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

Abstract

Resonance frequency of freestanding, square-shaped thick metal screens have been studied here in the wavelength range of infrared ͑IR͒ to mm ͑20 to 0.2 THz͒. 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.

Optical cages

“…Periodic metallo-dielectric structures (also known as screens, or metal meshes) have been studied in the past, and in particular in the long wavelength region -the wavelength region where the array pitch is of the order of, or smaller than the radiation wavelength [12][13][14]. This wavelength region is above the diffraction region.…”

Curved infrared screens

“…Square-shaped freestanding metal screens are commercially available and have been used as bandpass filters [2], reflectors for long IR wavelengths, Fabry-Perot etalons [3], and antennas [1]. In principle, such flat periodic structures enable transmission (or reflection) of specific IR bands by invoking standing wave surface modes: the incident beam is coupled to the surface waves by the periodic structure, which in turn supports standing waves [4]. The standing waves on both sides of the screen are coupled through waveguide modes in the structure opening.…”

“…This approach is quite intuitive and is substantiated below. Much insight has been gained in dealing with flat and tilted screens for various polarizations [4] and we will utilize it here.…”

“…The polarization state of the incident beam was determined by an IR polarizer. Typically, peak splitting occurs in these spectrometers even when the screen is placed flat at normal incidence [4]. Simulations for flat and tilted screens have been made using Ansoft HFSS and CST commercial packages; unfortunately, commercial packages cannot simulate curved structures effectively.…”

“…The last two equations are consistent if the Bragg reflection of the surface mode is made with the second (or higher) order of G. At normal incidence, the excited SP mode in these metal screens is doubly degenerate. Resonance occurs for wavelengths approximately matching the periodicity constant of the screen [4]. As the screen is tilted with respect to the incident beam, this degeneracy is lifted and a frequency gap is formed, hence the peak splitting for a p-polarized incident beam.…”