  We demonstrate an air-core single-mode hollow waveguide that uses Bragg
reflector structures in place of the vertical metal walls of the standard
rectangular waveguide or via holes of the so-called substrate integrated
waveguide. The high-order modes in the waveguide are substantially suppressed
by a modal-filtering effect, making the waveguide operate in the fundamental
mode over more than one octave. Numerical simulations show that the propagation
loss of the proposed waveguide can be lower than that of classic hollow
metallic rectangular waveguides at terahertz frequencies, benefiting from a
significant reduction in Ohmic loss. To facilitate fabrication and
characterization, a proof-of-concept 20 to 45 GHz waveguide is demonstrated,
which verifies the properties and advantages of the proposed waveguide. A zero
group-velocity dispersion point is observed at near the middle of the operating
band. This work offers a step towards a novel hybrid transmission-line medium
that can be used in a variety of functional components for broadband
millimeter-wave and terahertz applications.
