Soliton-like propagation of broadband terahertz pulses in a system of tunnel junctions
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SolitonLike Propagation of Broadband Terahertz Pulses in a System of Tunnel Junctions S. V. Sazonov National Research Centre Kurchatov Institute, pl. Akademika Kurchatova 1, Moscow, 123182 Russia email: [email protected] Received April 27, 2014
Abstract—Solitonlike 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 quasimonochro matic 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 solitonlike 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 selffocusing and defocusing of pulses have been revealed. A new type of local ized 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 tun neling transition is compensated by the defocusing effect of transitions to higher lying states under the con ditions of applicability of the geometrical optics approximation. DOI: 10.1134/S1063776114090192
1. INTRODUCTION Studies of generation of terahertz electromagnetic radiation have recently become more topical (see, e.g., [1]). The sensitivity of vibrational, rotational, vibrotational, tunneling, and other quantum transitions in various media to such a radiation provides the possi bility of efficiently using terahertz signals in spectros copy, safety systems, image reconstructions, etc. [2–4]. The method of generation based on the optical rec tification mechanism is one of the most efficient methods [5–20]. In this case, the spectrum of a tera hertz signal is broadband as a rule; i.e., the spectral width of the pulse is comparable to the central fre quency of its spectrum. The duration of a generated pulse is such that it contains about one period of oscil lations in the terahertz range [21–23]. Thus, a tera hertz signal obtained by the optical generation method has the properties of an ultrashort pulse [24–27]. In turn, this means that the interaction of such a pulse with matter cannot be theoretically considered in the approximation of slowly varying amplitudes standard for quasimonochromatic signals [28]. It is noteworthy that the interaction of terahertz radiation with matter is most poorly studied as com pared to radiation of other frequency ranges. For this reason, it is necessary to develop theoretical models describing such an interaction. Owing to a large spec tral width of a pulse, it interacts with a large number of different degrees of freedom of a medium. This c
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