  We study experimentally the free-space electro-magnetic field emitted from a
multimode 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 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 degree 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.
