An analysis is made of the deexcitation kinetics of strongly excited extended systems of initially uncorrelated two-level radiators. The possibility of partially cooperative states of the system is predicted when the radiation intensity acquires values intermediate between the spontaneous decay intensity and the Dicke superradiance intensity. The conditions for the onset of cooperative processes are obtained. The threshold for the oscillatory emission conditions is determined and the oscillation frequency is found. Modulation of the emission intensity is predicted with different modulation frequencies before and after the superradiant pulse. The modulation frequencies are determined. Formulas are derived for the superradiant pulse delay times and intensities. The possibility of weakly anisotropic cooperative emission is predicted.
An investigation is made of the dynamics of the superradiance of two-component quasiresonant media with similar frequencies of superradiant transitions. It is shown that variation of the detuning of the frequencies of the components can increase the peak intensity of a superradiance pulse and simultaneously reduce its temporal width. The range of the detuning of the frequencies of the components ensuring generation of the strongest and shortest superradiance pulse is determined. It is demonstrated that superradiance pulses emitted by two-component quasiresonant media may be delayed by a considerable time, which is significantly longer than the temporal width of the superradiance pulse itself.
In order to study the cooperative emission of radiation from an extended system of
N two-level atoms, we use the master-equation approach within the Born and Markov approximations. All the properties of the emission of radiation can be deduced from the diagonalization of the coupling matrix $\ensuremath{\Gamma}$, where an element ${\ensuremath{\Gamma}}_{\mathrm{jk}}$ gives the coupling between the atoms labeled
j and
k via the electromagnetic field. We choose a pencil-shaped volume and study analytically the coupling matrix in the limit for small and large Fresnel numbers. For Fresnel numbers of order unity numerical methods are used. Thus we prove that, for the pencil-shaped geometry, any elaborated model must take into account the interaction of the atoms with the infinity of electromagnetic field modes lying inside two small solid angles about the axis of the largest linear dimension, which we label the
z axis. In other words, the intensity of radiation emitted by any atom of the system depends on its position with respect to the others and this "propagation effect" cannot be neglected when interpreting superradiance experiments. For small Fresnel numbers the
z dependence predominates, in the opposite case the (
x,y) dependence does, while for Fresnel numbers of order unity the dependence of all three components must be taken into account. Preliminary calculations are given for which only the
z dependence is considered; for Fresnel numbers of magnitude of order unity or smaller, the effect of propagation along the
z axis displays large delays for the appearance of superradiance with respect to the time at which the superradiant pulse is maximum when the "propagation effects" are neglected (one-mode model).
In the theoretical stuqy of superradiance, the atoms or molecules are generally considered as two-level systems. However in many experimental situations, this simple assumption is not valid. It is the case, for example, in all experiments exhibiting superradiant beats [1-3]. Moreover, these beats are sometimes due to a specific property of superradiance : the existence of interferences between different atoms. Other examples of multilevel superradiance are encountered each time one observes several superradiant transitions sharing a common level [3,4]. In addition, superradiance is generally observed between degenerate levels, and it has been alreaqy shown experimentally that the level-degeneracy influences the properties of the superradiant light, in particular its polarization characteristics [5].