  Subwavelength-slotted parallel plate waveguides exhibit a localized
electromagnetic resonance bound to the slits at a frequency slightly below the
transverse electric cutoff [R. Merlin, Phys. Rev. X 2, 031015 (2012)]. The
resonance is long-lived and, as opposed to the vanishingly small transmission
shown by a single sub-wavelength aperture, it gives perfect transmission for
perfectly-conducting plates. We show that the aperture-supported resonances of
a pair of slotted copper plates have long lifetimes at THz frequencies. Finite
element method calculations show that these bound resonances can have quality
factors greater than 100. Effects of plate length and imperfect parallel
alignment are also discussed. Using THz time domain spectroscopy, we measured
the transmission of a broadband pulse through a test structure for several
plate separations. These results suggest that the slotted waveguide can
function as a highly compact THz spectrometer.
