Soliton-like propagation of broadband terahertz pulses in a system of tunnel junctions

Soliton-like regimes of propagation of broadband terahertz pulses in a system of tunnel junctions have been theoretically studied taking into account the effect of quantum states higher in energy. This study has been performed beyond the approximation of slowly varying amplitudes standard for quasimonochromatic signals. It has been shown that the role of higher lying states is fundamentally important and is not reduced only to correction effects. Previously unknown soliton-like regimes of propagation, as well as the corresponding dynamics of a medium, have been analyzed. Nonlinear refraction processes have been taken into account. Conditions for self-focusing and defocusing of pulses have been revealed. A new type of localized terahertz "bullets" with sharp boundaries at a nonequilibrium initial population of tunneling states has been predicted. The main difference of these bullets from those previously considered in an optical range is that diffraction does not affect the formation of the former. In this case, the focusing effect caused by a tunneling transition is compensated by the defocusing effect of transitions to higher lying states under the conditions of applicability of the geometrical optics approximation.