Mode superfluorescence of two-level and quasi-classical system in a bad cavity and without cavity

Summary form only given. We develop an analytic theory of mode superfluorescence (SF), i.e. collective spontaneous emission, in the bad-cavity limit when the medium relaxation is much weaker than the field relaxation. The corresponding superradiant class-D lasers and masers exhibit extreme spontaneous pulsation and emit powerful ultrashort SF pulses in the absence of any mode locking. It is possible both in quantum systems, say, with two energy levels (like Dicke atomic superradiance) and in quasi-classical systems with quasi-equidistant energy levels (for instance, vibrational or rotational levels in molecules, Rydberg levels in atoms, and Landau levels of electrons in magnetic field). We propose novel schemes of gas and solid lasers as well as original devices in semiconductor and vacuum electronics which can generate superradiant pulses as short as ten to hundred wavelengths without additional time-frequency compression.