  We demonstrate experimentally the resonant extinction of THz radiation by a
single plasmonic bowtie antenna, formed by two n-doped Si monomers with a
triangular shape and facing apexes. This demonstration is achieved by placing
the antenna at the output aperture of a conically tapered waveguide, which
enhances the intensity of the incident THz field at the antenna position by a
factor 10. The waveguide also suppresses the background radiation that
otherwise is transmitted without being scattered by the antenna. Bowtie
antennas, supporting localized surface plasmon polaritons, are relevant due to
their ability of resonantly enhancing the field intensity at the gap separating
the two triangular elements. This gap has subwavelength dimensions, which
allows the concentration of THz radiation beyond the diffraction limit. The
combination of a bowtie plasmonic antenna and a conical waveguide may serve as
a platform for far-field THz time-domain spectroscopy of single nanostructures
placed in the gap.
