Narrow-Linewidth and High-Transmission Terahertz Bandpass Filtering by Metallic Gratings

Accurate spectral resolving capability is important to THz spectroscopy. So far, the THz passband filters based on meta-material and surface plasmon suffer from the broad passband. In this paper, we propose a THz filter with calculated pass band of (Delta f/f) < 0.2% together with a peak transmission above 70% and demonstrate the experimental results with (Delta f/f) < 3% limited by the measurement system. The calculated value of Delta f/f is at least one order of magnitude smaller than the previous results. The unique filtering capability was attributed to the guided-mode resonance that is generated by metallic gratings and a waveguide. Compared to the dielectric gratings, the metallic gratings with a narrow air slit are proved to effectively suppress off-resonance wavelength transmission resulting in a bandpass filtering response. In addition, a weak waveguide is found to benefit a narrow-linewidth filtering. Fabrication method of metallic gratings on membrane in a size of several cm was developed and the experimental results show reasonably good agreement with the simulation for a filter with (Delta f/f) < 3% limited by the spectral resolution and detectability of our measurement system.