Terahertz waveguides

Quasi-optical techniques are used to efficiently couple freely propagating pulses of terahertz (THz) electromagnetic radiation into circular and rectangular metal waveguides. We have observed very dispersive, low-loss propagation over the frequency band from 0.65 to 3.5 THz with typical waveguide cross-section dimensions on the order of 300 µm and lengths of 25 mm. Classical waveguide theory is utilized to calculate the coupling coefficients into the modes of the waveguide for the incoming focused THz beam. It is shown that the linearly polarized incoming THz pulses significantly couple only into the ${\mathrm{TE}}_{11},$ ${\mathrm{TE}}_{12},$ and ${\mathrm{TM}}_{11}$ modes of the circular waveguide and the ${\mathrm{TE}}_{10}$ and ${\mathrm{TM}}_{12}$ modes of the rectangular guide. The propagation of the pulse through the guide is described as a linear superposition of the coupled propagating modes, each with a unique complex propagation vector. This picture explains in detail all the observed features of the THz pulse emerging from the waveguide. We demonstrate both theoretically and experimentally that it is possible to achieve ${\mathrm{TE}}_{10}$ single-mode coupling and propagation in a suitably sized rectangular waveguide for an incoming focused, linearly polarized THz pulse with a bandwidth covering many octaves in frequency and that overlaps more than 35 waveguide modes. Finally, to facilitate the application of these THz waveguides to THz time-domain spectroscopy of various configurations of dielectrics in the waveguide including surface layers, we present analytic results for the absorption and the dispersion of such layers.