  In this Letter we show that a single rectangular hole exhibits transmission
resonances that appear near the cutoff wavelength of the hole waveguide. For
light polarized with the electric field pointing along the short axis, it is
shown that the normalized-to-area transmittance at resonance is proportional to
the ratio between the long and short sides, and to the dielectric constant
inside the hole. Importantly, this resonant transmission process is accompanied
by a huge enhancement of the electric field at both entrance and exit
interfaces of the hole. These findings open the possibility of using
rectangular holes for spectroscopic purposes or for exploring non-linear
effects.
