Polarization waves and super-radiance in active media

The macroscopic electrodynamics of time-dependent coherent processes in active media is reviewed. Dicke super-radiance in the two-level model and cyclotron super-radiance in a model consisting of an electron beam in a magnetic field are investigated. Similar phenomena in other media are discussed. Particular attention is devoted to polarization waves, i.e., a type of normal wave that coexists with electromagnetic waves and in many ways determines the character of coherent effects. Super-radiance is considered as a dissipative instability of negative-energy polarization waves, which occurs in an active sample as a result of losses by emission of positive-energy electromagnetic waves into the ambient space. The method of phenomenological quantization is developed for unstable modes in active samples, and directly describes the quantum-mechanical properties of collective excitations. The procedure is used to analyze macroscopic quantum-mechanical fluctuations of super-radiance.