Square-shaped metal screens in the infrared to terahertz spectral region: Resonance frequency, band gap, and bandpass filter characteristics

Resonance frequency of freestanding, square-shaped thick metal screens have been studied here in the wavelength range of infrared (IR) to mm (20 to 0.2 THz). It was found that their peak transmission has a linear relationship to the screen's pitch. An experimental spectral feature, unaccounted for in typical simulations with plane parallel incident beams, was observed in the transmittance envelope for measurements in focused beams. In the past, this spectral feature was assigned to Wood's anomaly. Yet, unlike the latter, the observed spectral feature appears here in the long wavelength regime as well. We investigated this phenomenon for a large frequency range and assigned the spectral feature to the formation of a photonic band gap at oblique incidence. Many IR Fourier transform spectrometers use a noncollimated incident beam and such spectral features will appear whenever the local state of polarization includes components which are parallel to the plane of incidence.