode resonant radiation field withNtwo-level atoms

The interaction of a resonant radiation field with two-level atoms (in the long-wavelength limit) is described by a Hamiltonian trilinear in the boson operators. It is shown that the dynamics of the system can be determined nearly exactly if either at least one of the three modes is strongly populated for all times or any two modes are separately strong in two complementary time domains. Explicit results are given in two important specific cases: (i) pumping of a ground-state atomic system by a coherent radiation field and (ii) coherent spontaneous emission. The results are compared with previously known approximate results. Our results are also applicable to certain parametric processes such as parametric amplification, frequency conversion, Raman and Brillouin scattering, etc., where an identical trilinear Hamiltonian is used to describe these processes.

High‐frequency spontaneous emission of an electron beam injected into the ionospheric plasma

This paper gives a detailed quantitative analysis of the high‐frequency spontaneous wave emissions produced by an electron beam injected into the ionospheric plasma. A general discussion about individual‐collective and incoherent‐coherent characteristics of the emissions is given. Then attention is focused on the spontaneous coherent emission in three different modes: plasma waves, Bernstein modes, and electromagnetic whistler mode. Comparisons with experimental results obtained by several experiments (electron echo experiment, Araks) are made throughout the paper and suggest that many experimental features are well interpreted in terms of coherent emission.

Superradiance from three-level systems in atomic coherent state representation

A Fokker-Planck equation describing the coherent spontaneous emission from a system of 3-level atoms is derived using the atomic coherent states representation. The variables in this equation correspond directly to the number of atoms in the two excited states. The corresponding Langevin equations are discussed and their solutions for some special cases are presented.

Some comments on the use of phased angular momentum states in the theory of coherent spontaneous emission from large systems

We stress the importance of using phased atomic states when describing coherent spontaneous emission from large atomic systems. A sufficient criterion for the existence of a preferred direction of emission is deduced from dynamical arguments.

Photon statistics and coherence of stimulated superradiant pulses

A system of many #we-level atoms or molecules in a collective Dieke state (1) interacting with a single resonant mode of the quantized radiation field has often been adopted as a quantum-mechanical model for a point laser (2). In a previous paper (3) we reported exact dosed-form solutions and photon statistics of coherent spontaneous emission for the model atom-field state in the l imit of infinitely large co-operation number. The purpose of this paper is to report more general results based on this modeI. We give the exact closed-form solutions and photon statistics of coherent emission in the presence of an applied field for the model atom-field state in the same limit. For an initial atomic Dicke state interacting with an applied field in a coherent (Glauber) state (a), the emitted pulse can be desoribed in terms of Glauber states only when the levels of excitation of the initial Dicke states are (~ superradiant ~ (~). For an initially highly inverted system or low excited system, the photon distributions undergo large fluctuations. The model interaction Hamil tonian is given by

Exact closed form solutions and photon statistics of coherent spontaneous emission in a cavity

Exact closed form solutions are obtained for the coherent spontaneous emission by a large number of two-level atoms in a Dicke state. Depending on the initial states of the atoms, three types of photon statistics are obtained for the emitted radiation field. Possible implications for recent experimental results are discussed.

Coherent spontaneous emission from large systems

In this paper we discuss the conditions under which coherent spontaneous emission can take place in an arbitrary sample of linear dimensions smaller than a cooperation length. These conditions guarantee that the atoms—field interaction can be treated up to the second order in the Generalized Master Equation for the atoms. An equation describing coherent spontaneous emission is derived and discussed. A comparison is made between this and other known theories.

Superradiance Revisited

The basic features of coherent spontaneous emission, including its dependence on the square of the number of participating emitters, are reviewed. Analogies with magnetic resonance phenomena are exploited, and we explain the need to go beyond such analogies in order to treat optical emission. Some recent results in the theory of optical superradiance are discussed. The radiative decay process is treated as a quantum-classical interpolation problem, and explicit solutions are given. In addition to its time dependence, we give the angular distribution of radiated power and the expected intensity fluctuations, and we estimate the maximum number of emitters which can cooperate in a superradiant event. Dicke's “coherence brightening” criterion is discussed from the point of view of the present paper. A “failure” of the criterion is exhibited and explained. Two of the purely quantum aspects of superradiance are isolated and discussed briefly.