Scientific-Practical Material Research Centre, Belarus National Academy of Sciences, Belarus

We study simulton solutions for two-color pulses copropagating in multilevel medium with cascade transitions. Simulton regime of terahertz (THz) pulse propagation has been estimated for ammonia vapors expecting that pulses are resonant with lowest adjacent rotational transitions. In this case the problem reduces to investigation of THz pulse propagation in medium with two different multiplets consisting of three- and two- level subsystems, respectively. The first THz pulse is resonant with the lowest transition of three-level system and the second one is in resonance with the upper transition of three-level system as well as four two-level systems. Simulton trapping problem for shaped pulses with slowly varying envelopes is nonintegrable and it is significantly different from the simultons in optical spectral region. All relevant levels at room temperature are populated. Presence of two-level subsystems also complicates the trapping conditions. As follows from master equations for both pulses, obtained in the frame of operator approach, powerful two-humped and weak one-humped pulses may be trapped into simulton. Numerical modeling proves simulton stability at finite distances of propagation. Two-humped structure of the intense pulse is due to impact of weak pulse and different values of oscillator forces of adjacent transitions in three-level multiplets.