We numerically investigate the trade-offs between the dispersion properties, coupling efficiency, and geometrical constraints in dual-wire (twin-lead) terahertz (THz) waveguides. In particular, we show that their inherent linearly polarized quasi-transverse electromagnetic (TEM) modes exist for waveguide transverse dimensions comparable with the wavelength, enabling significant end-fire coupling (>10%) for numericalaperture limited Gaussian beams while supporting a relatively low-dispersion propagation of below 0.5 ps 2 /m, as desired for short-pulse time-domain spectroscopy applications. Starting from the dual-wire structure, we also demonstrate that low-dispersion tapers can be designed to improve coupling efficiency.OCIS Terahertz (THz) bandwidth has been attracting much interest in material characterization, owing to its demonstrated advantages in probing and recognizing different materials, for various potential applications in fields spanning from biology to security. These applications rely on the free space propagation of broadband singlecycle THz pulses, which enables time-domain spectroscopy (TDS), capable of retrieving the amplitude and phase information of the spectral signature of various compounds [1] . One of the recent challenges on the topic is the development of practical waveguides that are capable of transporting this signal to arbitrary locations.Many studies highlight that the most confining structures do not exhibit the wide bandwidth required by typical pulses used in THz-TDS. The strong dispersion scrambles the phase-time information and renders the extraction of the complex spectrum rather difficult. Most studies have been based either on conventional metallic guiding structures such as hollow circular waveguides [2] and hollow rectangular waveguides [3] ; or dielectric waveguides such as sapphire fibers [4] , plastic ribbon waveguides [5] , plastic photonic crystal fibers [6] , and parallel-plate photonic waveguides [7] ; or dielectric waveguides coated with metal sheets [8] . Some structures, such as metallic parallel-plate waveguides [9−11] , are known to support the propagation of almost dispersiveless transverse electromagnetic (TEM) or quasi-TEM modes at the expense of providing confinement in only one dimension, which largely limits their practical impact.Other structures, such as coaxial [12] and metal wire waveguides [13] , better known as Sommerfeld rods [14] , exhibit ring-shaped coaxial modes with a radial polarization distribution; hence, they have poor end-fire coupling efficiency when used with conventional THz sources, that generally output linearly polarized quasi-Gaussian beams.Recent studies report low loss propagation in THz twowire waveguides [15] with a wire separation of 4.7 mm [16] . However, mode analysis ( Fig. 1) confirms that when the conductors' separation exceeds three to four times the wavelength, the propagation mode no longer exists between the two wires; instead, there are individual Sommerfeld modes around each wire, resulting in negligible coupling w...
