Free-Space Subterahertz-Field Polarization Controlled by Selection of Waveguide Modes
Marc Westig (Kavli institute of nanoscience Delft, TU Delft - QN/Klapwijk Lab)
Holger Thierschmann (TU Delft - QN/Klapwijk Lab, Kavli institute of nanoscience Delft)
Allard Katan (TU Delft - Applied Sciences, Kavli institute of nanoscience Delft)
Matvey Finkel (TU Delft - QCD/DiCarlo Lab, Kavli institute of nanoscience Delft)
Teun M. Klapwijk (Kavli institute of nanoscience Delft, TU Delft - Applied Sciences)
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Abstract
We experimentally study the free-space electromagnetic field emitted from a multimode rectangular waveguide equipped with a diagonal-horn antenna. Using the frequency range of 215-580 GHz, a photomixer is used to launch a free-space circularly polarized electromagnetic field, exciting multiple modes at the input of the rectangular waveguide via an input diagonal-horn antenna. A second photomixer is used, together with a silicon mirror 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 that the polarization of the radiated field rotates by 45 if selectively exciting higher-order modes in the waveguide. Despite the higher-order modes, the radiated field appears to maintain a predominant Gaussian beam character, since an unidirectional coupling to a detector was possible, whereas the unidirectionality is independent of the frequency. We discuss a possible application of this finding.