Proof of concept for continuously-tunable terahertz bandpass filter based on a gradient metal-hole array

A continuously-tunable terahertz (THz) bandpass filter based on the resonant electromagnetic-wave transmission through a metal-hole array featuring a gradually changing period was developed and fabricated on a silicon substrate using optical lithography. A gradient geometry of the metal-hole array yields a wide tunability of the filter transmission, when operating with a focussed THz beam. The filter was studied numerically, using the finite element method, and experimentally, using the THz pulsed spectroscopy. We find that the central wavelength of the filter transmission band can be tuned in the wide range of λ_c = 400-800 μm with the relative bandwidth of Δλ/λ_c ≃ ~0.4. Finally, Kapton-based anti-reflection coating was applied to the filter flat side, in order to suppress an interference pattern in the filter transmission spectrum. We believe that the developed filter holds strong potential for multispectral THz imaging and sensing due to its conceptual simplicity and case of operation. Moreover, the presented filter concept can be translated to other spectral ranges, where appropriate technologies are available for the fabrication of gradient sub-wavelength metal-hole arrays.