A double layer FSS filter for sub-THz applications

 Ghavidel, A., M. Kokkonen, and S. Myllymäki. "A double layer FSS filter for sub-THz applications." Scientific Reports 11, no. 1 (2021): 1-8.

Abstract
This work presents the simulated and measured performance of single- and double-layer frequency selective surface filters for operation at sub-THz frequencies (250 GHz center frequency). They were composed of concentric square loops with a split as a unit cell resonator on top of a low dielectric permittivity, low thickness material (RT5880). Both a single layer filter and a cascaded two layer filter with varied distances were investigated. The simulated bandwidth for the cascaded filter was 27 GHz and 16 GHz and 9 GHz bandwidth measured with a THz-TDS and microwave system.

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Measurement using THz time-domain spectroscopy

A TeraPulse system (TeraPulse 4000, TeraView UK,  Fig. 7a, was used to measure the FSS filter’s impact on time-delay and transmittance and the results are shown in Fig. 7b,c. The single layer FSS filter was placed between two mirrors and the thickness of the FSS created a time-delay (around 0.1 picosecond) and reduced the amplitude to 0.9 compared to the reference signal amplitude (12.5). With a second layer, the time-delay increased to 0.5 ps and reduced the transmittance amplitude to 0.43. It was observed that each layer reduced the transmitted wave amplitude by around 30%. The bandwidth was measured to be ~ 12 GHz with one layer and ~ 16 GHz with two layers at 95% transmittance level.

Figure 7

(a) TeraPulse 4000 Spectroscopy system for measuring FSS filter, measured and normalized (b) measured pulse response of electric field amplitude where the refence signal was measured without the FSS layer and (c) measured signal transmittance of single- and double-layer unit cell.