Adiabatic coupling into Terahertz parallel-plate waveguides

We report on the coupling process of broadband terahertz radiation into metal parallel-plate waveguides with sub-wavelength gaps. Starting from well known quasi-optic coupling using silicon lenses, we developed a coupling scheme based on adiabatic coupling using metal flares. The metal surfaces change their shape slowly and smoothly with respect to the wavelength of terahertz radiation (300 µm). The undistorted pulse shape and dispersion-free propagation is maintained in the TEM geometry. The obtained coupling efficiencies for different waveguide geometries exceed previous values. For an optimized layout, also spectroscopic results of polycrystalline molecular samples are presented revealing high resolution absorption lines.