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 metamaterial and surface plasmon suffer from the broad passband. In this paper, we propose a THz filter with calculated pass band of (Δf/f) <; 0.2% together with a peak transmission above 70% and demonstrate the experimental results with (Δf/f) <; 0.3% limited by the measurement system. The calculated value of (Δf/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 (Δf/f) <; 0.3% limited by the spectral resolution and detectability of our measurement system.