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