  We consider arrays of square coaxial apertures in a gold layer and study
their diffractive behavior in the far infrared region. These structures exhibit
a resonant transmission enhancement that is used to design tunable bandpass
filters. We provide a study of their spectral features and show by a modal
analysis that the resonance peak is due to the excitation of leaky modes of the
open photonic structure. Fourier transform infrared (FTIR) spectrophotometry
transmission measurements of samples deposited on Si substrate show good
agreement with numerical results and demonstrate angular tolerance up to 30
degrees of the fabricated filters.
