Measuring TE1 mode Losses in Terahertz Parallel-Plate Waveguides

The parallel-plate waveguide (PPWG) has drawn considerable research interest [1–12], since it was first reported to support the undistorted propagation of broadband terahertz (THz) pulses. From that time, the transverse-electromagnetic (TEM) mode of the PPWG has been the popular choice for measurements due to its low loss, ease of quasi-optic coupling, and negligible dispersion as a result of the lack of a cutoff. Unlike the TEM mode, the lowest order transverse-electric (TE1) mode has a cutoff frequency, and thus this mode was largely unexplored in the THz regime until recently [13, 14]. This recent work showed that the cutoff frequency could be moved to lower frequencies to reduce the dispersion by increasing the plate separation, while matching the input beam size to the plate separation to realize dominantly single-mode propagation. This same work predicted the possibility of realizing ultra-low ohmic losses in the dB/km range by again utilizing the TE1 mode of this waveguide. These low ohmic losses could permit long-distance transport of THz radiation. However, with such long propagation distances a new concern arises: energy leakage out of the unconfined sides of the PPWG due to diffraction. In fact, this would be the dominant loss mechanism in the case under consideration, where the ohmic losses are virtually negligible. We addressed this diffraction problem in a recent article [15], where we showed that it is possible to inhibit diffraction losses for the TE1 mode by using a waveguide with slightly concave plates. Via a simple “bouncing plane wave” analysis, we demonstrated how to determine an ideal radius of curvature for the inner surfaces, for a waveguide operating at a given THz frequency. This is governed by a confocal condition, R 1⁄4 2bv .