Abstract <p>The modal properties of a terahertz (THz) pulse propagating in a parallel-plate waveguide partially filled with a nonlinear crystal – either lithium niobate (LiNbO₃) or 4<i>N</i>, <i>N</i> dimethylamino 4<i>N</i> methylstilbazolium tosylate (DAST), are investigated. THz pulses are generated remotely in the nonlinear crystal via optical rectification of a femtosecond Ti:sapphire laser pulse. A numerical analysis of the mode composition of the THz pulse in the structure of a “metallic parallel-plate waveguide partially filled with a nonlinear crystal” was performed using MATCAD and COMSOL software. The study demonstrates the possibility of achieving single-mode coupling between the input linearly polarized optical pulse and the THz pulse with an ultra-broad spectral bandwidth of 0.1–2.6 THz. By numerically determining the width and height of the nonlinear crystal for given waveguide dimensions and known dielectric permittivity (ε) of the crystal, conditions for single-mode propagation of the THz pulse were achieved. It is also shown that as the THz frequency increases, a greater portion of the pulse energy is confined within the nonlinear crystal. At the same time, the remainder propagates in the surrounding air-filled region.</p>