Nonlinear propagation of terahertz pulses in ammonia vapor

Abstract

We consider the propagation of terahertz (THz) pulses resonant with the lowest rotational levels in ammonia vapors at room temperature. We demonstrate a quasisimultonic regime of propagation for two-color THz pulses (their lossless simultaneous propagation with equal group velocities and stable envelopes at finite distances, like coupled solitons). The quasisimultons are formed by two pulses synchronized in time, the first one being intense and two humped with an area multiple of 4π , while the another one is weaker, one humped, and with an area much less than π. Two-and one-humped pulses have equal width and copropagate in ammonia vapor at the same group velocity. Moreover, the larger the dip of the two-hump pulse is, the weaker may be the one-humped pulse. The stability of this coupled state is provided by the fact that the two-humped pulse makes the medium transparent for the one-humped pulse. In its turn, the latter prevents the breakup of the former into separate pulses. It is established as well that a multihumped pulse with an odd or even number of humps and a hyperbolic secant pulse can behave like a simulton. In this case, the area of multihumped pulse equals to 2nπ , where n is the number of humps in the powerful pulse structure.