Bare metal wires are among the most promising waveguides for guiding terahertz (THz) surface plasmon polaritons. In this study, a thin-wall tube is proposed for coupling THz waves to a metal wire with ultrahigh efficiency, which results from three high mode matchings for the two waveguides: field distributions, polarization directions, and wave vectors. According to the mode-overlap calculation, the coupling efficiency can be always between 84% and 94% when the frequency of THz waves is in the range of 0.2-3 THz and the metal wire radius is 0.5 mm. The maximum efficiency is as high as 94% at 0.5 THz, which is much higher than that obtained by the previous methods. We further conclude that the optimal coupling efficiency can be obtained when the outer tube radius is equal to the wire radius and simultaneously the real propagation constants of modes in the two waveguides are the same. (C) 2016 Optical Society of America
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