An effective THz waveguide would exhibit both low loss and low dispersion. The principal difficulty has been the lack of materials well suited for guided propagation at THz frequencies. The best suited materials, those that are transparent and relatively dispersionless in the THz regime, are crystalline, which precludes their use for practical waveguides. Materials such as glasses and polymers that work well at optical frequencies exhibit unacceptably high absorptions losses at THz frequencies. Continuous wave (CW) THz applications often employ a confined geometry in which the THz radiation propagates through air inside of a metal tube. This is not practical, though, for a THz pulse because it suffers from group velocity dispersion that causes pulse reshaping and broadening. The parallel-plate metal waveguide developed by Grischkowsky et. al.[2] is an exception because it exhibits dispersionless propagation and very low losses.

It was recently shown that a simple cylindrical metal wire is a low-loss and relatively dispersionless THz waveguide[3]. The THz radiation propagates along the surface of the wire in a guided mode that is largely radially polarized. The loss is ultimately defined by the metal's finite conductivity. As the energy of the THz pulse moves along the wire, it causes the conduction electrons to oscillate, creating an effect known as a surface-plasmon polariton. At the end of the wire, the polariton radiates its energy into free space as a THz wave. This surface wave is often referred to as the Sommerfeld wave, named after the physicist who predicted it in the early 20th century. Shortly after the discovery of the wire's waveguiding capabilities, the first reports of a THz endoscope were published.

The most difficult step in developing the THz wire waveguide was to engineer a method to couple THz radiation to it. The linearly polarized THz pulses generated by a photoconductive antenna are poorly matched spatially to the radial mode of the waveguide. The first coupling configuration employed is depicted in Figure 2a. A second wire, the coupler, is placed approximately 500 microns from the wire waveguide but in a direction normal to it. The THz radiation is focused onto the gap between the two wires causing some of the radiation to scatter. While the amount of scattered radiation is very small, there is enough to excite the Sommerfeld wave.

Finite element method (FEM) simulations of this two-wire coupling configuration were performed to accurately determine the coupling efficiency. The plot in Figure 2b shows a simulated electric field at a frequency of 100 GHz incident on the wire gap. These simulations were completed using the commercial software COMSOL Multiphysics. The model depicted in Figure 2b consisted of more than 1 million mesh elements and was solved in the frequency domain using an iterative solver with matrix preconditioning. It can be seen in the simulation results that almost all of the incident radiation continues to propagate past the wire gap and only a small amount is coupled to the wire waveguide. Post-simulation calculations showed that less than 0.5% of the incident power is coupled to the waveguide.

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