Abstract-Free-space sub-terahertz field-polarization controlled by waveguide-mode-selection
We study experimentally the free-space electro-magnetic field emitted from a multi-mode rectangular waveguide equipped with a diagonal-horn antenna. Using the frequency range of 215-580 GHz, a photo mixer is used to launch a free-space circularly-polarized electro-magnetic field, exciting multiple modes at the input of the rectangular waveguide via an input diagonal-horn antenna. A second photo mixer is used, together with a silicon mirror acting as Fresnel scatterer, to act as a polarization-sensitive coherent detector to characterize the emitted field. We find that the radiated field, excited by the fundamental waveguide mode, is characterized by a linear polarization. In addition, we find, unexpectedly, that the polarization rotates by 45 degrees if selectively exciting higher-order modes in the waveguide. We discuss a possible application of this new finding.
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