The electrodynamics of active continuous media is applied to theoretically examine collective spontaneous emission regimes of dipole oscillator ensembles. Recent experiments in which the superfluorescence phenomenon has been observed are reviewed. The focus is on propagation and interaction effects experienced by the inhomogeneous waves of active center polarization and electromagnetic fields. The superradiant laser dynamics is examined and prospects for the realization of superradiant lasers using low-Q cavities are discussed.

Analytic theory, numerical simulation, and qualitative analysis of the threshold conditions, nonlinear dynamics, and spectral features of the superradiant emission and cooperative radiative behavior of a dense many-particle system in a low-Q Fabry-Perot cavity with distributed feedback of counter-propagating electromagnetic waves are given. Various systems with extreme spatial-spectral density of radiating particles as active media of superradiant lasers are discussed, including those with almost homogeneous broadening as well as strongly inhomogeneous broadening of a spectral line. In the case of experimental verification, the phenomenon of CW superradiant lasing will be promising in the information optoelectronics and condensed matter physics, in particular, for managing novel oscillators with complicated dynamical spectra and for creating unprecedented diagnostics of quantum coherent many-particle effects. Introduction. What is a class D laser?Recent experiments on pulsed superfluorescence and prospects of CW superradiant lasing in various active media (see Sect. 4.2) require a systematic analysis of the threshold conditions and the non-stationary regimes of a cooperative emission based on mode superradiance (SR). This emission takes place in the case when a photon lifetime is much shorter than a polarization lifetime of an active center. The latter situation means a low-Q cavity and a dense active medium and corresponds to the so-called class D lasers [6,28], which differ in many respects from the standard lasers of A, B, and C classes introduced by Arecchi and Harrison [3].The key difference is related to the collective dynamics of the active centers which totally alters the dynamical spectra of their population inversion and cavity Vl. V. Kocharovsky ( ) · V. V. Kocharovsky · E. R. Kocharovskaya IAP RAS, 46, Ulyanov str.,

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